ww/selfhost/cmd/wcc/check.ww

// selfhost/cmd/wcc/check.ww — minimal port of cmd/wcc/check.c.
//
// Status: name-resolution + primitive-type seeding only. Full type
// inference, conversion rules, tagged-union dispatch typing, return-
// type checking, etc. all live in cmd/wcc/check.c (937 lines) and
// will land here in subsequent commits.
//
// What this version does:
//   1. Creates a top scope and seeds it with primitive type names so
//      `i32`, `str`, `*u8` etc. resolve.
//   2. Walks the file's top-level decls (use/def/type/fn/let) and
//      installs Sym entries for each.
//   3. Recursively walks fn bodies; for every nkind.N_IDENT used as an
//      expression or as a type name, looks it up and counts the
//      resolved vs. unresolved.
//   4. Returns a summary the caller (wwdump -r) prints; the test
//      asserts unresolved == 0 on every selfhost fixture, which is
//      the floor signal that the frontend can name-resolve real ww.

package wcc;

import os;
import mem;
import tok;

type checker = struct {
	a:        *arena,
	tc:       *tctx,
	top:      *scope,
	cur:      *scope,
	nresolved:   i32,
	nunresolved: i32,
	errs:        i32,
	verbose:     i32,	// when non-zero, log each unresolved name
	fnret:       *node,	// enclosing fn's return type AST (for `?`)
	curmod:      str,	// importing-module bareword for the decl
				// currently being walked; "" for primary
				// compilation unit. Drives same-module
				// preference in bare-leaf lookups.
	file:        *node,	// N_FILE root; used by checkmoduleshadow
				// to consult the declaring source's own
				// `use` directives.
};

// seedprimitives — install the built-in type names so `i32`, `str`,
// etc. can be looked up like ordinary symbols.
fn seedprimitives(c: *checker) void = {
	scopedefine(c.top, "void",    skind.SK_TYPE, c.tc.tyvoid,    nil);
	scopedefine(c.top, "bool",    skind.SK_TYPE, c.tc.tybool,    nil);
	scopedefine(c.top, "rune",    skind.SK_TYPE, c.tc.tyrune,    nil);
	scopedefine(c.top, "i8",      skind.SK_TYPE, c.tc.tyi8,      nil);
	scopedefine(c.top, "i16",     skind.SK_TYPE, c.tc.tyi16,     nil);
	scopedefine(c.top, "i32",     skind.SK_TYPE, c.tc.tyi32,     nil);
	scopedefine(c.top, "i64",     skind.SK_TYPE, c.tc.tyi64,     nil);
	scopedefine(c.top, "u8",      skind.SK_TYPE, c.tc.tyu8,      nil);
	scopedefine(c.top, "u16",     skind.SK_TYPE, c.tc.tyu16,     nil);
	scopedefine(c.top, "u32",     skind.SK_TYPE, c.tc.tyu32,     nil);
	scopedefine(c.top, "u64",     skind.SK_TYPE, c.tc.tyu64,     nil);
	scopedefine(c.top, "int",     skind.SK_TYPE, c.tc.tyint,     nil);
	scopedefine(c.top, "uint",    skind.SK_TYPE, c.tc.tyuint,    nil);
	scopedefine(c.top, "uintptr", skind.SK_TYPE, c.tc.tyuintptr, nil);
	scopedefine(c.top, "f32",     skind.SK_TYPE, c.tc.tyf32,     nil);
	scopedefine(c.top, "f64",     skind.SK_TYPE, c.tc.tyf64,     nil);
	scopedefine(c.top, "str",     skind.SK_TYPE, c.tc.tystr,     nil);
	scopedefine(c.top, "never",   skind.SK_TYPE, c.tc.tynever,   nil);
	// #29: predeclare `type nomem = !void;` so user code needn't
	// declare it locally. Synthesize an nkind.N_TYPEDECL whose lhs is
	// nkind.N_TBANG{nkind.N_TNAME("void")} so varianterr and other
	// iserror-aware paths treat `nomem` identically to a user-written
	// alias. Mirrors cmd/wcc/check.c lookup_builtin returning
	// ty_nomem (NAMED, under=ty_void, iserror=1). Note: cgen owns a
	// separate alias chain — see collectaliases in cgen.ww for the
	// companion seed.
	let empty: str;
	let tnvoid: *node = newnode(c.a, nkind.N_TNAME, empty, 0, 0);
	tnvoid.str = "void";
	let bang: *node = newnode(c.a, nkind.N_TBANG, empty, 0, 0);
	bang.lhs = tnvoid;
	let nomemdecl: *node = newnode(c.a, nkind.N_TYPEDECL, empty, 0, 0);
	nomemdecl.str = "nomem";
	nomemdecl.lhs = bang;
	scopedefine(c.top, "nomem", skind.SK_TYPE, nil, nomemdecl);
	// `nil`, `true`, `false` are keywords — handled at the lex/parser
	// level, no symbol needed.
	// `len`, `alloc`, `free`, `append` are pseudo-builtins; scopedefine
	// them so their use sites resolve. The actual semantics live in cgen.
	scopedefine(c.top, "len",    skind.SK_FN, nil, nil);
	scopedefine(c.top, "alloc",  skind.SK_FN, nil, nil);
	scopedefine(c.top, "free",   skind.SK_FN, nil, nil);
	scopedefine(c.top, "append", skind.SK_FN, nil, nil);
	// #42: typed builtins folded to integer literals at check time —
	// `size(T)` / `align(T)` (arg is a type-expression planted by the
	// parser at lib/ww/parse/expr.ww:254-267) and `offset(e.f)` (arg is
	// an N_DOT). exprtype intercepts these and rewrites the N_CALL to
	// N_INTLIT so cgen never sees an unresolved size/align/offset symbol.
	// Mirrors cmd/wcc/check.c:907-955.
	scopedefine(c.top, "size",   skind.SK_FN, nil, nil);
	scopedefine(c.top, "align",  skind.SK_FN, nil, nil);
	scopedefine(c.top, "offset", skind.SK_FN, nil, nil);
};

// declmod — module-tag stamp for a top-level decl.
//
// The driver concatenates imported sources before the primary file and
// emits `// MODULE: foo` directives the lexer pins onto each decl's
// `module` field. We treat a decl as "imported" iff its module
// directive matches some `use IDENT;` bareword in this compilation
// unit. Primary-file decls return "" so they coexist (mod="") with
// imported decls of the same leaf name in scopelookupinmodule.
fn declmod(file: *node, d: *node) str = {
	let empty: str;
	if (d == nil) { return empty; };
	if (d.nmod.len == 0) { return empty; };
	if (file == nil) { return empty; };
	let u: *node = file.list;
	for (u != nil) {
		if (u.kind == nkind.N_USE) {
			if (streq(u.str, d.nmod)) { return d.nmod; };
		};
		u = u.next;
	};
	return empty;
};

