Add hidden __tag for union variables.

src/check_expr.c

@@ -422,6 +422,8 @@ isize check_fields(Checker *c, AstNode *node, AstNodeArray decls,
 			e->identifier = name;
 			if (str_eq(name_token.string, str_lit("_"))) {
 				fields[field_index++] = e;
+			} else if (str_eq(name_token.string, str_lit("__tag"))) {
+				error_node(name, "`__tag` is a reserved identifier for fields");
 			} else {
 				HashKey key = hash_string(name_token.string);
 				Entity **found = map_entity_get(&entity_map, key);
@@ -660,6 +662,10 @@ void check_union_type(Checker *c, Type *union_type, AstNode *node) {
 	union_type->Record.field_count         = field_count;
 	union_type->Record.are_offsets_set     = false;
 	union_type->Record.is_ordered          = true;
+	{
+		Entity *__tag = make_entity_field(c->allocator, NULL, make_token_ident(str_lit("__tag")), t_int, false, -1);
+		union_type->Record.union__tag = __tag;
+	}
 
 	for_array(i, ut->variants) {
 		AstNode *variant = ut->variants.e[i];

src/ir.c

@@ -2244,8 +2244,13 @@ irValue *ir_emit_struct_ep(irProcedure *proc, irValue *s, i32 index) {
 	} else if (is_type_union(t)) {
 		type_set_offsets(a, t);
 		GB_ASSERT(t->Record.field_count > 0);
-		GB_ASSERT(gb_is_between(index, 0, t->Record.field_count-1));
-		result_type = make_type_pointer(a, t->Record.fields[index]->type);
+		if (index == -1) {
+			index = t->Record.field_count+1;
+			result_type = t_int_ptr;
+		} else {
+			GB_ASSERT(gb_is_between(index, 0, t->Record.field_count-1));
+			result_type = make_type_pointer(a, t->Record.fields[index]->type);
+		}
 	} else if (is_type_tuple(t)) {
 		GB_ASSERT(t->Tuple.variable_count > 0);
 		GB_ASSERT(gb_is_between(index, 0, t->Tuple.variable_count-1));
@@ -2316,8 +2321,12 @@ irValue *ir_emit_struct_ev(irProcedure *proc, irValue *s, i32 index) {
 		result_type = t->Record.fields[index]->type;
 	} else if (is_type_union(t)) {
 		type_set_offsets(a, t);
-		GB_ASSERT(t->Record.field_count > 0);
-		GB_ASSERT(gb_is_between(index, 0, t->Record.field_count-1));
+		if (index == -1) {
+			index = t->Record.field_count+1;
+			result_type = t_int_ptr;
+		} else {
+			GB_ASSERT(gb_is_between(index, 0, t->Record.field_count-1));
+		}
 		result_type = t->Record.fields[index]->type;
 	} else if (is_type_tuple(t)) {
 		GB_ASSERT(t->Tuple.variable_count > 0);
@@ -2394,7 +2403,11 @@ irValue *ir_emit_deep_field_gep(irProcedure *proc, irValue *e, Selection sel) {
 			type = type->Record.fields[index]->type;
 			e = ir_emit_conv(proc, e, make_type_pointer(proc->module->allocator, type));
 		} else if (type->kind == Type_Record) {
-			type = type->Record.fields[index]->type;
+			if (index == -1) {
+				type = t_int;
+			} else {
+				type = type->Record.fields[index]->type;
+			}
 			e = ir_emit_struct_ep(proc, e, index);
 		} else if (type->kind == Type_Tuple) {
 			type = type->Tuple.variables[index]->type;

src/parser.c

@@ -1844,7 +1844,7 @@ AstNode *parse_operand(AstFile *f, bool lhs) {
 			return ast_proc_lit(f, type, NULL, tags, foreign_library, foreign_name, link_name);
 		}
 		if (tags != 0) {
-			syntax_error(token, "A procedure type cannot have tags");
+			// syntax_error(token, "A procedure type cannot have tags");
 		}
 
 		return type;

src/types.c

@@ -92,6 +92,8 @@ typedef struct TypeRecord {
 	// Entity_TypeName - union
 	Entity **variants;
 	i32      variant_count;
+	Entity * union__tag;
+
 
 	i64 *    offsets;
 	bool     are_offsets_set;
@@ -1415,6 +1417,15 @@ Selection lookup_field_with_selection(gbAllocator a, Type *type_, String field_n
 				sel.index.count = prev_count;
 			}
 		}
+		if (type->Record.kind == TypeRecord_Union) {
+			if (str_eq(field_name, str_lit("__tag"))) {
+				Entity *e = type->Record.union__tag;
+				GB_ASSERT(e != NULL);
+				selection_add_index(&sel, -1); // HACK(bill): Leaky memory
+				sel.entity = e;
+				return sel;
+			}
+		}
 	}
 
 	return sel;