Dart and analyzer work with old-style function type alias with generic type parameter in a strange way.

[iarkh]

Dart SDK version: 2.14.0-3.0.dev (dev) (Tue Apr 13 22:57:19 2021 -0700) on "windows_x64"

Please try to run the following code:

typedef void TEST<T extends void Function<TT>()>();
void testme<T extends void Function<TT>()>() { print(T); }

main() {
  TEST t = testme;

  dynamic t1 = t;
  dynamic t2 = testme;

  print(testme is TEST);
  print(t is TEST);
  print(t1 is TEST);
  print(t2 is TEST);

  t = t1;
  t = t2;
}

It's expected that a runtime type of t, t1 and t2 variables is the same (they points to the same Function object). However, this is not so - dart sample output is:

$> dart --enable-experiment=nonfunction-type-aliases,generic-metadata test.dart
false
true
true
false
Unhandled exception:
type '<T extends <X1>() => void>() => void' is not a subtype of type '() => void'
#0      main (file:///D:/DART/sdk/tests/co19/src/LanguageFeatures/Generic-functions-as-type-args/test.dart:16:3)
#1      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:283:19)
#2      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:184:12)

Analyzer silently passes with the example above.

See also Issue 45313, Issue 45322.

[eernstg]

That's not quite true. TEST<T> means void Function() for any T so the raw TEST also means void Function().

This implies that TEST t = testme; initializes t to the generic function instantiation of testme which has type void Function(), and when executed that instantiation of testme will print something that means void Function<TT>() (it probably prints something like '<TT>() => void').

So this means that t and t1 is the same function object, which is different from the result of evaluating testme with a context type that doesn't cause the generic function instantiation to take place (e.g., var t2 = testme; or dynamic t2 = testme;).

This explains why we get 'false', 'true', 'true', 'false', and also why t = t2 throws.

So it's all working as intended.