// srcimports — does the source file that contributed decl-module
// `modtag` carry `use <name>;`? Mirrors cstage's src_imports —
// `modtag.len == 0` means primary, matching declmod's empty-str
// return for primary-source decls.
fn srcimports(file: *node, modtag: str, name: str) bool = {
	if (file == nil) { return false; };
	if (name.len == 0) { return false; };
	let u: *node = file.list;
	for (u != nil) {
		if (u.kind == nkind.N_USE) {
			// Skip self-imports: lib/fmt/fmttest.ww carries
			// `use fmt;` while its module tag is also "fmt".
			// That directive doesn't introduce a foreign
			// module bareword and lib/fmt's own
			// `fn bsprintf(fmt: str, ...)` is not a shadow.
			if (u.nmod.len > 0) {
				if (streq(u.nmod, u.str)) {
					u = u.next;
					continue;
				};
			};
			let um: str = declmod(file, u);
			let m: bool = false;
			if (modtag.len == 0) {
				if (um.len == 0) { m = true; };
			} else { if (streq(um, modtag)) { m = true; }; };
			if (m) {
				if (streq(u.str, name)) { return true; };
			};
		};
		u = u.next;
	};
	return false;
};

// checkmoduleshadow — enforce "value names and module names are
// disjoint" at nested-scope binds. Mirrors cstage check_module_shadow
// (cmd/wcc/check.c). Fires for fn params / lets / forrange iters /
// mcase bindings whose name matches an in-scope `use foo;` import
// declared in the same source file. Top-level decls are exempt
// (their same-leaf-as-module pattern is the intentional coexistence
// shape — `use fnmatch; fn fnmatch(...)` etc.).
fn checkmoduleshadow(c: *checker, name: str, kindstr: str) void = {
	if (name.len == 0) { return; };
	if (c.cur == c.top) { return; };
	let seen: bool = false;
	let s: *scope = c.cur;
	for (s != nil) {
		let r: *sym = scopelookuplocal(s, name);
		if (r != nil) {
			if (r.skind == skind.SK_USE) {
				seen = true;
				s = nil;
			};
		};
		if (s != nil) { s = s.parent; };
	};
	if (!seen) { return; };
	if (!srcimports(c.file, c.curmod, name)) { return; };
	os.write(2, kindstr.ptr, kindstr.len: u64);
	os.write(2, " '".ptr, 2u64);
	os.write(2, name.ptr, name.len: u64);
	os.write(2, "' shadows imported module '".ptr, 27u64);
	os.write(2, name.ptr, name.len: u64);
	os.write(2, "'\n".ptr, 2u64);
	c.errs += 1;
};

// installdecl — install the top-level decl's name into the top scope.
// We don't compute its type yet (that's the resolve pass) — just bind
// the name so forward references resolve.
//
// Architectural note: wwstage uses COEXISTENCE rather than the cstage
// promote-SK_USE-in-place approach in cmd/wcc/check.c. SK_USE and any
// same-leaf SK_TYPE/SK_FN/SK_DEF/SK_VAR live as separate entries in
// the same scope-bucket, distinguished by `sym.mod`. The dot-prefix
// lookup in resolvewalk + scopelookupinmodule's mod-filter already
// disambiguate `fnmatch.flag` against an `fn fnmatch(...)` of the same
// leaf — no `use_alias` flag needed. So the cstage L1722-class bug
// (promotion missing use_alias) is structurally non-reachable here.
// Don't port the use_alias flag from cstage without first re-reading
// the architecture: adding a field to `sym` changes its size and risks
// the wwstage cgen amalloc-undersize trap (rob-pike). #11 (wwstage
// checkfile pass) will reconsider this when wwstage grows a real check
// pass on the cgen path.
// TODO(#11): cstage check.c errors on duplicate top-level type/def/fn
// (see cmd/wcc/check.c L1800/L1839/L1860 "duplicate <kind>") and on
// duplicate top-level let (cmd/wcc/check.c L1880, "duplicate let %s")
// once #32 lands. Wwstage's installdecl just drops the second insert
// silently. Add `if (s == nil) err(...)` here once #11 wires checkfile
// into w6c_ww. Silent-accept matches the deferred-check design — see
// test/wcc/708 and test/wcc/696 for the same cstage-only neg-case
// precedent.
fn installdecl(c: *checker, file: *node, d: *node) void = {
	if (d == nil) { return; };
	let k: nkind = d.kind;
	let nm: str = d.str;
	let mod: str = declmod(file, d);
	if (k == nkind.N_USE)      { scopedefine(c.top, nm, skind.SK_USE,  nil, d); return; };
	if (k == nkind.N_DEF)      { scopedefineinmodule(c.top, nm, mod, skind.SK_DEF,  nil, d); return; };
	if (k == nkind.N_TYPEDECL) { scopedefineinmodule(c.top, nm, mod, skind.SK_TYPE, nil, d); return; };
	if (k == nkind.N_FNDECL)   { scopedefineinmodule(c.top, nm, mod, skind.SK_FN,   nil, d); return; };
	if (k == nkind.N_LET)      { scopedefineinmodule(c.top, nm, mod, skind.SK_VAR,  nil, d); return; };
};

// resolvewalk — recursive AST walk that, for every nkind.N_IDENT and
// nkind.N_TNAME seen, looks up the name and bumps the resolved/unresolved
// counters. Local lets are installed in the current scope as soon as
// their init/type expressions have been walked (forward use of a let
// before its declaration would resolve to nothing — same semantics as
// the C checker's collect-then-resolve flow within a function).
// Also runs the typed checks (match exhaustiveness, ? subset) in
// the same pass — they need the same scope state.
fn resolvewalk(c: *checker, n: *node) void = {
	if (n == nil) { return; };
	let k: nkind = n.kind;

	// Typed checks fire on the way down so the scrutinee/operand
	// is examined before the arm bodies install new bindings.
	if (k == nkind.N_MATCH)   { checkmatchexhaust(c, n); };
	if (k == nkind.N_TRYPROP) { checktryprop(c, n); };
	if (k == nkind.N_TYPETEST)   { checkisas(c, n); };
	if (k == nkind.N_TYPEASSERT) { checkisas(c, n); };
	if (k == nkind.N_LET)     { checkletassign(c, n); };
	if (k == nkind.N_RETURN)  { checkretassign(c, n); };

	// `use IDENT;` — name is a module label, not a free ident.
	if (k == nkind.N_USE) { return; };

	if (k == nkind.N_IDENT) {
		let nm: str = n.str;
		if (nm.len > 0) {
			let s: *sym = scopelookupprefer(c.cur, c.curmod, nm);
			if (s == nil) {
				c.nunresolved += 1;
				if (c.verbose != 0) {
					os.write(2, "  unresolved id: ".ptr, 17u64);
					os.write(2, nm.ptr, nm.len: u64);
					os.write(2, "\n".ptr, 1u64);
				};
			} else { c.nresolved += 1; };
		};
	};

	if (k == nkind.N_TNAME) {
		let nm: str = n.str;
		if (nm.len > 0) {
			let s: *sym = scopelookupprefer(c.cur, c.curmod, nm);
			// `pkg.Type` — strip the last dot prefix and look up
			// the leaf with a mod filter so same-leaf-name types
			// from different imports (`bufio.stream` vs
			// `io.stream`) disambiguate to the right one.
			// Mirrors cmd/wcc/check.c resolve_typename.
			if (s == nil) {
				let dot: i32 = nm.len - 1;
				for (dot >= 0) {
					if (nm[dot] == 46u8) { break; };
					dot -= 1;
				};
				if (dot > 0) {
					let head: str;
					head.ptr = nm.ptr;
					head.len = dot;
					let m: *sym = scopelookup(c.cur, head);
					if (m != nil) {
						let leaf: str;
						leaf.ptr = nm.ptr + (dot + 1): u64;
						leaf.len = nm.len - (dot + 1);
						s = scopelookupinmodule(c.cur, head, leaf);
					};
				};
			};
			if (s == nil) {
				c.nunresolved += 1;
				if (c.verbose != 0) {
					os.write(2, "  unresolved tname: ".ptr, 20u64);
					os.write(2, nm.ptr, nm.len: u64);
					os.write(2, "\n".ptr, 1u64);
				};
			} else { c.nresolved += 1; };
		};
	};

