NNBD: Dart does not throw a compile error when declare a non-function typedef variable with unacceptable nullable type parameter

[iarkh]

Dart VM version: 2.9.0-18.0.dev (dev) (Mon Jun 22 16:07:30 2020 +0200) on "windows_x64"

The following source code example declares a A<num?> variable for typedef A<X extends num>

class C<X> {}
typedef A<X extends num> = C<X?>;

main() {
  A<num?>? aaa = throw "Here is me";
}

Num? is not assignable to num, so compile error should be thrown here. However, dart does not throw it.

Sample output is:

$> dart --enable-experiment=nonfunction-type-aliases,non-nullable test.dart
Unhandled exception:
Here is me
#0 main (file:///D:/DART/sdk/tests/co19/src/LanguageFeatures/Simple-bounds/static/type-aliases/test.dart:5:18)
#1 _startIsolate. (dart:isolate-patch/isolate_patch.dart:301:19)
#2 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:168:12)

[alexmarkov]

This looks like a front-end issue as there is a missing compile-time error.
/cc @johnniwinther

[johnniwinther]

Removing from NNBD since this requires nonfunction-type-aliases.

[leafpetersen]

@eernstg is this still an issue?

[eernstg]

Yes, the error message is still missing. The following variant avoids the throw (which helps avoid some potential confusion):

class C<X> {}
typedef A<X extends num> = C<X?>;

main() {
  A<num?>? aaa = null;
}

The following one does report the error, so this issue is associated with non-function type aliases:

class C<X> {}
typedef A<X extends num> = C<X?> Function();

main() {
  A<num?>? aaa = null;
}

[johnniwinther]

@eernstg Are you looking into this or should someone from the CFE take over?