Type inference doesn't work nicely with function types

[floitschG]

/cc @leafpetersen

I just ran into the following bug in the type inference:

// Wants a function that takes a function and returns a same-typed function.
void foo(T Function() Function<T>(T Function() f) parameterName) {}

// Using a function literal. -> the return type needs to be inferred.
foo(<T>(T Function() f) {
  if (true) return f;
  return () => f();
});

Clearly, the return type of the function literal is T Function().
Removing either return makes the whole thing work. But having both at the same time leads to an error.

[lrhn]

A runnable example: https://dartpad.dartlang.org/43bb3ebec177b1008f5c9308b53f5959
The error message that you get is the cryptic:

The argument type '<T₀>(() → T₀) → () → T' can't be assigned to the parameter type '<T>(() → T) → () → T'.

(But you do get bonus points for the non-ASCII chars, it's looking good!)

[leafpetersen]

Simplified example, that also changes the type variable name to check that it's not a dup of #30379:

main() {
  T Function() Function<T>() g =
      <S>() {
    S Function() f;
    if (f != null) return f;
    return () => f();
  };
}

[jmesserly]

possibly related bug: #29778 (comment)

[jmesserly]

Looking at this more, I'm pretty sure it's a dupe of #29778. I'll make sure these test cases are over there as well.