	// `for (let x .. slice) body` / `for (let (a, b) .. slice) body` —
	// each binding name becomes a fresh local. Walk the slice expr first
	// so its idents resolve before the bindings shadow anything, then
	// install bindings and walk the body/else.
	//
	// TODO(#11): cstage check.c (post-#32) errors `binding '%s'
	// redeclared in same scope` when the tuple-pattern lists the same
	// name twice (`for (let (a, a) .. xs)`). Wwstage's resolvewalk has
	// no per-block scope (see resolvefnbody's docstring) and is used
	// only by wwdump_ww as a diagnostic, so silent-accept here avoids
	// false-positives on legal cross-block shadow until #11 adds the
	// scoping infrastructure.
	if (k == nkind.N_FORRANGE) {
		if (n.lhs != nil) { resolvewalk(c, n.lhs); };
		if (n.list != nil) {
			let m: *node = n.list;
			for (m != nil) {
				let bnm: str = m.str;
				if (bnm.len > 0) {
					checkmoduleshadow(c, bnm, "binding");
					scopedefine(c.cur, bnm, skind.SK_VAR, nil, m);
				};
				m = m.next;
			};
		} else {
			let bnm: str = n.str;
			if (bnm.len > 0) {
				checkmoduleshadow(c, bnm, "binding");
				scopedefine(c.cur, bnm, skind.SK_VAR, nil, n);
			};
		};
		if (n.body != nil) { resolvewalk(c, n.body); };
		if (n.els  != nil) { resolvewalk(c, n.els);  };
		return;
	};

	// `match (e) { case let v: T => stmt; ... }` — the binding `v`
	// is declared by the case arm and visible inside its body. Push a
	// fresh scope so `case let e: str` doesn't collide with an outer
	// `let e: *T` (scopedefine drops same-scope dupes silently and
	// would leave references to `e` resolving to the outer type).
	// Mirrors cmd/wcc/check.c's newscope/saved-restore around cstmt.
	if (k == nkind.N_MCASE) {
		if (n.lhs  != nil) { resolvewalk(c, n.lhs);  };
		let outer: *scope = c.cur;
		c.cur = newscope(c.a, outer);
		let nm: str = n.str;
		if (nm.len > 0) {
			checkmoduleshadow(c, nm, "binding");
			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
		};
		if (n.body != nil) { resolvewalk(c, n.body); };
		c.cur = outer;
		return;
	};

	// #53: lexical block. Push a child scope so locals introduced by
	// inner-block lets (and the `let` install at the tail of this fn) go
	// out of scope at block exit. Without this, a deeply nested
	// `let i: u64 = 0u64;` survived to shadow a same-named outer
	// `let i: i32 = 1;` for the whole fn body, and exprtype handed
	// stale primitive types to checkletassign — silent miscompile
	// becomes a false-positive on the next driver (`wwdump_ww -r`
	// flagged the u64→i32 pair in selfhost/cmd/ww/enumeratedir).
	// Mirrors cstage cstmt N_BLOCK at cmd/wcc/check.c:1559-1566.
	if (k == nkind.N_BLOCK) {
		let outer: *scope = c.cur;
		c.cur = newscope(c.a, outer);
		let m: *node = n.list;
		for (m != nil) {
			resolvewalk(c, m);
			m = m.next;
		};
		c.cur = outer;
		return;
	};

	if (k == nkind.N_DOT) {
		// Walk only the base; the .field name is a member, not a
		// free identifier.
		if (n.lhs != nil) { resolvewalk(c, n.lhs); };
		return;
	};

	if (k == nkind.N_FIELD) {
		if (n.lhs != nil) { resolvewalk(c, n.lhs); };
		return;
	};

	if (k == nkind.N_TFIELD) {
		if (n.lhs != nil) { resolvewalk(c, n.lhs); };
		return;
	};

	// Walk children (mirroring ast.ww's printer descent order).
	if (n.attr != nil) { resolvewalk(c, n.attr); };
	if (n.lhs  != nil) { resolvewalk(c, n.lhs);  };
	if (n.rhs  != nil) { resolvewalk(c, n.rhs);  };
	if (n.cond != nil) { resolvewalk(c, n.cond); };
	if (n.body != nil) { resolvewalk(c, n.body); };
	if (n.els  != nil) { resolvewalk(c, n.els);  };
	if (n.list != nil) {
		let m: *node = n.list;
		for (m != nil) {
			resolvewalk(c, m);
			m = m.next;
		};
	};

	// #42: trigger the size/align/offset fold here so the mutation
	// fires regardless of context (if-conditions, expression statements,
	// etc.) — wwstage's exprtype is otherwise called only from
	// checkletassign / checkretassign / TRYPROP, and an unwrapped
	// `if (size(str) != 16)` would otherwise leave the N_CALL alone
	// and cgen would emit a stray `CALL size(SB)`. Mirrors cstage
	// cstmt's recursive cexpr discipline.
	if (k == nkind.N_CALL) {
		if (n.lhs != nil) {
			if (n.lhs.kind == nkind.N_IDENT) {
				let nm: str = n.lhs.str;
				if (streq(nm, "size") || streq(nm, "align") || streq(nm, "offset")) {
					let _t: *node = exprtype(c, n);
				};
			};
		};
	};

	// After walking children: a local `let X: T = init;` registers
	// `X` so subsequent statements can resolve it. Top-level lets
	// are installed in installdecl, so this duplicate install at
	// the file scope just no-ops (scopedefine returns nil on dup).
	//
	// Cross-block `let a; { let a; };` no longer trips dup-silence
	// since #53 added N_BLOCK push/pop above — the inner `a` lands in
	// the inner block's scope. Same-scope dup `let a=1; let a=2;`
	// still silent-accepts here; promoting that to an error stays
	// queued behind #11 (test/wcc/708 + test/wcc/696 are the cstage-
	// only neg-case precedent).
	if (k == nkind.N_LET) {
		let nm: str = n.str;
		if (nm.len > 0) {
			checkmoduleshadow(c, nm, "let");
			scopedefine(c.cur, nm, skind.SK_VAR, nil, n);
		};
	};
};

// ---- type-level helpers (AST-level, no resolved tinfo) --------------
//
// The selfhost check operates on AST type expressions rather than
// resolved Type structs. These helpers mirror what cmd/wcc/check.c
// does with tinfo, but only on the subset of cases this checker
// needs to enforce: tagged-union exhaustiveness, ? subset
// propagation, and !-flag semantics.

