Fix crash when proc return type is undeclared parapoly variable

Disallow the declaration of new parapoly variables in return types, when
the procedure's parapoly scope is itself. This happens if e.g.:
`foo :: proc() -> $T`.

Closes #3949, #4294, #4563

1 files changed, 8 insertions, 1 deletions

diff --git a/src/check_type.cpp b/src/check_type.cpp
index 13a6125ca..44108ccbe 100644
--- a/src/check_type.cpp
+++ b/src/check_type.cpp
@@ -2440,8 +2440,12 @@ gb_internal bool check_procedure_type(CheckerContext *ctx, Type *type, Ast *proc
 	bool success = true;
 	isize specialization_count = 0;
 	Type *params  = check_get_params(c, c->scope, pt->params, &variadic, &variadic_index, &success, &specialization_count, operands);
-	Type *results = check_get_results(c, c->scope, pt->results);
 
+	bool no_poly_return = c->disallow_polymorphic_return_types;
+	c->disallow_polymorphic_return_types = c->scope == c->polymorphic_scope;
+	// NOTE(zen3ger): if the parapoly scope is the current proc's scope, then the return types shall not declare new poly vars
+	Type *results = check_get_results(c, c->scope, pt->results);
+	c->disallow_polymorphic_return_types = no_poly_return;
 
 	isize param_count = 0;
 	isize result_count = 0;
@@ -3383,6 +3387,9 @@ gb_internal bool check_type_internal(CheckerContext *ctx, Ast *e, Type **type, T
 		}
 		Type *t = alloc_type_generic(ctx->scope, 0, token.string, specific);
 		if (ctx->allow_polymorphic_types) {
+			if (ctx->disallow_polymorphic_return_types) {
+				error(ident, "Undeclared polymorphic parameter '%.*s' in return type", LIT(token.string));
+			}
 			Scope *ps = ctx->polymorphic_scope;
 			Scope *s = ctx->scope;
 			Scope *entity_scope = s;