Generic function type as a type argument of the generic function type alias works incorrectly.

[iarkh]

Dart SDK version: 2.13.0-114.0.dev (dev) (Sat Mar 6 18:27:50 2021 -0800) on "windows_x64"

The following code example declares generic function typedef TEST_TYPEDEF and then try to assign TEST_TYPEDEF variable and check its type:

typedef TEST_TYPEDEF<TT extends T Function<T>(T)> = void Function<TTT extends TT>();
void  testme<TT extends T Function<T>(T)>() {}

main() {
  print(testme is TEST_TYPEDEF);
  TEST_TYPEDEF ttttt = testme;
  print(ttttt is TEST_TYPEDEF);
}

It's expected that if I succssffully assign testme to the variable of the type TEST_TYPEDEF, ttttt is TEST_TYPEDEF should report true. However, this is not so here: there are no errors and the example above twice prints false unexpectidly.

[eernstg]

Agreed, TEST_TYPEDEF means TEST_TYPEDEF<T Function<T>(T)> means void Function<TTT extends T Function<T>(T)>(), same as void Function<TT extends T Function<T>(T)>(), so we should get true, twice.

library /*isNonNullableByDefault*/;
import self as self;
import "dart:core" as core;

typedef TEST_TYPEDEF<invariant TT extends <T extends core::Object? = dynamic>(T%) → T% = <T extends core::Object? = dynamic>(T%) → T%> = <TTT extends TT = dynamic>() → void;
static method testme<TT extends <T extends core::Object? = dynamic>(T%) → T% = <T extends core::Object? = dynamic>(T%) → T%>() → void {}
static method main() → dynamic {
  core::print((#C1) is{ForNonNullableByDefault} <TTT extends <T extends core::Object? = dynamic>(T%) → T% = dynamic>() → void);
  <TTT extends <T extends core::Object? = dynamic>(T%) → T% = dynamic>() → void ttttt = #C1;
  core::print(ttttt is{ForNonNullableByDefault} <TTT extends <T extends core::Object? = dynamic>(T%) → T% = dynamic>() → void);
}
constants  {
  #C1 = tearoff self::testme
}

@johnniwinther, does this look like a front-end issue? I suspect it's a vm issue, but TTT extends TT = dynamic looks a bit surprising to me.

[a-siva]

//cc @crelier

[chloestefantsova]

@eernstg I think the suspicious TTT extends TT = dynamic part is because the default types (those the follow = in the declarations of type variables) don't really make sense for function types, only for classes and functions, because instantiate-to-bound doesn't apply to them. We see them there only because we use the same TypeParameter node kind for all of class, function, typedef, and function type type parameters. It likely doesn't affect the execution.

[eernstg]

OK, thanks!

[johnniwinther]

I don't think it's a front end issue.