// unwrapbang — strip an nkind.N_TBANG wrapper; leaves other nodes alone.
fn unwrapbang(n: *node) *node = {
	if (n == nil) { return nil; };
	if (n.kind == nkind.N_TBANG) { return n.lhs; };
	return n;
};

// resolvealias — if n is an nkind.N_TNAME pointing at a typedecl, return
// the typedecl's body (possibly recursively). Pass-through for any
// other node. The chain stops once we hit a non-nkind.N_TNAME node or a
// name we can't resolve.
fn resolvealias(c: *checker, n: *node) *node = {
	let cur: *node = n;
	for (cur != nil) {
		if (cur.kind != nkind.N_TNAME) { return cur; };
		let nm: str = cur.str;
		// #51: pkg.alias type refs land here as a single TNAME whose
		// str is the joined form (lib/ww/parse/parse.ww:258-265 in
		// parsetype). Split on the rightmost '.' and bind the leaf in
		// the head module's scope. Mirrors cstage resolve_typename
		// cmd/wcc/check.c:74-83 strrchr branch — without this the
		// raw `os.oserror` lookup misses and checkisas false-positives
		// every cross-module tagged scrutinee.
		let dotidx: i32 = -1;
		let i: i32 = 0;
		for (i < nm.len) {
			if (nm[i] == 46u8) { dotidx = i; };
			i += 1;
		};
		let s: *sym = nil;
		if (dotidx >= 0) {
			let head: str;
			head.ptr = nm.ptr;
			head.len = dotidx;
			let leaf: str;
			leaf.ptr = nm.ptr + ((dotidx + 1): u64);
			leaf.len = nm.len - dotidx - 1;
			s = scopelookupinmodule(c.cur, head, leaf);
		} else {
			s = scopelookup(c.cur, nm);
		};
		if (s == nil) { return cur; };
		if (s.skind != skind.SK_TYPE) { return cur; };
		let body: *node = nil;
		if (s.decl != nil) { body = s.decl.lhs; };
		if (body == nil) { return cur; };
		cur = unwrapbang(body);
	};
	return n;
};

// 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 typeeqast(a: *node, b: *node) bool = {
	let aa: *node = unwrapbang(a);
	let bb: *node = unwrapbang(b);
	if (aa == nil) { return bb == nil; };
	if (bb == nil) { return false; };
	if (aa.kind != bb.kind) { return false; };
	let k: nkind = aa.kind;
	if (k == nkind.N_TNAME) { return streq(aa.str, bb.str); };
	if (k == nkind.N_TPTR)  { return typeeqast(aa.lhs, bb.lhs); };
	if (k == nkind.N_TSLICE){ return typeeqast(aa.lhs, bb.lhs); };
	if (k == nkind.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;
};

// varianterr — does this variant carry the `!` mark? Either
// the variant itself is nkind.N_TBANG or it's an alias whose typedecl
// body is `!T`. Mirrors C check.c's iserror-after-NAMED rule.
fn varianterr(c: *checker, v: *node) bool = {
	if (v == nil) { return false; };
	if (v.kind == nkind.N_TBANG) { return true; };
	if (v.kind == nkind.N_TNAME) {
		let s: *sym = scopelookup(c.cur, v.str);
		if (s != nil) {
			if (s.skind == skind.SK_TYPE) {
				if (s.decl != nil) {
					if (s.decl.lhs != nil) {
						if (s.decl.lhs.kind == nkind.N_TBANG) {
							return true;
						};
					};
				};
			};
		};
	};
	return false;
};

// taggedhaserr — true iff any variant of `n` (assumed
// nkind.N_TTAGGED) is `!`-marked. Picks the explicit-flag semantics over
// the legacy "first variant = success" rule.
fn taggedhaserr(c: *checker, n: *node) bool = {
	let v: *node = n.list;
	for (v != nil) {
		if (varianterr(c, v)) { return true; };
		v = v.next;
	};
	return false;
};

// 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 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;
};

// scruttype — resolve the type expression for a match's
// scrutinee. Handles nkind.N_IDENT (look up local/param's declared
// type) and nkind.N_DOT (module-qualified ref). Returns nil if we
// can't statically determine the type. Used by exhaustiveness.
fn scruttype(c: *checker, e: *node) *node = {
	if (e == nil) { return nil; };
	if (e.kind == nkind.N_IDENT) {
		let s: *sym = scopelookup(c.cur, e.str);
		if (s == nil) { return nil; };
		if (s.decl == nil) { return nil; };
		// For nkind.N_LET / nkind.N_PARAM: declared type is decl.lhs.
		return s.decl.lhs;
	};
	// #51: `match (pkg.var)` / `pkg.var is T` — module-qualified ref.
	// lhs is N_IDENT (module bareword), str is the leaf. Bind via
	// scopelookupinmodule so the declared type carries the same
	// shape resolvealias' dotted-name branch now consumes. Falls
	// silently to nil when lhs is a value (struct-field access) —
	// the rest of the lenient-check contract.
	if (e.kind == nkind.N_DOT) {
		if (e.lhs == nil) { return nil; };
		if (e.lhs.kind != nkind.N_IDENT) { return nil; };
		let s: *sym = scopelookupinmodule(c.cur, e.lhs.str, e.str);
		if (s == nil) { return nil; };
		if (s.decl == nil) { return nil; };
		return s.decl.lhs;
	};
	return nil;
};

// mktname — fabricate an nkind.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, nkind.N_TNAME, "", 0, 0);
	n.str = nm;
	return n;
};

// #43: SSoT for primitive type byte sizes. astsize's N_TNAME-primitive
// arm and every wwstage cgen size walker (slotsize/fieldsize/letemit-
// size/elemsizeof/paramfieldsize) consult this table so a future
// ty_str.size bump (#1) lands in one place. Returns -1 for non-prim
// names; callers fall back to alias/struct/enum lookup. Cstage's
// equivalent SSoT is cmd/wcc/type.c:46-79 (ty_void/ty_bool/.../ty_str).
fn primtypesize(nm: str) i64 = {
	if (streq(nm, "void")) { return 0i64; };
	if (streq(nm, "bool")) { return 1i64; };
	if (streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
	if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
	if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
	if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64")) { return 8i64; };
	if (streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr")) { return 8i64; };
	if (streq(nm, "str")) { return 16i64; };
	return -1i64;
};

// #43: SSoT for slice header size (ptr+len+cap = 24B today). Mirrors
// cstage cmd/wcc/type.c:103 (ty_slice->size = 24). Bumping a slice's
// header layout in #34 touches only this constant.
fn tyslicesize() i64 = { return 24i64; };

