selfhost/cmd/wcc: isassignable arm 6 → recursive (cascade Commit 1, #39)

The concrete→tagged arm walked variants with typeeqast-only, rejecting
widenings that aren't strict surface-eq: NAMED-aliased variants, nested
tagged inside a variant, and concrete → variant after the wrap-induced
exprtype reshape that Commit 3 (#41, variadic wrap re-land) introduces.

Replace the walk with a recursive isassignable call per variant, mirroring
harec tagged_select_subtype (ref/harec/src/types.c:702-739, recursive
type_is_assignable at :718; invoked from the TAGGED arm at :1110-1112)
and the byte-id cstage precedent at cmd/wcc/type.c:298-299. The recursive
call's leading typeeqast (check.ww:2257) preserves the #55 surface-nominal
fast path; the #57 bare-vs-qualified TNAME residual is unchanged.

No new test: the new widen path is dormant pre-Commit-3; existing
tagged-union tests cover the surface-eq fast path. Commit 3's fmttest
exercise will exercise the recursive widen for free.

Closes 39 cascade errors that fire once #41 lands.

selfhost/cmd/w6c/main.combined.ww

@@ -9275,30 +9275,21 @@ fn isassignable(c: *checker, dst: *node, src: *node, confident: *bool) bool = {
 		return true;
 	};
 	// Tagged-union variant inclusion: src is one of dst's variants.
+	// Recursive isassignable mirrors cstage type_assignable
+	// (cmd/wcc/type.c:298-299) and harec tagged_select_subtype's
+	// recursive type_is_assignable call (ref/harec/src/types.c:702-739,
+	// :718; invoked from the TAGGED arm at :1110-1112). #39 cascade:
+	// the prior typeeqast-only walk rejected widenings that aren't
+	// strict surface-eq (NAMED-aliased variants, nested tagged inside
+	// a variant, concrete → variant after the wrap-induced exprtype
+	// reshape). #55 surface-nominal fast path is preserved by the
+	// recursive call's leading typeeqast (line 2257). #57 bare-vs-
+	// qualified TNAME residual unchanged.
 	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);
+			let innerconf: bool = false;
-			if (typeeqast(vraw, srcraw)) { return true; };
+			if (isassignable(c, v, src, &innerconf)) { return true; };
-			let vu: *node = resolvealias(c, vraw);
-			if (vu != nil) {
-				if (typeeqast(vu, su)) { return true; };
-			};
 			v = v.next;
 		};
 		return false;

selfhost/cmd/wcc/check.ww

@@ -2311,30 +2311,21 @@ fn isassignable(c: *checker, dst: *node, src: *node, confident: *bool) bool = {
 		return true;
 	};
 	// Tagged-union variant inclusion: src is one of dst's variants.
+	// Recursive isassignable mirrors cstage type_assignable
+	// (cmd/wcc/type.c:298-299) and harec tagged_select_subtype's
+	// recursive type_is_assignable call (ref/harec/src/types.c:702-739,
+	// :718; invoked from the TAGGED arm at :1110-1112). #39 cascade:
+	// the prior typeeqast-only walk rejected widenings that aren't
+	// strict surface-eq (NAMED-aliased variants, nested tagged inside
+	// a variant, concrete → variant after the wrap-induced exprtype
+	// reshape). #55 surface-nominal fast path is preserved by the
+	// recursive call's leading typeeqast (line 2257). #57 bare-vs-
+	// qualified TNAME residual unchanged.
 	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);
+			let innerconf: bool = false;
-			if (typeeqast(vraw, srcraw)) { return true; };
+			if (isassignable(c, v, src, &innerconf)) { return true; };
-			let vu: *node = resolvealias(c, vraw);
-			if (vu != nil) {
-				if (typeeqast(vu, su)) { return true; };
-			};
 			v = v.next;
 		};
 		return false;

selfhost/cmd/wwdump/main.combined.ww

@@ -9275,30 +9275,21 @@ fn isassignable(c: *checker, dst: *node, src: *node, confident: *bool) bool = {
 		return true;
 	};
 	// Tagged-union variant inclusion: src is one of dst's variants.
+	// Recursive isassignable mirrors cstage type_assignable
+	// (cmd/wcc/type.c:298-299) and harec tagged_select_subtype's
+	// recursive type_is_assignable call (ref/harec/src/types.c:702-739,
+	// :718; invoked from the TAGGED arm at :1110-1112). #39 cascade:
+	// the prior typeeqast-only walk rejected widenings that aren't
+	// strict surface-eq (NAMED-aliased variants, nested tagged inside
+	// a variant, concrete → variant after the wrap-induced exprtype
+	// reshape). #55 surface-nominal fast path is preserved by the
+	// recursive call's leading typeeqast (line 2257). #57 bare-vs-
+	// qualified TNAME residual unchanged.
 	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);
+			let innerconf: bool = false;
-			if (typeeqast(vraw, srcraw)) { return true; };
+			if (isassignable(c, v, src, &innerconf)) { return true; };
-			let vu: *node = resolvealias(c, vraw);
-			if (vu != nil) {
-				if (typeeqast(vu, su)) { return true; };
-			};
 			v = v.next;
 		};
 		return false;