selfhost/cmd/wcc: stamp e.type_ for N_INDEX (A.6.1.4)

Port cstage cmd/wcc/check.c:870-894 to exprtype. indexresult yields
the resolved element tnode for slice/array, u8 for str, T for *[N]T
(decay) and *T (generic), and []T for *[]T (no decay, mirrors the
Hare-faithful rule at check.c:883-885). The arm stamps e.type_ via
tinfofornode same shape as the A.6.1.3 BIN/UN pattern.

Phase 1 A.6 step 4 of 6 — slot the indexing result type into the
mandatory post-checker invariant. Cgenutil still derives index
stride from AST shape (indexbaseesz, elemsizeofc, indexvaluetnode);
A.6.3 will collapse those onto the now-stamped N_INDEX type_.

Verified 132/132 incl. 995_self_rebuild byte-identity.

selfhost/cmd/w6c/main.combined.ww

@@ -8271,10 +8271,36 @@ fn unoptype(c: *checker, e: *node) *node = {
 	return nil;
 };
 
+// indexresult — derive the result tnode of an N_INDEX expression.
+// Mirrors cstage cmd/wcc/check.c:870-894. Slice/array → elem; str →
+// u8; `*[N]T` decays to T (pointer-to-array); `*[]T` does NOT decay
+// (yields []T via the generic *U → U fallback — Hare-faithful, a
+// pointer-to-slice is a 1D array of slices, not of T); generic *T → T.
+fn indexresult(c: *checker, e: *node) *node = {
+	let basetn: *node = exprtype(c, e.lhs, nil);
+	let _idx: *node = exprtype(c, e.rhs, nil);
+	let u: *node = resolvealias(c, unwrapbang(basetn));
+	if (u == nil) { return nil; };
+	if (u.kind == nkind.N_TSLICE) { return u.lhs; };
+	if (u.kind == nkind.N_TARRAY) { return u.lhs; };
+	if (u.kind == nkind.N_TNAME) {
+		if (streq(u.str, "str")) { return mktname(c, "u8"); };
+	};
+	if (u.kind == nkind.N_TPTR) {
+		let inner: *node = u.lhs;
+		let iu: *node = resolvealias(c, unwrapbang(inner));
+		if (iu != nil && iu.kind == nkind.N_TARRAY) {
+			return iu.lhs;
+		};
+		return inner;
+	};
+	return nil;
+};
+
 // exprtype — best-effort type-AST inference for an expression
 // node. Handles literals, identifiers, calls, casts, binary/unary
-// ops; returns nil for shapes we don't statically know (struct
-// field access into non-primitive types, etc).
+// ops, indexing; returns nil for shapes we don't statically know
+// (struct field access into non-primitive types, etc).
 // `hint`: optional declared-type AST passed by the caller (let
 // target, assign target). nil = "no hint, derive from self". Threaded
 // for use by A.6.1's STRUCTLIT/ARRLIT arms which can't self-type and
@@ -8361,6 +8387,11 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 		e.type_ = tinfofornode(c, tn): *void;
 		return tn;
 	};
+	if (k == nkind.N_INDEX) {
+		let tn: *node = indexresult(c, e);
+		e.type_ = tinfofornode(c, tn): *void;
+		return tn;
+	};
 	if (k == nkind.N_CAST) {
 		// `expr: T` — explicit cast; the type expr is e.rhs. Mirrors
 		// cstage cmd/wcc/check.c:737 `n->type = resolve_type(c, n->rhs)`.

selfhost/cmd/wcc/check.ww

@@ -1339,10 +1339,36 @@ fn unoptype(c: *checker, e: *node) *node = {
 	return nil;
 };
 