// #42: AST-level layout helpers for the size(T)/align(T)/offset(e.f)
// fold. Mirror cstage resolve_type's size/align computation
// (cmd/wcc/check.c:286-528) on AST nodes — wwstage check.ww never
// materialises tinfo for user types so the fold has to walk the AST
// directly. Struct layout follows cstage check.c:471-526 (align each
// field, max align for the whole record, round size up to alignment).
fn astalign(c: *checker, t: *node) i64 = {
	if (t == nil) { return 1i64; };
	let k: nkind = t.kind;
	if (k == nkind.N_TBANG) { return astalign(c, t.lhs); };
	if (k == nkind.N_TPTR)   { return 8i64; };
	if (k == nkind.N_TSLICE) { return 8i64; };
	if (k == nkind.N_TCHAN)  { return 8i64; };
	if (k == nkind.N_TFN)    { return 8i64; };
	if (k == nkind.N_TARRAY) { return astalign(c, t.lhs); };
	if (k == nkind.N_TTAGGED) { return 8i64; };
	if (k == nkind.N_TTUPLE) {
		let m: i64 = 1i64;
		let p: *node = t.list;
		for (p != nil) {
			let pa: i64 = astalign(c, p);
			if (pa > m) { m = pa; };
			p = p.next;
		};
		return m;
	};
	if (k == nkind.N_TSTRUCT) {
		let m: i64 = 1i64;
		let f: *node = t.list;
		for (f != nil) {
			if (f.kind == nkind.N_TFIELD) {
				let fa: i64 = astalign(c, f.lhs);
				if (fa > m) { m = fa; };
			};
			f = f.next;
		};
		return m;
	};
	if (k == nkind.N_TENUM) {
		if (t.lhs != nil) { return astalign(c, t.lhs); };
		return 4i64;
	};
	if (k == nkind.N_TNAME) {
		let nm: str = t.str;
		if (streq(nm, "void") || streq(nm, "bool") || streq(nm, "i8") || streq(nm, "u8")) { return 1i64; };
		if (streq(nm, "i16") || streq(nm, "u16")) { return 2i64; };
		if (streq(nm, "i32") || streq(nm, "u32") || streq(nm, "f32") || streq(nm, "rune")) { return 4i64; };
		if (streq(nm, "i64") || streq(nm, "u64") || streq(nm, "f64") || streq(nm, "int") || streq(nm, "uint") || streq(nm, "uintptr") || streq(nm, "str")) { return 8i64; };
		let resolved: *node = resolvealias(c, t);
		if (resolved != nil && resolved != t) {
			return astalign(c, resolved);
		};
	};
	return 1i64;
};

fn astsize(c: *checker, t: *node) i64 = {
	if (t == nil) { return 0i64; };
	let k: nkind = t.kind;
	if (k == nkind.N_TBANG) { return astsize(c, t.lhs); };
	if (k == nkind.N_TPTR)   { return 8i64; };
	if (k == nkind.N_TSLICE) { return tyslicesize(); };
	if (k == nkind.N_TCHAN)  { return 8i64; };
	if (k == nkind.N_TFN)    { return 8i64; };
	if (k == nkind.N_TARRAY) {
		let elen: i64 = 0i64;
		if (t.rhs != nil) {
			if (t.rhs.kind == nkind.N_INTLIT) { elen = t.rhs.uval: i64; };
		};
		return astsize(c, t.lhs) * elen;
	};
	if (k == nkind.N_TTUPLE) {
		let total: i64 = 0i64;
		let p: *node = t.list;
		for (p != nil) {
			total += astsize(c, p);
			p = p.next;
		};
		return total;
	};
	if (k == nkind.N_TSTRUCT) {
		let off: i64 = 0i64;
		let maxal: i64 = 1i64;
		let f: *node = t.list;
		for (f != nil) {
			if (f.kind == nkind.N_TFIELD) {
				let fa: i64 = astalign(c, f.lhs);
				if (fa > maxal) { maxal = fa; };
				off = (off + fa - 1i64) & ~(fa - 1i64);
				off += astsize(c, f.lhs);
			};
			f = f.next;
		};
		return (off + maxal - 1i64) & ~(maxal - 1i64);
	};
	if (k == nkind.N_TTAGGED) {
		// 8 (tag) + max variant payload, rounded up to 8.
		let maxsz: i64 = 0i64;
		let v: *node = t.list;
		for (v != nil) {
			let sz: i64 = astsize(c, v);
			if (sz > maxsz) { maxsz = sz; };
			v = v.next;
		};
		let pad: i64 = (maxsz + 7i64) & ~7i64;
		return 8i64 + pad;
	};
	if (k == nkind.N_TENUM) {
		if (t.lhs != nil) { return astsize(c, t.lhs); };
		return 4i64;
	};
	if (k == nkind.N_TNAME) {
		let nm: str = t.str;
		let ps: i64 = primtypesize(nm);
		if (ps >= 0i64) { return ps; };
		let resolved: *node = resolvealias(c, t);
		if (resolved != nil && resolved != t) {
			return astsize(c, resolved);
		};
	};
	return 0i64;
};

// astoffset — byte offset of `dot.str` inside the struct type of
// `dot.lhs`. Mirrors cstage cmd/wcc/check.c:932-961: peel one N_TPTR
// (for `p.field` where p is *Struct), require N_TSTRUCT, walk fields
// honouring per-field alignment, return -1 if the field name is
// absent so the caller can flag the error and fold to 0.
fn astoffset(c: *checker, dot: *node) i64 = {
	if (dot == nil) { return -1i64; };
	if (dot.kind != nkind.N_DOT) { return -1i64; };
	let recv: *node = scruttype(c, dot.lhs);
	if (recv == nil) { return -1i64; };
	let rt: *node = resolvealias(c, unwrapbang(recv));
	if (rt == nil) { return -1i64; };
	if (rt.kind == nkind.N_TPTR) {
		rt = resolvealias(c, unwrapbang(rt.lhs));
	};
	if (rt == nil) { return -1i64; };
	if (rt.kind != nkind.N_TSTRUCT) { return -1i64; };
	let off: i64 = 0i64;
	let f: *node = rt.list;
	for (f != nil) {
		if (f.kind == nkind.N_TFIELD) {
			let fa: i64 = astalign(c, f.lhs);
			off = (off + fa - 1i64) & ~(fa - 1i64);
			if (streq(f.str, dot.str)) { return off; };
			off += astsize(c, f.lhs);
		};
		f = f.next;
	};
	return -1i64;
};

// arenau64tos — decimal string for the folded INTLIT's `str` field.
// Cstage uses aprintf("%llu") at the same site (cmd/wcc/check.c:921);
// wwstage cgen only reads `uval` for N_INTLIT codegen so `str` is
// just for the AST printer, but set it for parity with the parser's
// own literal-emit shape.
fn arenau64tos(a: *arena, v: u64) str = {
	let buf: *u8 = amalloc(a, 24u64): *u8;
	let i: i32 = 23;
	buf[i] = 0u8;
	if (v == 0u64) { i -= 1; buf[i] = 48u8; };
	let n: u64 = v;
	for (n > 0u64) {
		i -= 1;
		buf[i] = (48u64 + (n % 10u64)): u8;
		n /= 10u64;
	};
	let r: str;
	r.ptr = buf + (i: u64);
	r.len = 23 - i;
	return r;
};

// foldtointlit — mutate `n` in place to an N_INTLIT with value `v`.
// Used by the #42 size/align/offset intercepts so cgen sees the
// folded literal rather than an unresolved call. Mirrors cstage
// cmd/wcc/check.c:919-927 / :951-958.
fn foldtointlit(c: *checker, n: *node, v: i64) void = {
	n.kind = nkind.N_INTLIT;
	n.uval = v: u64;
	n.str = arenau64tos(c.a, v: u64);
	n.lhs = nil;
	n.list = nil;
	let empty: str;
	n.tsuffix = empty;
};

