From cb78abf9e9ac10cf44701a2631e40cb0af94ce88 Mon Sep 17 00:00:00 2001
From: Hojun-Cho <ghwns6743@gmail.com>
Date: Tue, 12 May 2026 03:49:09 +0900
Subject: [PATCH] selfhost: is/as validity + let/return assignability checks
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three more structural checks from C check.c ported to selfhost,
at the AST level (no resolved tinfo).

is/as validity: e is T / e as T require e's declared type to be a
tagged union and T to name a variant. Mirrors the case-variant
check that just landed.

let init-type and return-type assignability: a new exprtype helper
infers an AST type-node for literal/ident/call/cast/?/as/is
expressions; isassignable approximates C type_assignable on the
shapes we can resolve — exact match, untyped numeric → typed
numeric, untyped nil → ptr/slice/chan/fn, variant inclusion, and
two-primitive-mismatch.

isassignable returns (ok, confident). When confident=false the
check emits no error — better to miss a real bug than fire a
false positive on a binary-op expression we can't infer. This
keeps existing selfhost code clean while still catching the
common typo cases (let x: bool = 42; return "hi" from i32 fn).

Naming: all new helpers follow Plan 9 run-together convention per
CLAUDE.md (`typeeqast`, `isassignable`, `exprtype`, ...). Earlier
work that used snake_case helpers (`case_variant_in`,
`check_match_exhaustive`, ...) got the same treatment — bulk
renamed in this commit.

Five new rows in 950_selfcheck exercise the new checks
(is-not-a-variant, two let mismatches, return mismatch, plus the
case-variant row already there).
---
 selfhost/cmd/w6c/main.combined.ww    | 423 +++++++++++++++++++++++----
 selfhost/cmd/wcc/check.ww            | 423 +++++++++++++++++++++++----
 selfhost/cmd/wwdump/main.combined.ww | 423 +++++++++++++++++++++++----
 test/wcc/950_selfcheck.c             |  25 ++
 4 files changed, 1144 insertions(+), 150 deletions(-)

diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index f1b1877c..8f556376 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -3808,8 +3808,12 @@ fn resolvewalk(c: *checker, n: *node) void = {
 
 	// Typed checks fire on the way down so the scrutinee/operand
 	// is examined before the arm bodies install new bindings.
-	if (k == N_MATCH)   { check_match_exhaustive(c, n); };
-	if (k == N_TRYPROP) { check_tryprop(c, n); };
+	if (k == N_MATCH)   { checkmatchexhaust(c, n); };
+	if (k == N_TRYPROP) { checktryprop(c, n); };
+	if (k == N_TYPETEST)   { checkisas(c, n); };
+	if (k == N_TYPEASSERT) { checkisas(c, n); };
+	if (k == N_LET)     { checkletassign(c, n); };
+	if (k == N_RETURN)  { checkretassign(c, n); };
 
 	// `use IDENT;` — name is a module label, not a free ident.
 	if (k == N_USE) { return; };
@@ -3956,19 +3960,11 @@ fn resolvealias(c: *checker, n: *node) *node = {
 	return n;
 };
 
-// is_tagged_type — true if `n` (after alias resolution) is an
-// N_TTAGGED type expression.
-fn is_tagged_type(c: *checker, n: *node) bool = {
-	let u: *node = resolvealias(c, unwrapbang(n));
-	if (u == nil) { return false; };
-	return u.kind == N_TTAGGED;
-};
-
-// type_eq_ast — structural equality on AST type expressions, mod
+// typeeqast — structural equality on AST type expressions, mod
 // the `!` wrapper. Mirrors variant_match in cgen + check.c: NAMED
 // types compare by string (the closest stand-in for pointer
 // identity at the AST level); other nodes recurse by kind.
-fn type_eq_ast(a: *node, b: *node) bool = {
+fn typeeqast(a: *node, b: *node) bool = {
 	let aa: *node = unwrapbang(a);
 	let bb: *node = unwrapbang(b);
 	if (aa == nil) { return bb == nil; };
@@ -3976,19 +3972,19 @@ fn type_eq_ast(a: *node, b: *node) bool = {
 	if (aa.kind != bb.kind) { return false; };
 	let k: i32 = aa.kind;
 	if (k == N_TNAME) { return streq(aa.str, bb.str); };
-	if (k == N_TPTR)  { return type_eq_ast(aa.lhs, bb.lhs); };
-	if (k == N_TSLICE){ return type_eq_ast(aa.lhs, bb.lhs); };
-	if (k == N_TCHAN) { return type_eq_ast(aa.lhs, bb.lhs); };
+	if (k == N_TPTR)  { return typeeqast(aa.lhs, bb.lhs); };
+	if (k == N_TSLICE){ return typeeqast(aa.lhs, bb.lhs); };
+	if (k == N_TCHAN) { return typeeqast(aa.lhs, bb.lhs); };
 	// Conservative: anything else (struct/fn/tagged/tuple/array)
 	// fails the cheap check. Selfhost code doesn't currently rely
 	// on equality at these shapes for the targeted checks.
 	return false;
 };
 
-// variant_is_error — does this variant carry the `!` mark? Either
+// varianterr — does this variant carry the `!` mark? Either
 // the variant itself is N_TBANG or it's an alias whose typedecl
 // body is `!T`. Mirrors C check.c's iserror-after-NAMED rule.
-fn variant_is_error(c: *checker, v: *node) bool = {
+fn varianterr(c: *checker, v: *node) bool = {
 	if (v == nil) { return false; };
 	if (v.kind == N_TBANG) { return true; };
 	if (v.kind == N_TNAME) {
@@ -4008,35 +4004,35 @@ fn variant_is_error(c: *checker, v: *node) bool = {
 	return false;
 };
 
-// tagged_has_errflag — true iff any variant of `n` (assumed
+// taggedhaserr — true iff any variant of `n` (assumed
 // N_TTAGGED) is `!`-marked. Picks the explicit-flag semantics over
 // the legacy "first variant = success" rule.
-fn tagged_has_errflag(c: *checker, n: *node) bool = {
+fn taggedhaserr(c: *checker, n: *node) bool = {
 	let v: *node = n.list;
 	for (v != nil) {
-		if (variant_is_error(c, v)) { return true; };
+		if (varianterr(c, v)) { return true; };
 		v = v.next;
 	};
 	return false;
 };
 
-// is_error_variant — under flag-aware mode (any !-marked variant),
+// iserrvariant — under flag-aware mode (any !-marked variant),
 // returns true iff `v` is `!`-marked. Under legacy mode (no flags),
 // returns true iff `v` is not the first variant of `tagged`.
-fn is_error_variant(c: *checker, tagged: *node, v: *node) bool = {
-	if (tagged_has_errflag(c, tagged)) {
-		return variant_is_error(c, v);
+fn iserrvariant(c: *checker, tagged: *node, v: *node) bool = {
+	if (taggedhaserr(c, tagged)) {
+		return varianterr(c, v);
 	};
 	// Legacy: first variant of the union is success.
 	if (tagged.list == v) { return false; };
 	return true;
 };
 
-// scrutinee_type — resolve the type expression for a match's
+// scruttype — resolve the type expression for a match's
 // scrutinee. Handles N_IDENT (look up local/param's declared
 // type) and N_DOT (struct-field access). Returns nil if we
 // can't statically determine the type. Used by exhaustiveness.
-fn scrutinee_type(c: *checker, e: *node) *node = {
+fn scruttype(c: *checker, e: *node) *node = {
 	if (e == nil) { return nil; };
 	if (e.kind == N_IDENT) {
 		let s: *sym = scopelookup(c.cur, e.str);
@@ -4048,25 +4044,264 @@ fn scrutinee_type(c: *checker, e: *node) *node = {
 	return nil;
 };
 
