v3: keep refined C extern redeclarations in the signature tables (#27978)

Author: medvednikov

vlib/v3/types/checker.v

@@ -5189,10 +5189,19 @@ fn (mut tc TypeChecker) register_fn_signature(name string, ret_type Type, params
 
 // register_fn_name_alias updates register fn name alias state for types.
 fn (mut tc TypeChecker) register_fn_name_alias(name string, ret_type Type, params []Type, shared_params []bool, is_variadic bool, implicit_veb_ctx bool) {
-	if owner_module := tc.fn_type_modules[name] {
-		if owner_module != tc.cur_module && tc.cur_module !in ['', 'main', 'builtin']
-			&& !name.starts_with('${tc.cur_module}.') {
-			return
+	// C externs live in one global namespace and modules routinely redeclare
+	// them with refined types (builtin: `C.pthread_join(thread voidptr, ...)`,
+	// v3.workers: `C.pthread_join(thread C.pthread_t, ...)`). The
+	// first-registration guard below must not drop those refinements: cgen's
+	// module-blind parameter lookup would then see `voidptr` and take the
+	// address of struct-typed handles (gen-2 compilers panicked with
+	// `failed to join compiler worker 0`).
+	if !name.starts_with('C.') {
+		if owner_module := tc.fn_type_modules[name] {
+			if owner_module != tc.cur_module && tc.cur_module !in ['', 'main', 'builtin']
+				&& !name.starts_with('${tc.cur_module}.') {
+				return
+			}
 		}
 	}
 	tc.fn_ret_types[name] = ret_type
@@ -5204,8 +5213,15 @@ fn (mut tc TypeChecker) register_fn_name_alias(name string, ret_type Type, param
 		tc.fn_shared_params.delete(name)
 	}
 	if tc.cur_file.len > 0 {
-		tc.fn_type_files[name] = tc.cur_file
-		tc.fn_type_modules[name] = tc.cur_module
+		// A refined C-extern redeclaration updates the signature tables above
+		// but must not steal ownership: is_builtin_unsafe_c_call keys the
+		// unsafe-block requirement for the C.m*/C.s* memory externs off the
+		// builtin owner, so a module redeclaring C.malloc (json does) would
+		// otherwise suppress that diagnostic program-wide.
+		if !name.starts_with('C.') || name !in tc.fn_type_modules {
+			tc.fn_type_files[name] = tc.cur_file
+			tc.fn_type_modules[name] = tc.cur_module
+		}
 	}
 	tc.fn_variadic[name] = is_variadic
 	if implicit_veb_ctx || tc.fn_implicit_veb_ctx.len > 0 {