// 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: nkind = e.kind;
	if (k == nkind.N_INTLIT)   { return mktname(c, "untyped_int"); };
	if (k == nkind.N_FLOATLIT) { return mktname(c, "untyped_float"); };
	if (k == nkind.N_STRLIT)   { return mktname(c, "str"); };
	if (k == nkind.N_RUNELIT)  { return mktname(c, "rune"); };
	if (k == nkind.N_TRUE)     { return mktname(c, "bool"); };
	if (k == nkind.N_FALSE)    { return mktname(c, "bool"); };
	if (k == nkind.N_VOIDLIT)  { return mktname(c, "void"); };
	if (k == nkind.N_NIL)      { return mktname(c, "untyped_nil"); };
	if (k == nkind.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 == nkind.N_CAST) {
		// `expr: T` — explicit cast; the type expr is e.rhs.
		return e.rhs;
	};
	if (k == nkind.N_CALL) {
		let callee: *node = e.lhs;
		if (callee == nil) { return nil; };
		// #31: synthesize the `alloc(value)` / `alloc([], n)` builtin
		// return shape so checkletassign sees the same `(*T | nomem)` /
		// `([]T | nomem)` cstage's check.c stamps at L981-1006. Without
		// this, exprtype returns the seeded decl's nil lhs and the let
		// silently accepts `let p: *T = alloc(v);` — rule 10 trap.
		// Same-module gate mirrors cstage's `c->cur_mod &&
		// scope_lookup_in_module(...)` check from task #23.
		if (callee.kind == nkind.N_IDENT) {
			if (streq(callee.str, "alloc")) {
				let shadowed: bool = false;
				if (c.curmod.len > 0) {
					if (scopelookupinmodule(c.cur, c.curmod, "alloc") != nil) {
						shadowed = true;
					};
				};
				if (!shadowed) {
					if (e.list != nil) {
						// Slice form: `alloc([], n)`.
						if (e.list.kind == nkind.N_ARRLIT) {
							if (e.list.list == nil) {
								if (e.list.next != nil) {
									if (e.list.next.next == nil) {
										let sl: *node = newnode(c.a, nkind.N_TSLICE, "", 0, 0);
										sl.lhs = mktname(c, "u8");
										let nome: *node = mktname(c, "nomem");
										sl.next = nome;
										let tt: *node = newnode(c.a, nkind.N_TTAGGED, "", 0, 0);
										tt.list = sl;
										return tt;
									};
								};
							};
						};
						// Value form: `alloc(value)`.
						if (e.list.next == nil) {
							let argt: *node = exprtype(c, e.list);
							let ptr: *node = newnode(c.a, nkind.N_TPTR, "", 0, 0);
							ptr.lhs = argt;
							let nome: *node = mktname(c, "nomem");
							ptr.next = nome;
							let tt: *node = newnode(c.a, nkind.N_TTAGGED, "", 0, 0);
							tt.list = ptr;
							return tt;
						};
					};
				};
			};
		};
		// #42: size(T) / align(T) / offset(e.f) typed-builtin intercepts.
		// Fold the N_CALL in place to an N_INTLIT so cgen never sees an
		// unresolved size/align/offset symbol. Same-module shadow gate
		// mirrors the alloc precedent (#23) so a user `fn size(...)`
		// inside this module suppresses the builtin. Mirrors cstage
		// cmd/wcc/check.c:907-960.
		if (callee.kind == nkind.N_IDENT) {
			let bname: str = callee.str;
			let issize: bool = streq(bname, "size");
			let isalign: bool = streq(bname, "align");
			let isoffset: bool = streq(bname, "offset");
			if (issize || isalign || isoffset) {
				let shadowed: bool = false;
				if (c.curmod.len > 0) {
					if (scopelookupinmodule(c.cur, c.curmod, bname) != nil) {
						shadowed = true;
					};
				};
				if (!shadowed) {
					if (e.list != nil) {
						if (issize) {
							let v: i64 = astsize(c, e.list);
							foldtointlit(c, e, v);
							return mktname(c, "i32");
						};
						if (isalign) {
							let v: i64 = astalign(c, e.list);
							foldtointlit(c, e, v);
							return mktname(c, "i32");
						};
						// offset(e.f): the arg is a value expression
						// (N_DOT), parsed via parsearglist — not a
						// type expression.
						if (isoffset) {
							if (e.list.next == nil && e.list.kind == nkind.N_DOT) {
								let off: i64 = astoffset(c, e.list);
								if (off < 0i64) {
									os.write(2, "offset: no field '".ptr, 18u64);
									os.write(2, e.list.str.ptr, e.list.str.len: u64);
									os.write(2, "'\n".ptr, 2u64);
									c.errs += 1;
									off = 0i64;
								};
								foldtointlit(c, e, off);
								return mktname(c, "i32");
							};
						};
					};
				};
			};
		};
		let nm: str;
		nm.ptr = nil; nm.len = 0;
		if (callee.kind == nkind.N_IDENT) { nm = callee.str; };
		if (callee.kind == nkind.N_DOT)   { nm = callee.str; };
		if (nm.len == 0) { return nil; };
		// #56: bare-leaf N_IDENT calls go through scopelookupprefer so
		// `foo()` inside module M binds to M.foo rather than another
		// module's same-leaf foo at the head of the flat scope bucket.
		// Mirrors cstage cexpr N_IDENT routing through
		// scope_lookup_prefer with c->cur_mod. N_DOT keeps the bare
		// scopelookup — its module-qualified resolution is a separate
		// gap (parser stores the leaf in callee.str; mod is in
		// callee.lhs.str, not consumed here yet).
		let s: *sym = nil;
		if (callee.kind == nkind.N_IDENT) {
			s = scopelookupprefer(c.cur, c.curmod, nm);
		} else {
			s = scopelookup(c.cur, nm);
		};
		if (s == nil) { return nil; };
		if (s.skind != 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 == nkind.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 != nkind.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 == nkind.N_TRYUNW) {
		// `e!` abort-on-error unwrap; success variant is what the
		// receiver gets, identical to `?` shape modulo control flow.
		// #31: required so `let p: *T = alloc(v)!;` resolves to *T.
		let opt: *node = exprtype(c, e.lhs);
		let ou: *node = resolvealias(c, unwrapbang(opt));
		if (ou == nil) { return nil; };
		if (ou.kind != nkind.N_TTAGGED) { return nil; };
		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 == nkind.N_TYPEASSERT) {
		// `e as T` → T
		return e.rhs;
	};
	if (k == nkind.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 != nkind.N_TNAME) { return false; };
	return streq(t.str, "untyped_int");
};

fn isuntypedfloat(t: *node) bool = {
	if (t == nil) { return false; };
	if (t.kind != nkind.N_TNAME) { return false; };
	return streq(t.str, "untyped_float");
};

fn isuntypednil(t: *node) bool = {
	if (t == nil) { return false; };
	if (t.kind != nkind.N_TNAME) { return false; };
	return streq(t.str, "untyped_nil");
};