+// mktname — fabricate an N_TNAME node with str = `nm`. Used by
+// exprtype to return primitive type nodes for literal
+// expressions. The arena keeps them around as long as the checker.
+fn mktname(c: *checker, nm: str) *node = {
+	let n: *node = newnode(c.a, N_TNAME, "", 0, 0);
+	n.str = nm;
+	return n;
+};
+
+// exprtype — best-effort type-AST inference for an expression
+// node. Handles literals, identifiers, calls, and casts; returns
+// nil for shapes we don't statically know (binary ops, struct
+// field access into non-primitive types, etc).
+fn exprtype(c: *checker, e: *node) *node = {
+	if (e == nil) { return nil; };
+	let k: i32 = e.kind;
+	if (k == N_INTLIT)   { return mktname(c, "untyped_int"); };
+	if (k == N_FLOATLIT) { return mktname(c, "untyped_float"); };
+	if (k == N_STRLIT)   { return mktname(c, "str"); };
+	if (k == N_RUNELIT)  { return mktname(c, "rune"); };
+	if (k == N_TRUE)     { return mktname(c, "bool"); };
+	if (k == N_FALSE)    { return mktname(c, "bool"); };
+	if (k == N_VOIDLIT)  { return mktname(c, "void"); };
+	if (k == N_NIL)      { return mktname(c, "untyped_nil"); };
+	if (k == N_IDENT) {
+		let s: *sym = scopelookup(c.cur, e.str);
+		if (s == nil) { return nil; };
+		if (s.decl == nil) { return nil; };
+		return s.decl.lhs;
+	};
+	if (k == N_CAST) {
+		// `expr: T` — explicit cast; the type expr is e.rhs.
+		return e.rhs;
+	};
+	if (k == N_CALL) {
+		let callee: *node = e.lhs;
+		if (callee == nil) { return nil; };
+		let nm: str;
+		nm.ptr = nil; nm.len = 0;
+		if (callee.kind == N_IDENT) { nm = callee.str; };
+		if (callee.kind == N_DOT)   { nm = callee.str; };
+		if (nm.len == 0) { return nil; };
+		let s: *sym = scopelookup(c.cur, nm);
+		if (s == nil) { return nil; };
+		if (s.skind != SK_FN) { return nil; };
+		if (s.decl == nil) { return nil; };
+		return s.decl.lhs;	// fn-decl's lhs is the return type
+	};
+	if (k == N_TRYPROP) {
+		// success unwrap: the success-variant type of operand's
+		// tagged union.
+		let opt: *node = exprtype(c, e.lhs);
+		let ou: *node = resolvealias(c, unwrapbang(opt));
+		if (ou == nil) { return nil; };
+		if (ou.kind != N_TTAGGED) { return nil; };
+		// Hare semantics: success = first non-error variant if
+		// any !-flag is present; else first variant.
+		if (taggedhaserr(c, ou)) {
+			let v: *node = ou.list;
+			for (v != nil) {
+				if (!iserrvariant(c, ou, v)) { return v; };
+				v = v.next;
+			};
+			return nil;
+		};
+		return ou.list;
+	};
+	if (k == N_TYPEASSERT) {
+		// `e as T` → T
+		return e.rhs;
+	};
+	if (k == N_TYPETEST) {
+		// `e is T` → bool
+		return mktname(c, "bool");
+	};
+	return nil;
+};
+
+// isuntypedint / is_str_like / is_bool_like — helpers used
+// by the assignability check below to allow common AST shapes
+// through without needing real type inference.
+fn isuntypedint(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_int");
+};
+
+fn isuntypedfloat(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_float");
+};
+
+fn isuntypednil(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_nil");
+};
+
+fn isnumerictname(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	let s: str = t.str;
+	if (streq(s, "i8"))  { return true; };
+	if (streq(s, "i16")) { return true; };
+	if (streq(s, "i32")) { return true; };
+	if (streq(s, "i64")) { return true; };
+	if (streq(s, "u8"))  { return true; };
+	if (streq(s, "u16")) { return true; };
+	if (streq(s, "u32")) { return true; };
+	if (streq(s, "u64")) { return true; };
+	if (streq(s, "int"))  { return true; };
+	if (streq(s, "uint")) { return true; };
+	if (streq(s, "uintptr")) { return true; };
+	if (streq(s, "rune")) { return true; };
+	if (streq(s, "f32"))  { return true; };
+	if (streq(s, "f64"))  { return true; };
+	return false;
+};
+
+fn isstrtname(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "str");
+};
+
+// isassignable — AST-level approximation of C check.c
+// type_assignable. Returns true when we know the assignment is
+// OK, false only when we're confident it isn't, and "skip" (true)
+// when we can't tell — to avoid false positives. The trailing bool
+// `confident` lets the caller decide whether to emit an error
+// when the result is false: if !confident, the caller should not
+// flag it.
+fn isassignable(c: *checker, dst: *node, src: *node, confident: *bool) bool = {
+	*confident = false;
+	if (dst == nil) { return true; };	// no declared target
+	if (src == nil) { return true; };	// unknown src type
+	*confident = true;
+	let du: *node = resolvealias(c, unwrapbang(dst));
+	let su: *node = resolvealias(c, unwrapbang(src));
+	if (du == nil) { *confident = false; return true; };
+	if (su == nil) { *confident = false; return true; };
+	if (typeeqast(du, su)) { return true; };
+	// untyped numeric → any numeric named type.
+	if (isuntypedint(su)) {
+		if (isnumerictname(du)) { return true; };
+		// (T | ...) tagged: only OK if some variant accepts untyped_int.
+		if (du.kind == N_TTAGGED) {
+			let v: *node = du.list;
+			for (v != nil) {
+				let vu: *node = resolvealias(c, unwrapbang(v));
+				if (vu != nil) {
+					if (isnumerictname(vu)) { return true; };
+				};
+				v = v.next;
+			};
+			*confident = false;
+			return true;
+		};
+		// Known non-numeric primitive: confidently wrong.
+		if (du.kind == N_TNAME) {
+			if (streq(du.str, "bool")) { return false; };
+			if (streq(du.str, "void")) { return false; };
+			if (streq(du.str, "str"))  { return false; };
+		};
+		// Unknown shapes: stay quiet.
+		*confident = false;
+		return true;
+	};
+	if (isuntypedfloat(su)) {
+		if (isnumerictname(du)) { return true; };
+		if (du.kind == N_TNAME) {
+			if (streq(du.str, "bool")) { return false; };
+			if (streq(du.str, "void")) { return false; };
+			if (streq(du.str, "str"))  { return false; };
+		};
+		*confident = false;
+		return true;
+	};
+	if (isuntypednil(su)) {
+		// nil → ptr/slice/chan/fn/nullable
+		if (du.kind == N_TPTR)   { return true; };
+		if (du.kind == N_TSLICE) { return true; };
+		if (du.kind == N_TCHAN)  { return true; };
+		if (du.kind == N_TFN)    { return true; };
+		// nullable `(*T | void)` — already accepted by typeeqast
+		// when matched whole; nil is OK there too.
+		if (du.kind == N_TTAGGED) {
+			let v: *node = du.list;
+			for (v != nil) {
+				if (v.kind == N_TPTR)  { return true; };
+				if (v.kind == N_TSLICE){ return true; };
+				v = v.next;
+			};
+		};
+		*confident = false;
+		return true;
+	};
+	// Tagged-union variant inclusion: src is one of dst's variants.
+	if (du.kind == N_TTAGGED && su.kind != N_TTAGGED) {
+		let v: *node = du.list;
+		for (v != nil) {
+			let vu: *node = resolvealias(c, unwrapbang(v));
+			if (vu != nil) {
+				if (typeeqast(vu, su)) { return true; };
+			};
+			v = v.next;
+		};
+		return false;
+	};
+	// tagged → tagged: structural variant list compare. Skip
+	// (don't be confident) — common when forwarding a fallible
+	// return through another fn with the same shape but possibly
+	// a different surface spelling.
+	if (du.kind == N_TTAGGED && su.kind == N_TTAGGED) {
+		*confident = false;
+		return true;
+	};
+	// Two known primitives with different names are confidently
+	// incompatible. `i32 ↔ bool`, `str ↔ i32`, etc.
+	if (du.kind == N_TNAME && su.kind == N_TNAME) {
+		let known_d: bool = isnumerictname(du) || isstrtname(du);
+		if (!known_d) { if (streq(du.str, "bool")) { known_d = true; }; };
+		if (!known_d) { if (streq(du.str, "void")) { known_d = true; }; };
+		let known_s: bool = isnumerictname(su) || isstrtname(su);
+		if (!known_s) { if (streq(su.str, "bool")) { known_s = true; }; };
+		if (!known_s) { if (streq(su.str, "void")) { known_s = true; }; };
+		if (known_d) {
+			if (known_s) {
+				// Both primitives, different names → no.
+				return false;
+			};
+		};
+	};
+	// Anything else: don't claim confidence.
+	*confident = false;
+	return true;
+};
+
 // ---- match exhaustiveness --------------------------------------------
 //
 // For every match arm, verify that every variant of the scrutinee's
 // tagged-union type is handled by some case (or a default arm
 // exists). Multi-pattern `case A | B =>` covers all alts.
 
-fn case_covers(c: *checker, cs: *node, want: *node) bool = {
+fn casecovers(c: *checker, cs: *node, want: *node) bool = {
 	if (cs.lhs != nil) {
-		if (type_eq_ast(cs.lhs, want)) { return true; };
+		if (typeeqast(cs.lhs, want)) { return true; };
 	};
 	let alt: *node = cs.list;
 	for (alt != nil) {
-		if (type_eq_ast(alt, want)) { return true; };
+		if (typeeqast(alt, want)) { return true; };
 		alt = alt.next;
 	};
 	return false;
 };
 
-fn err_match_variant(c: *checker, n: *node, vname: *node) void = {
+fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
 	os.write(2, "match: variant not handled".ptr, 26u64);
 	if (vname != nil) {
 		if (vname.kind == N_TNAME) {
@@ -4079,19 +4314,19 @@ fn err_match_variant(c: *checker, n: *node, vname: *node) void = {
 	c.errs += 1;
 };
 
-// case_variant_in — true iff `pat` (a `case T` pattern, including
+// casevariantin — true iff `pat` (a `case T` pattern, including
 // each alt of a multi-pattern) names a variant of the tagged
 // union `tagged`.
-fn case_variant_in(tagged: *node, pat: *node) bool = {
+fn casevariantin(tagged: *node, pat: *node) bool = {
 	let v: *node = tagged.list;
 	for (v != nil) {
-		if (type_eq_ast(v, pat)) { return true; };
+		if (typeeqast(v, pat)) { return true; };
 		v = v.next;
 	};
 	return false;
 };
 
-fn err_bad_case_variant(c: *checker, pat: *node) void = {
+fn errbadcase(c: *checker, pat: *node) void = {
 	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
 	if (pat != nil) {
 		if (pat.kind == N_TNAME) {
@@ -4104,10 +4339,10 @@ fn err_bad_case_variant(c: *checker, pat: *node) void = {
 	c.errs += 1;
 };
 
