Allow @tagName on enum literals

Closes #4214

src/ir.cpp

@@ -21389,9 +21389,9 @@ static IrInstruction *ir_analyze_union_tag(IrAnalyze *ira, IrInstruction *source
     if (type_is_invalid(value->value->type))
         return ira->codegen->invalid_instruction;
 
-    if (value->value->type->id == ZigTypeIdEnum) {
+    if (value->value->type->id == ZigTypeIdEnum ||
+        value->value->type->id == ZigTypeIdEnumLiteral)
         return value;
-    }
 
     if (value->value->type->id != ZigTypeIdUnion) {
         ir_add_error(ira, value,
@@ -22352,6 +22352,14 @@ static IrInstruction *ir_analyze_instruction_enum_tag_name(IrAnalyze *ira, IrIns
     if (type_is_invalid(target->value->type))
         return ira->codegen->invalid_instruction;
 
+    if (target->value->type->id == ZigTypeIdEnumLiteral) {
+        IrInstruction *result = ir_const(ira, &instruction->base, nullptr);
+        Buf *field_name = target->value->data.x_enum_literal;
+        ZigValue *array_val = create_const_str_lit(ira->codegen, field_name)->data.x_ptr.data.ref.pointee;
+        init_const_slice(ira->codegen, result->value, array_val, 0, buf_len(field_name), true);
+        return result;
+    }
+
     assert(target->value->type->id == ZigTypeIdEnum);
 
     if (instr_is_comptime(target)) {

test/stage1/behavior/enum.zig

@@ -1094,3 +1094,8 @@ test "enum with one member default to u0 tag type" {
     };
     comptime expect(@TagType(E0) == u0);
 }
+
+test "tagName on enum literals" {
+    expect(mem.eql(u8, @tagName(.FooBar), "FooBar"));
+    comptime expect(mem.eql(u8, @tagName(.FooBar), "FooBar"));
+}