// isinttypeast — int-typed AST node. Either a primitive int name
// (i8..i64/u8..u64/int/uint/uintptr/rune) or an N_TENUM. Floats are
// excluded so the enum↔int reinterpret in checkisas (#52) refuses a
// surprise `enum as f64` shape. Mirrors cstage's type_isint
// (cmd/wcc/type.c) restricted to the kinds reachable from AST.
fn isinttypeast(t: *node) bool = {
	if (t == nil) { return false; };
	if (t.kind == nkind.N_TENUM) { return true; };
	if (t.kind != nkind.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; };
	return false;
};

fn isnumerictname(t: *node) bool = {
	if (t == nil) { return false; };
	if (t.kind != nkind.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 != nkind.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 == nkind.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 == nkind.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 == nkind.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 == nkind.N_TPTR)   { return true; };
		if (du.kind == nkind.N_TSLICE) { return true; };
		if (du.kind == nkind.N_TCHAN)  { return true; };
		if (du.kind == nkind.N_TFN)    { return true; };
		// nullable `(*T | void)` — already accepted by typeeqast
		// when matched whole; nil is OK there too.
		if (du.kind == nkind.N_TTAGGED) {
			let v: *node = du.list;
			for (v != nil) {
				if (v.kind == nkind.N_TPTR)  { return true; };
				if (v.kind == nkind.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 == nkind.N_TTAGGED && su.kind != nkind.N_TTAGGED) {
		// #55: nominal-first compare. Cstage variant_match
		// (cmd/wcc/check.c:90-100) takes NAMED types as
		// pointer-identical, so two `err`s match before bodies
		// are resolved. Wwstage's typeeqast already does
		// string-nominal on N_TNAME, but pre-fix this branch
		// resolvealias-d both v and src to their bodies (e.g.
		// N_TSTRUCT), and typeeqast's conservative struct arm
		// returned false — `return e;` inside `fn f() (void | err)`
		// got flagged. Compare surface forms first; fall through
		// to resolved compare only when the surface mismatches
		// (covers structurally-anonymous variant cases that
		// resolvealias actually disambiguates). Residual: bare-vs-
		// qualified TNAME (`(void | modM.err)` variant vs bare `err`
		// inside modM) still misses both arms — filed as #57.
		let srcraw: *node = unwrapbang(src);
		let v: *node = du.list;
		for (v != nil) {
			let vraw: *node = unwrapbang(v);
			if (typeeqast(vraw, srcraw)) { return true; };
			let vu: *node = resolvealias(c, vraw);
			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 == nkind.N_TTAGGED && su.kind == nkind.N_TTAGGED) {
		*confident = false;
		return true;
	};
	// tagged → non-tagged: requires `?` / `!` / match to project a
	// variant. #31: this is what traps `let p: *T = alloc(v);`
	// where the builtin returns `(*T | nomem)` and the LHS is bare.
	if (su.kind == nkind.N_TTAGGED && du.kind != nkind.N_TTAGGED) {
		return false;
	};
	// Two known primitives with different names are confidently
	// incompatible. `i32 ↔ bool`, `str ↔ i32`, etc.
	if (du.kind == nkind.N_TNAME && su.kind == nkind.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 casecovers(c: *checker, cs: *node, want: *node) bool = {
	if (cs.lhs != nil) {
		if (typeeqast(cs.lhs, want)) { return true; };
	};
	let alt: *node = cs.list;
	for (alt != nil) {
		if (typeeqast(alt, want)) { return true; };
		alt = alt.next;
	};
	return false;
};

fn errmatchvariant(c: *checker, n: *node, vname: *node) void = {
	os.write(2, "match: variant not handled".ptr, 26u64);
	if (vname != nil) {
		if (vname.kind == nkind.N_TNAME) {
			os.write(2, " (".ptr, 2u64);
			os.write(2, vname.str.ptr, vname.str.len: u64);
			os.write(2, ")".ptr, 1u64);
		};
	};
	os.write(2, "\n".ptr, 1u64);
	c.errs += 1;
};

// casevariantin — true iff `pat` (a `case T` pattern, including
// each alt of a multi-pattern) names a variant of the tagged
// union `tagged`.
fn casevariantin(tagged: *node, pat: *node) bool = {
	let v: *node = tagged.list;
	for (v != nil) {
		if (typeeqast(v, pat)) { return true; };
		v = v.next;
	};
	return false;
};

fn errbadcase(c: *checker, pat: *node) void = {
	os.write(2, "case: not a variant of scrutinee".ptr, 32u64);
	if (pat != nil) {
		if (pat.kind == nkind.N_TNAME) {
			os.write(2, " (".ptr, 2u64);
			os.write(2, pat.str.ptr, pat.str.len: u64);
			os.write(2, ")".ptr, 1u64);
		};
	};
	os.write(2, "\n".ptr, 1u64);
	c.errs += 1;
};

fn checkmatchexhaust(c: *checker, n: *node) void = {
	if (n == nil) { return; };
	if (n.lhs == nil) { return; };
	let st: *node = scruttype(c, n.lhs);
	let u: *node = resolvealias(c, unwrapbang(st));
	if (u == nil) { return; };
	if (u.kind != nkind.N_TTAGGED) { return; };
	// Validity: every `case T` pattern (and multi-pattern alts)
	// must name a variant of u. Catches typos and dead arms that
	// the dispatch would never reach.
	let cs0: *node = n.list;
	for (cs0 != nil) {
		if (cs0.lhs != nil) {
			if (!casevariantin(u, cs0.lhs)) {
				errbadcase(c, cs0.lhs);
			};
			let alt: *node = cs0.list;
			for (alt != nil) {
				if (!casevariantin(u, alt)) {
					errbadcase(c, alt);
				};
				alt = alt.next;
			};
		};
		cs0 = cs0.next;
	};
	// Default arm absorbs anything; skip exhaustiveness.
	let cs: *node = n.list;
	for (cs != nil) {
		if (cs.lhs == nil) { return; };	// default
		cs = cs.next;
	};
	// For each variant of u, look for a covering case.
	let v: *node = u.list;
	for (v != nil) {
		let covered: bool = false;
		let cs2: *node = n.list;
		for (cs2 != nil) {
			if (casecovers(c, cs2, v)) {
				covered = true;
				cs2 = nil;
			} else {
				cs2 = cs2.next;
			};
		};
		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 == nkind.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 == nkind.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
	// #45: alloc([], n) defers element type to the let-init context
	// (Hare-style). exprtype's alloc-slice branch synthesizes
	// ([]u8 | nomem) / []u8 (for the ?/! wrap) with no LHS context;
	// when the let declares []T, retype src to []T / ([]T | nomem)
	// so isassignable sees exact equality. cgenstmt cglet drives the
	// element size from n.lhs already (cmd/wcc/cgenstmt.ww), so this
	// stays symmetric with cstage check.c clet's parallel retype.
	if (n.lhs.kind == nkind.N_TSLICE) {
		let wrapped: bool = false;
		let inner: *node = n.rhs;
		if (inner.kind == nkind.N_TRYPROP) {
			wrapped = true;
			inner = inner.lhs;
		} else { if (inner.kind == nkind.N_TRYUNW) {
			wrapped = true;
			inner = inner.lhs;
		}; };
		if (inner != nil && inner.kind == nkind.N_CALL) {
			let callee: *node = inner.lhs;
			let a0: *node = inner.list;
			let a1: *node = nil;
			let a2: *node = nil;
			if (a0 != nil) { a1 = a0.next; };
			if (a1 != nil) { a2 = a1.next; };
			if (callee != nil
			    && callee.kind == nkind.N_IDENT
			    && streq(callee.str, "alloc")
			    && a0 != nil && a0.kind == nkind.N_ARRLIT
			    && a0.list == nil
			    && a1 != nil && a2 == nil) {
				let shadowed: bool = false;
				if (c.curmod.len > 0) {
					if (scopelookupinmodule(c.cur, c.curmod, "alloc") != nil) {
						shadowed = true;
					};
				};
				if (!shadowed) {
					let sl: *node = newnode(c.a, nkind.N_TSLICE, "", 0, 0);
					sl.lhs = n.lhs.lhs;
					if (wrapped) {
						src = sl;
					} else {
						let nome: *node = mktname(c, "nomem");
						sl.next = nome;
						let tt: *node = newnode(c.a, nkind.N_TTAGGED, "", 0, 0);
						tt.list = sl;
						src = tt;
					};
				};
			};
		};
	};
	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; };
	// #52: enum ↔ int reinterpret (`enum as intT` / `intT as enum`).
	// Mirrors cstage cmd/wcc/check.c:1346-1357 — N_TYPEASSERT with an
	// enum on either side and integer types on both reinterprets in
	// the same register, no tag check involved. Returns early before
	// the tagged-union gate so lib/time/instant.ww `(c as i32)` and
	// the lib/os syscall casts stop false-positiving. `is` (TYPETEST)
	// stays rejected on non-tagged operands — cstage cmd/wcc/check.c
	// gates the bypass on N_TYPEASSERT only.
	if (n.kind == nkind.N_TYPEASSERT) {
		let v: *node = resolvealias(c, unwrapbang(n.rhs));
		let lhsenum: bool = false;
		let rhsenum: bool = false;
		if (u != nil) { if (u.kind == nkind.N_TENUM) { lhsenum = true; }; };
		if (v != nil) { if (v.kind == nkind.N_TENUM) { rhsenum = true; }; };
		if (lhsenum || rhsenum) {
			if (isinttypeast(u)) { if (isinttypeast(v)) { return; }; };
		};
	};
	if (u.kind != nkind.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 == nkind.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
// enclosing fn's return-type variants. Mirrors C check.c. Operand
// is nkind.N_TRYPROP; its lhs is the value-bearing expr; we look at the
// expr's *declared* type for nkind.N_IDENT/nkind.N_CALL cases.