-fn check_match_exhaustive(c: *checker, n: *node) void = {
+fn checkmatchexhaust(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
-	let st: *node = scrutinee_type(c, n.lhs);
+	let st: *node = scruttype(c, n.lhs);
 	let u: *node = resolvealias(c, unwrapbang(st));
 	if (u == nil) { return; };
 	if (u.kind != N_TTAGGED) { return; };
@@ -4117,13 +4352,13 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 	let cs0: *node = n.list;
 	for (cs0 != nil) {
 		if (cs0.lhs != nil) {
-			if (!case_variant_in(u, cs0.lhs)) {
-				err_bad_case_variant(c, cs0.lhs);
+			if (!casevariantin(u, cs0.lhs)) {
+				errbadcase(c, cs0.lhs);
 			};
 			let alt: *node = cs0.list;
 			for (alt != nil) {
-				if (!case_variant_in(u, alt)) {
-					err_bad_case_variant(c, alt);
+				if (!casevariantin(u, alt)) {
+					errbadcase(c, alt);
 				};
 				alt = alt.next;
 			};
@@ -4142,18 +4377,106 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 		let covered: bool = false;
 		let cs2: *node = n.list;
 		for (cs2 != nil) {
-			if (case_covers(c, cs2, v)) {
+			if (casecovers(c, cs2, v)) {
 				covered = true;
 				cs2 = nil;
 			} else {
 				cs2 = cs2.next;
 			};
 		};
-		if (!covered) { err_match_variant(c, n, v); };
+		if (!covered) { errmatchvariant(c, n, v); };
 		v = v.next;
 	};
 };
 
+// ---- let init / return assignability --------------------------------
+//
+// AST-level approximation: when we can infer src's type and dst is
+// explicitly declared, verify isassignable. We only emit an error
+// when isassignable says "false with confidence." If we can't tell
+// (binary ops, complex exprs we don't infer), we stay quiet — full
+// type inference lives only on the C side.
+
+fn errnotassign(c: *checker, dst: *node, src: *node, where: str) void = {
+	os.write(2, where.ptr, where.len: u64);
+	os.write(2, ": not assignable".ptr, 16u64);
+	if (src != nil) {
+		if (src.kind == N_TNAME) {
+			os.write(2, " (".ptr, 2u64);
+			os.write(2, src.str.ptr, src.str.len: u64);
+			os.write(2, " → ".ptr, 5u64);
+			if (dst != nil) {
+				if (dst.kind == N_TNAME) {
+					os.write(2, dst.str.ptr, dst.str.len: u64);
+				};
+			};
+			os.write(2, ")".ptr, 1u64);
+		};
+	};
+	os.write(2, "\n".ptr, 1u64);
+	c.errs += 1;
+};
+
+fn checkletassign(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) { return; };	// no declared type, nothing to check
+	if (n.rhs == nil) { return; };	// no init
+	let src: *node = exprtype(c, n.rhs);
+	if (src == nil) { return; };	// can't infer
+	let conf: bool = false;
+	let ok: bool = isassignable(c, n.lhs, src, &conf);
+	if (!conf) { return; };
+	if (!ok) { errnotassign(c, n.lhs, src, "let"); };
+};
+
+fn checkretassign(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) {
+		// bare `return;` — OK iff fnret is void or a tagged union
+		// with a void variant. Skip flagging for now; cgen handles
+		// the void-variant tag synthesis already.
+		return;
+	};
+	if (c.fnret == nil) { return; };
+	let src: *node = exprtype(c, n.lhs);
+	if (src == nil) { return; };
+	let conf: bool = false;
+	let ok: bool = isassignable(c, c.fnret, src, &conf);
+	if (!conf) { return; };
+	if (!ok) { errnotassign(c, c.fnret, src, "return"); };
+};
+
+// ---- is / as validity ------------------------------------------------
+//
+// `e is T` and `e as T` require that e's declared type be a tagged
+// union and that T name one of its variants. Operates on AST type
+// expressions; falls back silently when we can't determine e's
+// type (matches the case-variant rule for match).
+fn checkisas(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	// e is in n.lhs (value), T is in n.rhs (type expr).
+	let st: *node = scruttype(c, n.lhs);
+	let u: *node = resolvealias(c, unwrapbang(st));
+	if (u == nil) { return; };
+	if (u.kind != N_TTAGGED) {
+		os.write(2, "is/as: operand is not a tagged union\n".ptr, 37u64);
+		c.errs += 1;
+		return;
+	};
+	let want: *node = n.rhs;
+	if (want == nil) { return; };
+	if (!casevariantin(u, want)) {
+		os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+		if (want.kind == N_TNAME) {
+			os.write(2, " (".ptr, 2u64);
+			os.write(2, want.str.ptr, want.str.len: u64);
+			os.write(2, ")".ptr, 1u64);
+		};
+		os.write(2, "\n".ptr, 1u64);
+		c.errs += 1;
+	};
+};
+
 // ---- ? subset propagation --------------------------------------------
 //
 // For `expr?`, the operand's error subset must be a subset of the
@@ -4161,7 +4484,7 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 // is N_TRYPROP; its lhs is the value-bearing expr; we look at the
 // expr's *declared* type for N_IDENT/N_CALL cases.
 
-fn expr_type_for_tryprop(c: *checker, e: *node) *node = {
+fn exprtypeoftry(c: *checker, e: *node) *node = {
 	if (e == nil) { return nil; };
 	if (e.kind == N_IDENT) {
 		let s: *sym = scopelookup(c.cur, e.str);
@@ -4188,20 +4511,20 @@ fn expr_type_for_tryprop(c: *checker, e: *node) *node = {
 	return nil;
 };
 
-fn check_tryprop(c: *checker, n: *node) void = {
+fn checktryprop(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
-	let t: *node = expr_type_for_tryprop(c, n.lhs);
+	let t: *node = exprtypeoftry(c, n.lhs);
 	let u: *node = resolvealias(c, unwrapbang(t));
 	if (u == nil) { return; };
 	if (u.kind != N_TTAGGED) { return; };
 	// Does the operand have any error variants?
-	let has_err: bool = false;
+	let haserr: bool = false;
 	let v: *node = u.list;
 	for (v != nil) {
-		if (is_error_variant(c, u, v)) { has_err = true; };
+		if (iserrvariant(c, u, v)) { haserr = true; };
 		v = v.next;
 	};
-	if (!has_err) { return; };
+	if (!haserr) { return; };
 	// Enclosing fn must return a tagged union with each operand
 	// error variant present.
 	let r: *node = resolvealias(c, unwrapbang(c.fnret));
@@ -4217,11 +4540,11 @@ fn check_tryprop(c: *checker, n: *node) void = {
 	};
 	let ev: *node = u.list;
 	for (ev != nil) {
-		if (is_error_variant(c, u, ev)) {
+		if (iserrvariant(c, u, ev)) {
 			let found: bool = false;
 			let rv: *node = r.list;
 			for (rv != nil) {
-				if (type_eq_ast(rv, ev)) {
+				if (typeeqast(rv, ev)) {
 					found = true;
 					rv = nil;
 				} else { rv = rv.next; };
diff --git a/selfhost/cmd/wcc/check.ww b/selfhost/cmd/wcc/check.ww
index feff0a0e..0ea5ff51 100644
--- a/selfhost/cmd/wcc/check.ww
+++ b/selfhost/cmd/wcc/check.ww
@@ -91,8 +91,12 @@ fn resolvewalk(c: *checker, n: *node) void = {
 
 	// Typed checks fire on the way down so the scrutinee/operand
 	// is examined before the arm bodies install new bindings.
-	if (k == N_MATCH)   { check_match_exhaustive(c, n); };
-	if (k == N_TRYPROP) { check_tryprop(c, n); };
+	if (k == N_MATCH)   { checkmatchexhaust(c, n); };
+	if (k == N_TRYPROP) { checktryprop(c, n); };
+	if (k == N_TYPETEST)   { checkisas(c, n); };
+	if (k == N_TYPEASSERT) { checkisas(c, n); };
+	if (k == N_LET)     { checkletassign(c, n); };
+	if (k == N_RETURN)  { checkretassign(c, n); };
 
 	// `use IDENT;` — name is a module label, not a free ident.
 	if (k == N_USE) { return; };
@@ -239,19 +243,11 @@ fn resolvealias(c: *checker, n: *node) *node = {
 	return n;
 };
 
-// is_tagged_type — true if `n` (after alias resolution) is an
-// N_TTAGGED type expression.
-fn is_tagged_type(c: *checker, n: *node) bool = {
-	let u: *node = resolvealias(c, unwrapbang(n));
-	if (u == nil) { return false; };
-	return u.kind == N_TTAGGED;
-};
-
-// type_eq_ast — structural equality on AST type expressions, mod
+// typeeqast — structural equality on AST type expressions, mod
 // the `!` wrapper. Mirrors variant_match in cgen + check.c: NAMED
 // types compare by string (the closest stand-in for pointer
 // identity at the AST level); other nodes recurse by kind.
-fn type_eq_ast(a: *node, b: *node) bool = {
+fn typeeqast(a: *node, b: *node) bool = {
 	let aa: *node = unwrapbang(a);
 	let bb: *node = unwrapbang(b);
 	if (aa == nil) { return bb == nil; };
@@ -259,19 +255,19 @@ fn type_eq_ast(a: *node, b: *node) bool = {
 	if (aa.kind != bb.kind) { return false; };
 	let k: i32 = aa.kind;
 	if (k == N_TNAME) { return streq(aa.str, bb.str); };
-	if (k == N_TPTR)  { return type_eq_ast(aa.lhs, bb.lhs); };
-	if (k == N_TSLICE){ return type_eq_ast(aa.lhs, bb.lhs); };
-	if (k == N_TCHAN) { return type_eq_ast(aa.lhs, bb.lhs); };
+	if (k == N_TPTR)  { return typeeqast(aa.lhs, bb.lhs); };
+	if (k == N_TSLICE){ return typeeqast(aa.lhs, bb.lhs); };
+	if (k == N_TCHAN) { return typeeqast(aa.lhs, bb.lhs); };
 	// Conservative: anything else (struct/fn/tagged/tuple/array)
 	// fails the cheap check. Selfhost code doesn't currently rely
 	// on equality at these shapes for the targeted checks.
 	return false;
 };
 
