Fix `using` struct cycle check

src/check_type.cpp

@@ -27,6 +27,9 @@ void populate_using_array_index(CheckerContext *ctx, Ast *node, AstField *field,
 }
 
 void populate_using_entity_scope(CheckerContext *ctx, Ast *node, AstField *field, Type *t) {
+	if (t == nullptr) {
+		return;
+	}
 	t = base_type(type_deref(t));
 	gbString str = nullptr;
 	defer (gb_string_free(str));
@@ -40,7 +43,7 @@ void populate_using_entity_scope(CheckerContext *ctx, Ast *node, AstField *field
 			GB_ASSERT(f->kind == Entity_Variable);
 			String name = f->token.string;
 			Entity *e = scope_lookup_current(ctx->scope, name);
-			if (e != nullptr && name != "_" && e != f) {
+			if (e != nullptr && name != "_") {
 				// TODO(bill): Better type error
 				if (str != nullptr) {
 					error(e->token, "'%.*s' is already declared in '%s'", LIT(name), str);
@@ -59,7 +62,7 @@ void populate_using_entity_scope(CheckerContext *ctx, Ast *node, AstField *field
 			Entity *f = t->BitField.fields[i];
 			String name = f->token.string;
 			Entity *e = scope_lookup_current(ctx->scope, name);
-			if (e != nullptr && name != "_" && e != f) {
+			if ((e != nullptr && name != "_") && (e != f)) {
 				// TODO(bill): Better type error
 				if (str != nullptr) {
 					error(e->token, "'%.*s' is already declared in '%s'", LIT(name), str);