fn exprtypeoftry(c: *checker, e: *node) *node = {
	if (e == nil) { return nil; };
	if (e.kind == nkind.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 (e.kind == nkind.N_CALL) {
		// callee return type lookup: callee is e.lhs (nkind.N_IDENT or
		// nkind.N_DOT). We need the fn-decl's lhs (return-type AST).
		let callee: *node = e.lhs;
		if (callee == nil) { return nil; };
		let nm: str;
		nm.ptr = nil; nm.len = 0;
		if (callee.kind == nkind.N_IDENT) { nm = callee.str; };
		if (callee.kind == nkind.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 != skind.SK_FN) { return nil; };
		if (s.decl == nil) { return nil; };
		return s.decl.lhs;
	};
	return nil;
};

fn checktryprop(c: *checker, n: *node) void = {
	if (n == nil) { return; };
	let t: *node = exprtypeoftry(c, n.lhs);
	let u: *node = resolvealias(c, unwrapbang(t));
	if (u == nil) { return; };
	if (u.kind != nkind.N_TTAGGED) { return; };
	// Does the operand have any error variants?
	let haserr: bool = false;
	let v: *node = u.list;
	for (v != nil) {
		if (iserrvariant(c, u, v)) { haserr = true; };
		v = v.next;
	};
	if (!haserr) { return; };
	// Enclosing fn must return a tagged union with each operand
	// error variant present.
	let r: *node = resolvealias(c, unwrapbang(c.fnret));
	if (r == nil) {
		os.write(2, "?: enclosing fn has no tagged-union return\n".ptr, 43u64);
		c.errs += 1;
		return;
	};
	if (r.kind != nkind.N_TTAGGED) {
		os.write(2, "?: enclosing fn return is not tagged\n".ptr, 37u64);
		c.errs += 1;
		return;
	};
	let ev: *node = u.list;
	for (ev != nil) {
		if (iserrvariant(c, u, ev)) {
			let found: bool = false;
			let rv: *node = r.list;
			for (rv != nil) {
				if (typeeqast(rv, ev)) {
					found = true;
					rv = nil;
				} else { rv = rv.next; };
			};
			if (!found) {
				os.write(2, "?: error variant not in enclosing return\n".ptr, 41u64);
				c.errs += 1;
			};
		};
		ev = ev.next;
	};
};

// install_param — when entering a fn body, define its params in a
// fresh local scope.
//
// TODO(#11): cstage check.c (post-#32) errors `param '%s' redeclared`
// when two params share a name. The fn body's scope IS fresh here
// (resolvefnbody opens it before calling us), so guarding scopedefine's
// nil return would be sound — but we defer until #11 wires checkfile
// into w6c_ww so the diagnostic class lands as a single coordinated
// step rather than dribbling in. Matches the cstage-only neg-case
// precedent at test/wcc/708 + test/wcc/696.
fn installparams(c: *checker, params: *node) void = {
	let p: *node = params;
	for (p != nil) {
		if (p.kind == nkind.N_PARAM) {
			let nm: str = p.str;
			if (nm.len > 0) {
				checkmoduleshadow(c, nm, "param");
				scopedefine(c.cur, nm, skind.SK_PARAM, nil, p);
			};
		};
		p = p.next;
	};
};

// resolvefnbody — open a child scope for the fn, install its params,
// then walk the body. Local lets installed by walk_stmt (a future
// extension); for the current pass we just resolve-walk without
// per-statement scopes.
fn resolvefnbody(c: *checker, fnnode: *node) void = {
	let outer: *scope = c.cur;
	c.cur = newscope(c.a, c.cur);
	installparams(c, fnnode.list);
	let prevret: *node = c.fnret;
	c.fnret = fnnode.lhs;	// return type AST, used by `?` check
	if (fnnode.body != nil) {
		resolvewalk(c, fnnode.body);
	};
	c.fnret = prevret;
	c.cur = outer;
};

export fn checkinit(c: *checker, a: *arena, tc: *tctx) void = {
	c.a = a;
	c.tc = tc;
	c.top = newscope(a, nil);
	c.cur = c.top;
	c.nresolved = 0;
	c.nunresolved = 0;
	c.errs = 0;
	c.verbose = 0;
	c.fnret = nil;
	let empty: str;
	c.curmod = empty;
	c.file = nil;
	seedprimitives(c);
};

export fn checkfile(c: *checker, file: *node) void = {
	if (file == nil) { return; };
	if (file.kind != nkind.N_FILE) { return; };
	c.file = file;

	// Pass 1: install all top-level names.
	let d: *node = file.list;
	for (d != nil) {
		installdecl(c, file, d);
		d = d.next;
	};

	// Pass 2: walk decl bodies/types and resolve identifiers.
	// Track the per-decl module bareword so bare-leaf lookups inside
	// the body prefer same-module entries over alphabetically-earlier
	// same-leaf imports.
	d = file.list;
	for (d != nil) {
		c.curmod = declmod(file, d);
		let k: nkind = d.kind;
		if (k == nkind.N_FNDECL) {
			if (d.lhs != nil) { resolvewalk(c, d.lhs); };	// return type
			resolvefnbody(c, d);
		} else { if (k == nkind.N_DEF) {
			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
			if (d.rhs != nil) { resolvewalk(c, d.rhs); };
		} else { if (k == nkind.N_TYPEDECL) {
			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
		} else { if (k == nkind.N_LET) {
			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
			if (d.rhs != nil) { resolvewalk(c, d.rhs); };
		};};};};
		d = d.next;
	};
	let empty: str;
	c.curmod = empty;

};