-// variant_is_error — does this variant carry the `!` mark? Either
+// varianterr — does this variant carry the `!` mark? Either
 // the variant itself is N_TBANG or it's an alias whose typedecl
 // body is `!T`. Mirrors C check.c's iserror-after-NAMED rule.
-fn variant_is_error(c: *checker, v: *node) bool = {
+fn varianterr(c: *checker, v: *node) bool = {
 	if (v == nil) { return false; };
 	if (v.kind == N_TBANG) { return true; };
 	if (v.kind == N_TNAME) {
@@ -291,35 +287,35 @@ fn variant_is_error(c: *checker, v: *node) bool = {
 	return false;
 };
 
-// tagged_has_errflag — true iff any variant of `n` (assumed
+// taggedhaserr — true iff any variant of `n` (assumed
 // N_TTAGGED) is `!`-marked. Picks the explicit-flag semantics over
 // the legacy "first variant = success" rule.
-fn tagged_has_errflag(c: *checker, n: *node) bool = {
+fn taggedhaserr(c: *checker, n: *node) bool = {
 	let v: *node = n.list;
 	for (v != nil) {
-		if (variant_is_error(c, v)) { return true; };
+		if (varianterr(c, v)) { return true; };
 		v = v.next;
 	};
 	return false;
 };
 
-// is_error_variant — under flag-aware mode (any !-marked variant),
+// iserrvariant — under flag-aware mode (any !-marked variant),
 // returns true iff `v` is `!`-marked. Under legacy mode (no flags),
 // returns true iff `v` is not the first variant of `tagged`.
-fn is_error_variant(c: *checker, tagged: *node, v: *node) bool = {
-	if (tagged_has_errflag(c, tagged)) {
-		return variant_is_error(c, v);
+fn iserrvariant(c: *checker, tagged: *node, v: *node) bool = {
+	if (taggedhaserr(c, tagged)) {
+		return varianterr(c, v);
 	};
 	// Legacy: first variant of the union is success.
 	if (tagged.list == v) { return false; };
 	return true;
 };
 
-// scrutinee_type — resolve the type expression for a match's
+// scruttype — resolve the type expression for a match's
 // scrutinee. Handles N_IDENT (look up local/param's declared
 // type) and N_DOT (struct-field access). Returns nil if we
 // can't statically determine the type. Used by exhaustiveness.
-fn scrutinee_type(c: *checker, e: *node) *node = {
+fn scruttype(c: *checker, e: *node) *node = {
 	if (e == nil) { return nil; };
 	if (e.kind == N_IDENT) {
 		let s: *sym = scopelookup(c.cur, e.str);
@@ -331,25 +327,264 @@ fn scrutinee_type(c: *checker, e: *node) *node = {
 	return nil;
 };
 
+// mktname — fabricate an N_TNAME node with str = `nm`. Used by
+// exprtype to return primitive type nodes for literal
+// expressions. The arena keeps them around as long as the checker.
+fn mktname(c: *checker, nm: str) *node = {
+	let n: *node = newnode(c.a, N_TNAME, "", 0, 0);
+	n.str = nm;
+	return n;
+};
+
+// exprtype — best-effort type-AST inference for an expression
+// node. Handles literals, identifiers, calls, and casts; returns
+// nil for shapes we don't statically know (binary ops, struct
+// field access into non-primitive types, etc).
+fn exprtype(c: *checker, e: *node) *node = {
+	if (e == nil) { return nil; };
+	let k: i32 = e.kind;
+	if (k == N_INTLIT)   { return mktname(c, "untyped_int"); };
+	if (k == N_FLOATLIT) { return mktname(c, "untyped_float"); };
+	if (k == N_STRLIT)   { return mktname(c, "str"); };
+	if (k == N_RUNELIT)  { return mktname(c, "rune"); };
+	if (k == N_TRUE)     { return mktname(c, "bool"); };
+	if (k == N_FALSE)    { return mktname(c, "bool"); };
+	if (k == N_VOIDLIT)  { return mktname(c, "void"); };
+	if (k == N_NIL)      { return mktname(c, "untyped_nil"); };
+	if (k == N_IDENT) {
+		let s: *sym = scopelookup(c.cur, e.str);
+		if (s == nil) { return nil; };
+		if (s.decl == nil) { return nil; };
+		return s.decl.lhs;
+	};
+	if (k == N_CAST) {
+		// `expr: T` — explicit cast; the type expr is e.rhs.
+		return e.rhs;
+	};
+	if (k == N_CALL) {
+		let callee: *node = e.lhs;
+		if (callee == nil) { return nil; };
+		let nm: str;
+		nm.ptr = nil; nm.len = 0;
+		if (callee.kind == N_IDENT) { nm = callee.str; };
+		if (callee.kind == N_DOT)   { nm = callee.str; };
+		if (nm.len == 0) { return nil; };
+		let s: *sym = scopelookup(c.cur, nm);
+		if (s == nil) { return nil; };
+		if (s.skind != SK_FN) { return nil; };
+		if (s.decl == nil) { return nil; };
+		return s.decl.lhs;	// fn-decl's lhs is the return type
+	};
+	if (k == N_TRYPROP) {
+		// success unwrap: the success-variant type of operand's
+		// tagged union.
+		let opt: *node = exprtype(c, e.lhs);
+		let ou: *node = resolvealias(c, unwrapbang(opt));
+		if (ou == nil) { return nil; };
+		if (ou.kind != N_TTAGGED) { return nil; };
+		// Hare semantics: success = first non-error variant if
+		// any !-flag is present; else first variant.
+		if (taggedhaserr(c, ou)) {
+			let v: *node = ou.list;
+			for (v != nil) {
+				if (!iserrvariant(c, ou, v)) { return v; };
+				v = v.next;
+			};
+			return nil;
+		};
+		return ou.list;
+	};
+	if (k == N_TYPEASSERT) {
+		// `e as T` → T
+		return e.rhs;
+	};
+	if (k == N_TYPETEST) {
+		// `e is T` → bool
+		return mktname(c, "bool");
+	};
+	return nil;
+};
+
+// isuntypedint / is_str_like / is_bool_like — helpers used
+// by the assignability check below to allow common AST shapes
+// through without needing real type inference.
+fn isuntypedint(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_int");
+};
+
+fn isuntypedfloat(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_float");
+};
+
+fn isuntypednil(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_nil");
+};
+
+fn isnumerictname(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	let s: str = t.str;
+	if (streq(s, "i8"))  { return true; };
+	if (streq(s, "i16")) { return true; };
+	if (streq(s, "i32")) { return true; };
+	if (streq(s, "i64")) { return true; };
+	if (streq(s, "u8"))  { return true; };
+	if (streq(s, "u16")) { return true; };
+	if (streq(s, "u32")) { return true; };
+	if (streq(s, "u64")) { return true; };
+	if (streq(s, "int"))  { return true; };
+	if (streq(s, "uint")) { return true; };
+	if (streq(s, "uintptr")) { return true; };
+	if (streq(s, "rune")) { return true; };
+	if (streq(s, "f32"))  { return true; };
+	if (streq(s, "f64"))  { return true; };
+	return false;
+};
+
+fn isstrtname(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "str");
+};
+
+// isassignable — AST-level approximation of C check.c
+// type_assignable. Returns true when we know the assignment is
+// OK, false only when we're confident it isn't, and "skip" (true)
+// when we can't tell — to avoid false positives. The trailing bool
+// `confident` lets the caller decide whether to emit an error
+// when the result is false: if !confident, the caller should not
+// flag it.
+fn isassignable(c: *checker, dst: *node, src: *node, confident: *bool) bool = {
+	*confident = false;
+	if (dst == nil) { return true; };	// no declared target
+	if (src == nil) { return true; };	// unknown src type
+	*confident = true;
+	let du: *node = resolvealias(c, unwrapbang(dst));
+	let su: *node = resolvealias(c, unwrapbang(src));
+	if (du == nil) { *confident = false; return true; };
+	if (su == nil) { *confident = false; return true; };
+	if (typeeqast(du, su)) { return true; };
+	// untyped numeric → any numeric named type.
+	if (isuntypedint(su)) {
+		if (isnumerictname(du)) { return true; };
+		// (T | ...) tagged: only OK if some variant accepts untyped_int.
+		if (du.kind == N_TTAGGED) {
+			let v: *node = du.list;
+			for (v != nil) {
+				let vu: *node = resolvealias(c, unwrapbang(v));
+				if (vu != nil) {
+					if (isnumerictname(vu)) { return true; };
+				};
+				v = v.next;
+			};
+			*confident = false;
+			return true;
+		};
+		// Known non-numeric primitive: confidently wrong.
+		if (du.kind == N_TNAME) {
+			if (streq(du.str, "bool")) { return false; };
+			if (streq(du.str, "void")) { return false; };
+			if (streq(du.str, "str"))  { return false; };
+		};
+		// Unknown shapes: stay quiet.
+		*confident = false;
+		return true;
+	};
+	if (isuntypedfloat(su)) {
+		if (isnumerictname(du)) { return true; };
+		if (du.kind == N_TNAME) {
+			if (streq(du.str, "bool")) { return false; };
+			if (streq(du.str, "void")) { return false; };
+			if (streq(du.str, "str"))  { return false; };
+		};
+		*confident = false;
+		return true;
+	};
+	if (isuntypednil(su)) {
+		// nil → ptr/slice/chan/fn/nullable
+		if (du.kind == N_TPTR)   { return true; };
+		if (du.kind == N_TSLICE) { return true; };
+		if (du.kind == N_TCHAN)  { return true; };
+		if (du.kind == N_TFN)    { return true; };
+		// nullable `(*T | void)` — already accepted by typeeqast
+		// when matched whole; nil is OK there too.
+		if (du.kind == N_TTAGGED) {
+			let v: *node = du.list;
+			for (v != nil) {
+				if (v.kind == N_TPTR)  { return true; };
+				if (v.kind == N_TSLICE){ return true; };
+				v = v.next;
+			};
+		};
+		*confident = false;
+		return true;
+	};
+	// Tagged-union variant inclusion: src is one of dst's variants.
+	if (du.kind == N_TTAGGED && su.kind != N_TTAGGED) {
+		let v: *node = du.list;
+		for (v != nil) {
+			let vu: *node = resolvealias(c, unwrapbang(v));
+			if (vu != nil) {
+				if (typeeqast(vu, su)) { return true; };
+			};
+			v = v.next;
+		};
+		return false;
+	};
+	// tagged → tagged: structural variant list compare. Skip
+	// (don't be confident) — common when forwarding a fallible
+	// return through another fn with the same shape but possibly
+	// a different surface spelling.
+	if (du.kind == N_TTAGGED && su.kind == N_TTAGGED) {
+		*confident = false;
+		return true;
+	};
+	// Two known primitives with different names are confidently
+	// incompatible. `i32 ↔ bool`, `str ↔ i32`, etc.
+	if (du.kind == N_TNAME && su.kind == N_TNAME) {
+		let known_d: bool = isnumerictname(du) || isstrtname(du);
+		if (!known_d) { if (streq(du.str, "bool")) { known_d = true; }; };
+		if (!known_d) { if (streq(du.str, "void")) { known_d = true; }; };
+		let known_s: bool = isnumerictname(su) || isstrtname(su);
+		if (!known_s) { if (streq(su.str, "bool")) { known_s = true; }; };
+		if (!known_s) { if (streq(su.str, "void")) { known_s = true; }; };
+		if (known_d) {
+			if (known_s) {
+				// Both primitives, different names → no.
+				return false;
+			};
+		};
+	};
+	// Anything else: don't claim confidence.
+	*confident = false;
+	return true;
+};
+
 // ---- match exhaustiveness --------------------------------------------
 //
 // For every match arm, verify that every variant of the scrutinee's
 // tagged-union type is handled by some case (or a default arm
 // exists). Multi-pattern `case A | B =>` covers all alts.
 
