[check-type] fix faulty #no_nil variants error

- when checking the variants of a union type we will skip adding
  the variants if we have an unspecialized polymorphic, hence our
  union_type variants will be empty and have a count of 0

- so when checking if we violate the #no_nil error, if we are in the
  unspecialized polymorphic case and there exists at least one variant
  in the original variants then we should not raise this error

- test checks that we do not raise the error anymore, and that we still
  detect the #no_nil error in the described circumstances

1 files changed, 6 insertions, 0 deletions

diff --git a/src/check_type.cpp b/src/check_type.cpp
index bbfc25a12..a69dcdadc 100644
--- a/src/check_type.cpp
+++ b/src/check_type.cpp
@@ -729,6 +729,12 @@ gb_internal void check_union_type(CheckerContext *ctx, Type *union_type, Ast *no
 	union_type->Union.kind = ut->kind;
 	switch (ut->kind) {
 	case UnionType_no_nil:
+		if (union_type->Union.is_polymorphic && poly_operands == nullptr) {
+			GB_ASSERT(variants.count == 0);
+			if (ut->variants.count != 1) {
+				break;
+			}
+		}
 		if (variants.count < 2) {
 			error(ut->align, "A union with #no_nil must have at least 2 variants");
 		}