[Enhanced enums] Runtime error when printing enum with mixin

[sgrekhov]

The following code produces runtime error

mixin M1 on Enum {
  int mixedInMethod1(int v) => v;
}

enum E with M1 {
  e1,
  e2,
  e3;
}

main() {
  print(E.e1);
}

Output is

Unhandled exception:
Invalid argument (object): toString method returned 'null': Instance of 'E'
#0      _StringBase._interpolateSingle (dart:core-patch/string_patch.dart:834:7)
#1      print (dart:core/print.dart:9:26)
#2      main (file:///C:/Users/sgrek/Work/Google/co19/LanguageFeatures/Enhanced-Enum/semantics_A03_t05.dart:47:3)
#3      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:297:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:192:12)

Process finished with exit code 255

Tested on Dart SDK version: 2.17.0-69.0.dev (dev) (Fri Jan 28 00:44:27 2022 -0800) on "windows_x64"

[a-siva]

//cc @alexmarkov

[alexmarkov]

For some reason CFE didn't generate the body of toString method in E. The empty body just returns null, hence the exception is thrown.

Kernel:

  abstract class M1 extends core::Enum /*isMixinDeclaration*/  {
    method mixedInMethod1(core::int v) → core::int
      return v;
  }
  abstract class _E&_Enum&M1 extends core::_Enum implements tes::M1 /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
    const synthetic constructor •(core::int index, core::String _name) → tes::_E&_Enum&M1
      : super core::_Enum::•(index, _name)
      ;
    method mixedInMethod1(core::int v) → core::int
      return v;
  }
  class E extends tes::_E&_Enum&M1 /*isEnum*/  {
    static const field core::List<tes::E> values = #C10;
    static const field tes::E e1 = #C3;
    static const field tes::E e2 = #C6;
    static const field tes::E e3 = #C9;
    const constructor •(core::int index, core::String name) → tes::E
      : super tes::_E&_Enum&M1::•(index, name)
      ;
    method toString() → core::String
      ;
  }
  static method main() → dynamic {
    core::print(#C3);
  }

@chloestefantsova Could you please take a look?