-fn case_covers(c: *checker, cs: *node, want: *node) bool = {
+fn casecovers(c: *checker, cs: *node, want: *node) bool = {
 	if (cs.lhs != nil) {
-		if (type_eq_ast(cs.lhs, want)) { return true; };
+		if (typeeqast(cs.lhs, want)) { return true; };
 	};
 	let alt: *node = cs.list;
 	for (alt != nil) {
-		if (type_eq_ast(alt, want)) { return true; };
+		if (typeeqast(alt, want)) { return true; };
 		alt = alt.next;
 	};
 	return false;
 };
 
-fn err_match_variant(c: *checker, n: *node, vname: *node) void = {
+fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
 	os.write(2, "match: variant not handled".ptr, 26u64);
 	if (vname != nil) {
 		if (vname.kind == N_TNAME) {
@@ -362,19 +597,19 @@ fn err_match_variant(c: *checker, n: *node, vname: *node) void = {
 	c.errs += 1;
 };
 
-// case_variant_in — true iff `pat` (a `case T` pattern, including
+// casevariantin — true iff `pat` (a `case T` pattern, including
 // each alt of a multi-pattern) names a variant of the tagged
 // union `tagged`.
-fn case_variant_in(tagged: *node, pat: *node) bool = {
+fn casevariantin(tagged: *node, pat: *node) bool = {
 	let v: *node = tagged.list;
 	for (v != nil) {
-		if (type_eq_ast(v, pat)) { return true; };
+		if (typeeqast(v, pat)) { return true; };
 		v = v.next;
 	};
 	return false;
 };
 
-fn err_bad_case_variant(c: *checker, pat: *node) void = {
+fn errbadcase(c: *checker, pat: *node) void = {
 	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
 	if (pat != nil) {
 		if (pat.kind == N_TNAME) {
@@ -387,10 +622,10 @@ fn err_bad_case_variant(c: *checker, pat: *node) void = {
 	c.errs += 1;
 };
 
-fn check_match_exhaustive(c: *checker, n: *node) void = {
+fn checkmatchexhaust(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
-	let st: *node = scrutinee_type(c, n.lhs);
+	let st: *node = scruttype(c, n.lhs);
 	let u: *node = resolvealias(c, unwrapbang(st));
 	if (u == nil) { return; };
 	if (u.kind != N_TTAGGED) { return; };
@@ -400,13 +635,13 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 	let cs0: *node = n.list;
 	for (cs0 != nil) {
 		if (cs0.lhs != nil) {
-			if (!case_variant_in(u, cs0.lhs)) {
-				err_bad_case_variant(c, cs0.lhs);
+			if (!casevariantin(u, cs0.lhs)) {
+				errbadcase(c, cs0.lhs);
 			};
 			let alt: *node = cs0.list;
 			for (alt != nil) {
-				if (!case_variant_in(u, alt)) {
-					err_bad_case_variant(c, alt);
+				if (!casevariantin(u, alt)) {
+					errbadcase(c, alt);
 				};
 				alt = alt.next;
 			};
@@ -425,18 +660,106 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 		let covered: bool = false;
 		let cs2: *node = n.list;
 		for (cs2 != nil) {
-			if (case_covers(c, cs2, v)) {
+			if (casecovers(c, cs2, v)) {
 				covered = true;
 				cs2 = nil;
 			} else {
 				cs2 = cs2.next;
 			};
 		};
-		if (!covered) { err_match_variant(c, n, v); };
+		if (!covered) { errmatchvariant(c, n, v); };
 		v = v.next;
 	};
 };
 
+// ---- let init / return assignability --------------------------------
+//
+// AST-level approximation: when we can infer src's type and dst is
+// explicitly declared, verify isassignable. We only emit an error
+// when isassignable says "false with confidence." If we can't tell
+// (binary ops, complex exprs we don't infer), we stay quiet — full
+// type inference lives only on the C side.
+
+fn errnotassign(c: *checker, dst: *node, src: *node, where: str) void = {
+	os.write(2, where.ptr, where.len: u64);
+	os.write(2, ": not assignable".ptr, 16u64);
+	if (src != nil) {
+		if (src.kind == N_TNAME) {
+			os.write(2, " (".ptr, 2u64);
+			os.write(2, src.str.ptr, src.str.len: u64);
+			os.write(2, " → ".ptr, 5u64);
+			if (dst != nil) {
+				if (dst.kind == N_TNAME) {
+					os.write(2, dst.str.ptr, dst.str.len: u64);
+				};
+			};
+			os.write(2, ")".ptr, 1u64);
+		};
+	};
+	os.write(2, "\n".ptr, 1u64);
+	c.errs += 1;
+};
+
+fn checkletassign(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) { return; };	// no declared type, nothing to check
+	if (n.rhs == nil) { return; };	// no init
+	let src: *node = exprtype(c, n.rhs);
+	if (src == nil) { return; };	// can't infer
+	let conf: bool = false;
+	let ok: bool = isassignable(c, n.lhs, src, &conf);
+	if (!conf) { return; };
+	if (!ok) { errnotassign(c, n.lhs, src, "let"); };
+};
+
+fn checkretassign(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) {
+		// bare `return;` — OK iff fnret is void or a tagged union
+		// with a void variant. Skip flagging for now; cgen handles
+		// the void-variant tag synthesis already.
+		return;
+	};
+	if (c.fnret == nil) { return; };
+	let src: *node = exprtype(c, n.lhs);
+	if (src == nil) { return; };
+	let conf: bool = false;
+	let ok: bool = isassignable(c, c.fnret, src, &conf);
+	if (!conf) { return; };
+	if (!ok) { errnotassign(c, c.fnret, src, "return"); };
+};
+
+// ---- is / as validity ------------------------------------------------
+//
+// `e is T` and `e as T` require that e's declared type be a tagged
+// union and that T name one of its variants. Operates on AST type
+// expressions; falls back silently when we can't determine e's
+// type (matches the case-variant rule for match).
+fn checkisas(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	// e is in n.lhs (value), T is in n.rhs (type expr).
+	let st: *node = scruttype(c, n.lhs);
+	let u: *node = resolvealias(c, unwrapbang(st));
+	if (u == nil) { return; };
+	if (u.kind != N_TTAGGED) {
+		os.write(2, "is/as: operand is not a tagged union\n".ptr, 37u64);
+		c.errs += 1;
+		return;
+	};
+	let want: *node = n.rhs;
+	if (want == nil) { return; };
+	if (!casevariantin(u, want)) {
+		os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+		if (want.kind == N_TNAME) {
+			os.write(2, " (".ptr, 2u64);
+			os.write(2, want.str.ptr, want.str.len: u64);
+			os.write(2, ")".ptr, 1u64);
+		};
+		os.write(2, "\n".ptr, 1u64);
+		c.errs += 1;
+	};
+};
+
 // ---- ? subset propagation --------------------------------------------
 //
 // For `expr?`, the operand's error subset must be a subset of the
@@ -444,7 +767,7 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 // is N_TRYPROP; its lhs is the value-bearing expr; we look at the
 // expr's *declared* type for N_IDENT/N_CALL cases.
 