+// indexresult — derive the result tnode of an N_INDEX expression.
+// Mirrors cstage cmd/wcc/check.c:870-894. Slice/array → elem; str →
+// u8; `*[N]T` decays to T (pointer-to-array); `*[]T` does NOT decay
+// (yields []T via the generic *U → U fallback — Hare-faithful, a
+// pointer-to-slice is a 1D array of slices, not of T); generic *T → T.
+fn indexresult(c: *checker, e: *node) *node = {
+	let basetn: *node = exprtype(c, e.lhs, nil);
+	let _idx: *node = exprtype(c, e.rhs, nil);
+	let u: *node = resolvealias(c, unwrapbang(basetn));
+	if (u == nil) { return nil; };
+	if (u.kind == nkind.N_TSLICE) { return u.lhs; };
+	if (u.kind == nkind.N_TARRAY) { return u.lhs; };
+	if (u.kind == nkind.N_TNAME) {
+		if (streq(u.str, "str")) { return mktname(c, "u8"); };
+	};
+	if (u.kind == nkind.N_TPTR) {
+		let inner: *node = u.lhs;
+		let iu: *node = resolvealias(c, unwrapbang(inner));
+		if (iu != nil && iu.kind == nkind.N_TARRAY) {
+			return iu.lhs;
+		};
+		return inner;
+	};
+	return nil;
+};
+
 // exprtype — best-effort type-AST inference for an expression
 // node. Handles literals, identifiers, calls, casts, binary/unary
-// ops; returns nil for shapes we don't statically know (struct
-// field access into non-primitive types, etc).
+// ops, indexing; returns nil for shapes we don't statically know
+// (struct field access into non-primitive types, etc).
 // `hint`: optional declared-type AST passed by the caller (let
 // target, assign target). nil = "no hint, derive from self". Threaded
 // for use by A.6.1's STRUCTLIT/ARRLIT arms which can't self-type and
@@ -1429,6 +1455,11 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 		e.type_ = tinfofornode(c, tn): *void;
 		return tn;
 	};
+	if (k == nkind.N_INDEX) {
+		let tn: *node = indexresult(c, e);
+		e.type_ = tinfofornode(c, tn): *void;
+		return tn;
+	};
 	if (k == nkind.N_CAST) {
 		// `expr: T` — explicit cast; the type expr is e.rhs. Mirrors
 		// cstage cmd/wcc/check.c:737 `n->type = resolve_type(c, n->rhs)`.

selfhost/cmd/wwdump/main.combined.ww

@@ -8271,10 +8271,36 @@ fn unoptype(c: *checker, e: *node) *node = {
 	return nil;
 };
 
+// indexresult — derive the result tnode of an N_INDEX expression.
+// Mirrors cstage cmd/wcc/check.c:870-894. Slice/array → elem; str →
+// u8; `*[N]T` decays to T (pointer-to-array); `*[]T` does NOT decay
+// (yields []T via the generic *U → U fallback — Hare-faithful, a
+// pointer-to-slice is a 1D array of slices, not of T); generic *T → T.
+fn indexresult(c: *checker, e: *node) *node = {
+	let basetn: *node = exprtype(c, e.lhs, nil);
+	let _idx: *node = exprtype(c, e.rhs, nil);
+	let u: *node = resolvealias(c, unwrapbang(basetn));
+	if (u == nil) { return nil; };
+	if (u.kind == nkind.N_TSLICE) { return u.lhs; };
+	if (u.kind == nkind.N_TARRAY) { return u.lhs; };
+	if (u.kind == nkind.N_TNAME) {
+		if (streq(u.str, "str")) { return mktname(c, "u8"); };
+	};
+	if (u.kind == nkind.N_TPTR) {
+		let inner: *node = u.lhs;
+		let iu: *node = resolvealias(c, unwrapbang(inner));
+		if (iu != nil && iu.kind == nkind.N_TARRAY) {
+			return iu.lhs;
+		};
+		return inner;
+	};
+	return nil;
+};
+
 // exprtype — best-effort type-AST inference for an expression
 // node. Handles literals, identifiers, calls, casts, binary/unary
-// ops; returns nil for shapes we don't statically know (struct
-// field access into non-primitive types, etc).
+// ops, indexing; returns nil for shapes we don't statically know
+// (struct field access into non-primitive types, etc).
 // `hint`: optional declared-type AST passed by the caller (let
 // target, assign target). nil = "no hint, derive from self". Threaded
 // for use by A.6.1's STRUCTLIT/ARRLIT arms which can't self-type and
@@ -8361,6 +8387,11 @@ fn exprtype(c: *checker, e: *node, hint: *node) *node = {
 		e.type_ = tinfofornode(c, tn): *void;
 		return tn;
 	};
+	if (k == nkind.N_INDEX) {
+		let tn: *node = indexresult(c, e);
+		e.type_ = tinfofornode(c, tn): *void;
+		return tn;
+	};
 	if (k == nkind.N_CAST) {
 		// `expr: T` — explicit cast; the type expr is e.rhs. Mirrors
 		// cstage cmd/wcc/check.c:737 `n->type = resolve_type(c, n->rhs)`.