Instantiating an alias of a user-defined type was not properly caught #2798

releasenotes.md

@@ -101,6 +101,7 @@
 - Casting const bytes to vector with different element size was broken #2787
 - Unable to access fields of a const inline enum with an aggregate underlying type. #2802
 - Using an optional type as generic parameter was not properly caught #2799
+- Instantiating an alias of a user-defined type was not properly caught #2798
 
 ### Stdlib changes
 - Add `ThreadPool` join function to wait for all threads to finish in the pool without destroying the threads.

src/compiler/sema_types.c

@@ -442,15 +442,15 @@ INLINE bool sema_resolve_generic_type(SemaContext *context, TypeInfo *type_info)
 	{
 		RETURN_SEMA_ERROR(inner, "Parameterization required a concrete type name here.");
 	}
-	if (inner->resolve_status == RESOLVE_DONE)
+	switch (inner->resolve_status)
 	{
-		if (!type_is_user_defined(inner->type))
-		{
-			RETURN_SEMA_ERROR(inner, "A user defined type was expected here, not %s.", type_quoted_error_string(inner->type));
-		}
+		case RESOLVE_DONE:
+			RETURN_SEMA_ERROR(inner, "A user-defined generic type was expected here, but the type was %s.", type_quoted_error_string(inner->type));
+		case RESOLVE_RUNNING:
+			RETURN_SEMA_ERROR(inner, "Resolving the type %s entered a recursive loop.", type_quoted_error_string(inner->type));
+		default:
+			break;
 	}
-	ASSERT_SPAN(inner, inner->resolve_status == RESOLVE_NOT_DONE);
-
 	bool was_recursive = false;
 	if (compiler.generic_depth >= MAX_GENERIC_DEPTH)
 	{

test/test_suite/generic/generic_instantiate_alias.c3

@@ -0,0 +1,9 @@
+import std;
+fn void a()
+{
+	usz{} i = Z; // #error: A user-defined generic type was expected here, but the type was 'usz' (ulong)
+}
+fn int main()
+{
+	return 0;
+}