-fn expr_type_for_tryprop(c: *checker, e: *node) *node = {
+fn exprtypeoftry(c: *checker, e: *node) *node = {
 	if (e == nil) { return nil; };
 	if (e.kind == N_IDENT) {
 		let s: *sym = scopelookup(c.cur, e.str);
@@ -471,20 +794,20 @@ fn expr_type_for_tryprop(c: *checker, e: *node) *node = {
 	return nil;
 };
 
-fn check_tryprop(c: *checker, n: *node) void = {
+fn checktryprop(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
-	let t: *node = expr_type_for_tryprop(c, n.lhs);
+	let t: *node = exprtypeoftry(c, n.lhs);
 	let u: *node = resolvealias(c, unwrapbang(t));
 	if (u == nil) { return; };
 	if (u.kind != N_TTAGGED) { return; };
 	// Does the operand have any error variants?
-	let has_err: bool = false;
+	let haserr: bool = false;
 	let v: *node = u.list;
 	for (v != nil) {
-		if (is_error_variant(c, u, v)) { has_err = true; };
+		if (iserrvariant(c, u, v)) { haserr = true; };
 		v = v.next;
 	};
-	if (!has_err) { return; };
+	if (!haserr) { return; };
 	// Enclosing fn must return a tagged union with each operand
 	// error variant present.
 	let r: *node = resolvealias(c, unwrapbang(c.fnret));
@@ -500,11 +823,11 @@ fn check_tryprop(c: *checker, n: *node) void = {
 	};
 	let ev: *node = u.list;
 	for (ev != nil) {
-		if (is_error_variant(c, u, ev)) {
+		if (iserrvariant(c, u, ev)) {
 			let found: bool = false;
 			let rv: *node = r.list;
 			for (rv != nil) {
-				if (type_eq_ast(rv, ev)) {
+				if (typeeqast(rv, ev)) {
 					found = true;
 					rv = nil;
 				} else { rv = rv.next; };
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index 6de333ca..7a61b3fc 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -3808,8 +3808,12 @@ fn resolvewalk(c: *checker, n: *node) void = {
 
 	// Typed checks fire on the way down so the scrutinee/operand
 	// is examined before the arm bodies install new bindings.
-	if (k == N_MATCH)   { check_match_exhaustive(c, n); };
-	if (k == N_TRYPROP) { check_tryprop(c, n); };
+	if (k == N_MATCH)   { checkmatchexhaust(c, n); };
+	if (k == N_TRYPROP) { checktryprop(c, n); };
+	if (k == N_TYPETEST)   { checkisas(c, n); };
+	if (k == N_TYPEASSERT) { checkisas(c, n); };
+	if (k == N_LET)     { checkletassign(c, n); };
+	if (k == N_RETURN)  { checkretassign(c, n); };
 
 	// `use IDENT;` — name is a module label, not a free ident.
 	if (k == N_USE) { return; };
@@ -3956,19 +3960,11 @@ fn resolvealias(c: *checker, n: *node) *node = {
 	return n;
 };
 
-// is_tagged_type — true if `n` (after alias resolution) is an
-// N_TTAGGED type expression.
-fn is_tagged_type(c: *checker, n: *node) bool = {
-	let u: *node = resolvealias(c, unwrapbang(n));
-	if (u == nil) { return false; };
-	return u.kind == N_TTAGGED;
-};
-
-// type_eq_ast — structural equality on AST type expressions, mod
+// typeeqast — structural equality on AST type expressions, mod
 // the `!` wrapper. Mirrors variant_match in cgen + check.c: NAMED
 // types compare by string (the closest stand-in for pointer
 // identity at the AST level); other nodes recurse by kind.
-fn type_eq_ast(a: *node, b: *node) bool = {
+fn typeeqast(a: *node, b: *node) bool = {
 	let aa: *node = unwrapbang(a);
 	let bb: *node = unwrapbang(b);
 	if (aa == nil) { return bb == nil; };
@@ -3976,19 +3972,19 @@ fn type_eq_ast(a: *node, b: *node) bool = {
 	if (aa.kind != bb.kind) { return false; };
 	let k: i32 = aa.kind;
 	if (k == N_TNAME) { return streq(aa.str, bb.str); };
-	if (k == N_TPTR)  { return type_eq_ast(aa.lhs, bb.lhs); };
-	if (k == N_TSLICE){ return type_eq_ast(aa.lhs, bb.lhs); };
-	if (k == N_TCHAN) { return type_eq_ast(aa.lhs, bb.lhs); };
+	if (k == N_TPTR)  { return typeeqast(aa.lhs, bb.lhs); };
+	if (k == N_TSLICE){ return typeeqast(aa.lhs, bb.lhs); };
+	if (k == N_TCHAN) { return typeeqast(aa.lhs, bb.lhs); };
 	// Conservative: anything else (struct/fn/tagged/tuple/array)
 	// fails the cheap check. Selfhost code doesn't currently rely
 	// on equality at these shapes for the targeted checks.
 	return false;
 };
 
-// variant_is_error — does this variant carry the `!` mark? Either
+// varianterr — does this variant carry the `!` mark? Either
 // the variant itself is N_TBANG or it's an alias whose typedecl
 // body is `!T`. Mirrors C check.c's iserror-after-NAMED rule.
-fn variant_is_error(c: *checker, v: *node) bool = {
+fn varianterr(c: *checker, v: *node) bool = {
 	if (v == nil) { return false; };
 	if (v.kind == N_TBANG) { return true; };
 	if (v.kind == N_TNAME) {
@@ -4008,35 +4004,35 @@ fn variant_is_error(c: *checker, v: *node) bool = {
 	return false;
 };
 
-// tagged_has_errflag — true iff any variant of `n` (assumed
+// taggedhaserr — true iff any variant of `n` (assumed
 // N_TTAGGED) is `!`-marked. Picks the explicit-flag semantics over
 // the legacy "first variant = success" rule.
-fn tagged_has_errflag(c: *checker, n: *node) bool = {
+fn taggedhaserr(c: *checker, n: *node) bool = {
 	let v: *node = n.list;
 	for (v != nil) {
-		if (variant_is_error(c, v)) { return true; };
+		if (varianterr(c, v)) { return true; };
 		v = v.next;
 	};
 	return false;
 };
 
-// is_error_variant — under flag-aware mode (any !-marked variant),
+// iserrvariant — under flag-aware mode (any !-marked variant),
 // returns true iff `v` is `!`-marked. Under legacy mode (no flags),
 // returns true iff `v` is not the first variant of `tagged`.
-fn is_error_variant(c: *checker, tagged: *node, v: *node) bool = {
-	if (tagged_has_errflag(c, tagged)) {
-		return variant_is_error(c, v);
+fn iserrvariant(c: *checker, tagged: *node, v: *node) bool = {
+	if (taggedhaserr(c, tagged)) {
+		return varianterr(c, v);
 	};
 	// Legacy: first variant of the union is success.
 	if (tagged.list == v) { return false; };
 	return true;
 };
 
-// scrutinee_type — resolve the type expression for a match's
+// scruttype — resolve the type expression for a match's
 // scrutinee. Handles N_IDENT (look up local/param's declared
 // type) and N_DOT (struct-field access). Returns nil if we
 // can't statically determine the type. Used by exhaustiveness.
-fn scrutinee_type(c: *checker, e: *node) *node = {
+fn scruttype(c: *checker, e: *node) *node = {
 	if (e == nil) { return nil; };
 	if (e.kind == N_IDENT) {
 		let s: *sym = scopelookup(c.cur, e.str);
@@ -4048,25 +4044,264 @@ fn scrutinee_type(c: *checker, e: *node) *node = {
 	return nil;
 };
 
