Fix `type_union_tag_offset` when all members are zero sized

src/check_builtin.cpp

@@ -6032,12 +6032,13 @@ gb_internal bool check_builtin_procedure(CheckerContext *c, Operand *operand, As
 			
 			// NOTE(jakubtomsu): forces calculation of variant_block_size
 			type_size_of(u);
-			i64 tag_offset = u->Union.variant_block_size;
-			GB_ASSERT(tag_offset > 0);
+			// NOTE(Jeroen): A tag offset of zero is perfectly fine if all members of the union are empty structs.
+			//               What matters is that the tag size is > 0.
+			GB_ASSERT(u->Union.tag_size > 0);
 			
 			operand->mode = Addressing_Constant;
 			operand->type = t_untyped_integer;
-			operand->value = exact_value_i64(tag_offset);
+			operand->value = exact_value_i64(u->Union.variant_block_size);
 		}
 		break;
 

src/types.cpp

@@ -4108,10 +4108,10 @@ gb_internal i64 type_size_of_internal(Type *t, TypePath *path) {
 		}
 
 		i64 max = 0;
-		i64 field_size = 0;
 
 		for_array(i, t->Union.variants) {
 			Type *variant_type = t->Union.variants[i];
+
 			i64 size = type_size_of_internal(variant_type, path);
 			if (max < size) {
 				max = size;
@@ -4130,7 +4130,7 @@ gb_internal i64 type_size_of_internal(Type *t, TypePath *path) {
 			size = align_formula(max, tag_size);
 			// NOTE(bill): Calculate the padding between the common fields and the tag
 			t->Union.tag_size = cast(i16)tag_size;
-			t->Union.variant_block_size = size - field_size;
+			t->Union.variant_block_size = size;
 
 			size += tag_size;
 		}