+// mktname — fabricate an N_TNAME node with str = `nm`. Used by
+// exprtype to return primitive type nodes for literal
+// expressions. The arena keeps them around as long as the checker.
+fn mktname(c: *checker, nm: str) *node = {
+	let n: *node = newnode(c.a, N_TNAME, "", 0, 0);
+	n.str = nm;
+	return n;
+};
+
+// exprtype — best-effort type-AST inference for an expression
+// node. Handles literals, identifiers, calls, and casts; returns
+// nil for shapes we don't statically know (binary ops, struct
+// field access into non-primitive types, etc).
+fn exprtype(c: *checker, e: *node) *node = {
+	if (e == nil) { return nil; };
+	let k: i32 = e.kind;
+	if (k == N_INTLIT)   { return mktname(c, "untyped_int"); };
+	if (k == N_FLOATLIT) { return mktname(c, "untyped_float"); };
+	if (k == N_STRLIT)   { return mktname(c, "str"); };
+	if (k == N_RUNELIT)  { return mktname(c, "rune"); };
+	if (k == N_TRUE)     { return mktname(c, "bool"); };
+	if (k == N_FALSE)    { return mktname(c, "bool"); };
+	if (k == N_VOIDLIT)  { return mktname(c, "void"); };
+	if (k == N_NIL)      { return mktname(c, "untyped_nil"); };
+	if (k == N_IDENT) {
+		let s: *sym = scopelookup(c.cur, e.str);
+		if (s == nil) { return nil; };
+		if (s.decl == nil) { return nil; };
+		return s.decl.lhs;
+	};
+	if (k == N_CAST) {
+		// `expr: T` — explicit cast; the type expr is e.rhs.
+		return e.rhs;
+	};
+	if (k == N_CALL) {
+		let callee: *node = e.lhs;
+		if (callee == nil) { return nil; };
+		let nm: str;
+		nm.ptr = nil; nm.len = 0;
+		if (callee.kind == N_IDENT) { nm = callee.str; };
+		if (callee.kind == N_DOT)   { nm = callee.str; };
+		if (nm.len == 0) { return nil; };
+		let s: *sym = scopelookup(c.cur, nm);
+		if (s == nil) { return nil; };
+		if (s.skind != SK_FN) { return nil; };
+		if (s.decl == nil) { return nil; };
+		return s.decl.lhs;	// fn-decl's lhs is the return type
+	};
+	if (k == N_TRYPROP) {
+		// success unwrap: the success-variant type of operand's
+		// tagged union.
+		let opt: *node = exprtype(c, e.lhs);
+		let ou: *node = resolvealias(c, unwrapbang(opt));
+		if (ou == nil) { return nil; };
+		if (ou.kind != N_TTAGGED) { return nil; };
+		// Hare semantics: success = first non-error variant if
+		// any !-flag is present; else first variant.
+		if (taggedhaserr(c, ou)) {
+			let v: *node = ou.list;
+			for (v != nil) {
+				if (!iserrvariant(c, ou, v)) { return v; };
+				v = v.next;
+			};
+			return nil;
+		};
+		return ou.list;
+	};
+	if (k == N_TYPEASSERT) {
+		// `e as T` → T
+		return e.rhs;
+	};
+	if (k == N_TYPETEST) {
+		// `e is T` → bool
+		return mktname(c, "bool");
+	};
+	return nil;
+};
+
+// isuntypedint / is_str_like / is_bool_like — helpers used
+// by the assignability check below to allow common AST shapes
+// through without needing real type inference.
+fn isuntypedint(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_int");
+};
+
+fn isuntypedfloat(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_float");
+};
+
+fn isuntypednil(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "untyped_nil");
+};
+
+fn isnumerictname(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	let s: str = t.str;
+	if (streq(s, "i8"))  { return true; };
+	if (streq(s, "i16")) { return true; };
+	if (streq(s, "i32")) { return true; };
+	if (streq(s, "i64")) { return true; };
+	if (streq(s, "u8"))  { return true; };
+	if (streq(s, "u16")) { return true; };
+	if (streq(s, "u32")) { return true; };
+	if (streq(s, "u64")) { return true; };
+	if (streq(s, "int"))  { return true; };
+	if (streq(s, "uint")) { return true; };
+	if (streq(s, "uintptr")) { return true; };
+	if (streq(s, "rune")) { return true; };
+	if (streq(s, "f32"))  { return true; };
+	if (streq(s, "f64"))  { return true; };
+	return false;
+};
+
+fn isstrtname(t: *node) bool = {
+	if (t == nil) { return false; };
+	if (t.kind != N_TNAME) { return false; };
+	return streq(t.str, "str");
+};
+
+// isassignable — AST-level approximation of C check.c
+// type_assignable. Returns true when we know the assignment is
+// OK, false only when we're confident it isn't, and "skip" (true)
+// when we can't tell — to avoid false positives. The trailing bool
+// `confident` lets the caller decide whether to emit an error
+// when the result is false: if !confident, the caller should not
+// flag it.
+fn isassignable(c: *checker, dst: *node, src: *node, confident: *bool) bool = {
+	*confident = false;
+	if (dst == nil) { return true; };	// no declared target
+	if (src == nil) { return true; };	// unknown src type
+	*confident = true;
+	let du: *node = resolvealias(c, unwrapbang(dst));
+	let su: *node = resolvealias(c, unwrapbang(src));
+	if (du == nil) { *confident = false; return true; };
+	if (su == nil) { *confident = false; return true; };
+	if (typeeqast(du, su)) { return true; };
+	// untyped numeric → any numeric named type.
+	if (isuntypedint(su)) {
+		if (isnumerictname(du)) { return true; };
+		// (T | ...) tagged: only OK if some variant accepts untyped_int.
+		if (du.kind == N_TTAGGED) {
+			let v: *node = du.list;
+			for (v != nil) {
+				let vu: *node = resolvealias(c, unwrapbang(v));
+				if (vu != nil) {
+					if (isnumerictname(vu)) { return true; };
+				};
+				v = v.next;
+			};
+			*confident = false;
+			return true;
+		};
+		// Known non-numeric primitive: confidently wrong.
+		if (du.kind == N_TNAME) {
+			if (streq(du.str, "bool")) { return false; };
+			if (streq(du.str, "void")) { return false; };
+			if (streq(du.str, "str"))  { return false; };
+		};
+		// Unknown shapes: stay quiet.
+		*confident = false;
+		return true;
+	};
+	if (isuntypedfloat(su)) {
+		if (isnumerictname(du)) { return true; };
+		if (du.kind == N_TNAME) {
+			if (streq(du.str, "bool")) { return false; };
+			if (streq(du.str, "void")) { return false; };
+			if (streq(du.str, "str"))  { return false; };
+		};
+		*confident = false;
+		return true;
+	};
+	if (isuntypednil(su)) {
+		// nil → ptr/slice/chan/fn/nullable
+		if (du.kind == N_TPTR)   { return true; };
+		if (du.kind == N_TSLICE) { return true; };
+		if (du.kind == N_TCHAN)  { return true; };
+		if (du.kind == N_TFN)    { return true; };
+		// nullable `(*T | void)` — already accepted by typeeqast
+		// when matched whole; nil is OK there too.
+		if (du.kind == N_TTAGGED) {
+			let v: *node = du.list;
+			for (v != nil) {
+				if (v.kind == N_TPTR)  { return true; };
+				if (v.kind == N_TSLICE){ return true; };
+				v = v.next;
+			};
+		};
+		*confident = false;
+		return true;
+	};
+	// Tagged-union variant inclusion: src is one of dst's variants.
+	if (du.kind == N_TTAGGED && su.kind != N_TTAGGED) {
+		let v: *node = du.list;
+		for (v != nil) {
+			let vu: *node = resolvealias(c, unwrapbang(v));
+			if (vu != nil) {
+				if (typeeqast(vu, su)) { return true; };
+			};
+			v = v.next;
+		};
+		return false;
+	};
+	// tagged → tagged: structural variant list compare. Skip
+	// (don't be confident) — common when forwarding a fallible
+	// return through another fn with the same shape but possibly
+	// a different surface spelling.
+	if (du.kind == N_TTAGGED && su.kind == N_TTAGGED) {
+		*confident = false;
+		return true;
+	};
+	// Two known primitives with different names are confidently
+	// incompatible. `i32 ↔ bool`, `str ↔ i32`, etc.
+	if (du.kind == N_TNAME && su.kind == N_TNAME) {
+		let known_d: bool = isnumerictname(du) || isstrtname(du);
+		if (!known_d) { if (streq(du.str, "bool")) { known_d = true; }; };
+		if (!known_d) { if (streq(du.str, "void")) { known_d = true; }; };
+		let known_s: bool = isnumerictname(su) || isstrtname(su);
+		if (!known_s) { if (streq(su.str, "bool")) { known_s = true; }; };
+		if (!known_s) { if (streq(su.str, "void")) { known_s = true; }; };
+		if (known_d) {
+			if (known_s) {
+				// Both primitives, different names → no.
+				return false;
+			};
+		};
+	};
+	// Anything else: don't claim confidence.
+	*confident = false;
+	return true;
+};
+
 // ---- match exhaustiveness --------------------------------------------
 //
 // For every match arm, verify that every variant of the scrutinee's
 // tagged-union type is handled by some case (or a default arm
 // exists). Multi-pattern `case A | B =>` covers all alts.
 
-fn case_covers(c: *checker, cs: *node, want: *node) bool = {
+fn casecovers(c: *checker, cs: *node, want: *node) bool = {
 	if (cs.lhs != nil) {
-		if (type_eq_ast(cs.lhs, want)) { return true; };
+		if (typeeqast(cs.lhs, want)) { return true; };
 	};
 	let alt: *node = cs.list;
 	for (alt != nil) {
-		if (type_eq_ast(alt, want)) { return true; };
+		if (typeeqast(alt, want)) { return true; };
 		alt = alt.next;
 	};
 	return false;
 };
 
-fn err_match_variant(c: *checker, n: *node, vname: *node) void = {
+fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
 	os.write(2, "match: variant not handled".ptr, 26u64);
 	if (vname != nil) {
 		if (vname.kind == N_TNAME) {
@@ -4079,19 +4314,19 @@ fn err_match_variant(c: *checker, n: *node, vname: *node) void = {
 	c.errs += 1;
 };
 
-// case_variant_in — true iff `pat` (a `case T` pattern, including
+// casevariantin — true iff `pat` (a `case T` pattern, including
 // each alt of a multi-pattern) names a variant of the tagged
 // union `tagged`.
-fn case_variant_in(tagged: *node, pat: *node) bool = {
+fn casevariantin(tagged: *node, pat: *node) bool = {
 	let v: *node = tagged.list;
 	for (v != nil) {
-		if (type_eq_ast(v, pat)) { return true; };
+		if (typeeqast(v, pat)) { return true; };
 		v = v.next;
 	};
 	return false;
 };
 
-fn err_bad_case_variant(c: *checker, pat: *node) void = {
+fn errbadcase(c: *checker, pat: *node) void = {
 	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
 	if (pat != nil) {
 		if (pat.kind == N_TNAME) {
@@ -4104,10 +4339,10 @@ fn err_bad_case_variant(c: *checker, pat: *node) void = {
 	c.errs += 1;
 };
 
-fn check_match_exhaustive(c: *checker, n: *node) void = {
+fn checkmatchexhaust(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
 	if (n.lhs == nil) { return; };
-	let st: *node = scrutinee_type(c, n.lhs);
+	let st: *node = scruttype(c, n.lhs);
 	let u: *node = resolvealias(c, unwrapbang(st));
 	if (u == nil) { return; };
 	if (u.kind != N_TTAGGED) { return; };
@@ -4117,13 +4352,13 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 	let cs0: *node = n.list;
 	for (cs0 != nil) {
 		if (cs0.lhs != nil) {
-			if (!case_variant_in(u, cs0.lhs)) {
-				err_bad_case_variant(c, cs0.lhs);
+			if (!casevariantin(u, cs0.lhs)) {
+				errbadcase(c, cs0.lhs);
 			};
 			let alt: *node = cs0.list;
 			for (alt != nil) {
-				if (!case_variant_in(u, alt)) {
-					err_bad_case_variant(c, alt);
+				if (!casevariantin(u, alt)) {
+					errbadcase(c, alt);
 				};
 				alt = alt.next;
 			};
@@ -4142,18 +4377,106 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 		let covered: bool = false;
 		let cs2: *node = n.list;
 		for (cs2 != nil) {
-			if (case_covers(c, cs2, v)) {
+			if (casecovers(c, cs2, v)) {
 				covered = true;
 				cs2 = nil;
 			} else {
 				cs2 = cs2.next;
 			};
 		};
-		if (!covered) { err_match_variant(c, n, v); };
+		if (!covered) { errmatchvariant(c, n, v); };
 		v = v.next;
 	};
 };
 
+// ---- let init / return assignability --------------------------------
+//
+// AST-level approximation: when we can infer src's type and dst is
+// explicitly declared, verify isassignable. We only emit an error
+// when isassignable says "false with confidence." If we can't tell
+// (binary ops, complex exprs we don't infer), we stay quiet — full
+// type inference lives only on the C side.
+
+fn errnotassign(c: *checker, dst: *node, src: *node, where: str) void = {
+	os.write(2, where.ptr, where.len: u64);
+	os.write(2, ": not assignable".ptr, 16u64);
+	if (src != nil) {
+		if (src.kind == N_TNAME) {
+			os.write(2, " (".ptr, 2u64);
+			os.write(2, src.str.ptr, src.str.len: u64);
+			os.write(2, " → ".ptr, 5u64);
+			if (dst != nil) {
+				if (dst.kind == N_TNAME) {
+					os.write(2, dst.str.ptr, dst.str.len: u64);
+				};
+			};
+			os.write(2, ")".ptr, 1u64);
+		};
+	};
+	os.write(2, "\n".ptr, 1u64);
+	c.errs += 1;
+};
+
+fn checkletassign(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) { return; };	// no declared type, nothing to check
+	if (n.rhs == nil) { return; };	// no init
+	let src: *node = exprtype(c, n.rhs);
+	if (src == nil) { return; };	// can't infer
+	let conf: bool = false;
+	let ok: bool = isassignable(c, n.lhs, src, &conf);
+	if (!conf) { return; };
+	if (!ok) { errnotassign(c, n.lhs, src, "let"); };
+};
+
+fn checkretassign(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) {
+		// bare `return;` — OK iff fnret is void or a tagged union
+		// with a void variant. Skip flagging for now; cgen handles
+		// the void-variant tag synthesis already.
+		return;
+	};
+	if (c.fnret == nil) { return; };
+	let src: *node = exprtype(c, n.lhs);
+	if (src == nil) { return; };
+	let conf: bool = false;
+	let ok: bool = isassignable(c, c.fnret, src, &conf);
+	if (!conf) { return; };
+	if (!ok) { errnotassign(c, c.fnret, src, "return"); };
+};
+
+// ---- is / as validity ------------------------------------------------
+//
+// `e is T` and `e as T` require that e's declared type be a tagged
+// union and that T name one of its variants. Operates on AST type
+// expressions; falls back silently when we can't determine e's
+// type (matches the case-variant rule for match).
+fn checkisas(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	// e is in n.lhs (value), T is in n.rhs (type expr).
+	let st: *node = scruttype(c, n.lhs);
+	let u: *node = resolvealias(c, unwrapbang(st));
+	if (u == nil) { return; };
+	if (u.kind != N_TTAGGED) {
+		os.write(2, "is/as: operand is not a tagged union\n".ptr, 37u64);
+		c.errs += 1;
+		return;
+	};
+	let want: *node = n.rhs;
+	if (want == nil) { return; };
+	if (!casevariantin(u, want)) {
+		os.write(2, "is/as: not a variant of operand".ptr, 31u64);
+		if (want.kind == N_TNAME) {
+			os.write(2, " (".ptr, 2u64);
+			os.write(2, want.str.ptr, want.str.len: u64);
+			os.write(2, ")".ptr, 1u64);
+		};
+		os.write(2, "\n".ptr, 1u64);
+		c.errs += 1;
+	};
+};
+
 // ---- ? subset propagation --------------------------------------------
 //
 // For `expr?`, the operand's error subset must be a subset of the
@@ -4161,7 +4484,7 @@ fn check_match_exhaustive(c: *checker, n: *node) void = {
 // is N_TRYPROP; its lhs is the value-bearing expr; we look at the
 // expr's *declared* type for N_IDENT/N_CALL cases.
 
-fn expr_type_for_tryprop(c: *checker, e: *node) *node = {
+fn exprtypeoftry(c: *checker, e: *node) *node = {
 	if (e == nil) { return nil; };
 	if (e.kind == N_IDENT) {
 		let s: *sym = scopelookup(c.cur, e.str);
@@ -4188,20 +4511,20 @@ fn expr_type_for_tryprop(c: *checker, e: *node) *node = {
 	return nil;
 };
 
-fn check_tryprop(c: *checker, n: *node) void = {
+fn checktryprop(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
-	let t: *node = expr_type_for_tryprop(c, n.lhs);
+	let t: *node = exprtypeoftry(c, n.lhs);
 	let u: *node = resolvealias(c, unwrapbang(t));
 	if (u == nil) { return; };
 	if (u.kind != N_TTAGGED) { return; };
 	// Does the operand have any error variants?
-	let has_err: bool = false;
+	let haserr: bool = false;
 	let v: *node = u.list;
 	for (v != nil) {
-		if (is_error_variant(c, u, v)) { has_err = true; };
+		if (iserrvariant(c, u, v)) { haserr = true; };
 		v = v.next;
 	};
-	if (!has_err) { return; };
+	if (!haserr) { return; };
 	// Enclosing fn must return a tagged union with each operand
 	// error variant present.
 	let r: *node = resolvealias(c, unwrapbang(c.fnret));
@@ -4217,11 +4540,11 @@ fn check_tryprop(c: *checker, n: *node) void = {
 	};
 	let ev: *node = u.list;
 	for (ev != nil) {
-		if (is_error_variant(c, u, ev)) {
+		if (iserrvariant(c, u, ev)) {
 			let found: bool = false;
 			let rv: *node = r.list;
 			for (rv != nil) {
-				if (type_eq_ast(rv, ev)) {
+				if (typeeqast(rv, ev)) {
 					found = true;
 					rv = nil;
 				} else { rv = rv.next; };
diff --git a/test/wcc/950_selfcheck.c b/test/wcc/950_selfcheck.c
index eba30de4..f01e3451 100644
--- a/test/wcc/950_selfcheck.c
+++ b/test/wcc/950_selfcheck.c
@@ -103,6 +103,23 @@ static const struct row rows[] = {
 	  "  };\n"
 	  "};\n",
 	  "case: not a variant" },
+	/* is T where T isn't a variant of the operand */
+	{ "fn pick() (i32 | str) = { return 1; };\n"
+	  "fn caller() void = {\n"
+	  "  let v: (i32 | str) = pick();\n"
+	  "  if (v is f64) { };\n"
+	  "};\n",
+	  "is/as: not a variant" },
+	/* let init-type mismatch on primitives */
+	{ "fn caller() void = {\n"
+	  "  let x: bool = 42;\n"
+	  "};\n",
+	  "let: not assignable" },
+	/* return type mismatch */
+	{ "fn caller() i32 = {\n"
+	  "  return \"hi\";\n"
+	  "};\n",
+	  "return: not assignable" },
 };
 
 int
@@ -136,6 +153,14 @@ main(void)
 		    "?: enclosing fn has no tagged-union return") != NULL);
 		err_present = err_present || (err && strstr(err,
 		    "case: not a variant") != NULL);
+		err_present = err_present || (err && strstr(err,
+		    "is/as: not a variant") != NULL);
+		err_present = err_present || (err && strstr(err,
+		    "is/as: operand is not a tagged union") != NULL);
+		err_present = err_present || (err && strstr(err,
+		    "let: not assignable") != NULL);
+		err_present = err_present || (err && strstr(err,
+		    "return: not assignable") != NULL);
 		int ok;
 		if (expected_no_err) ok = !err_present;
 		else                 ok = got_match;