[vm] Incorrect FutureOr normalization on substitution

[askeksa]

On the VM,

import 'dart:async';

void printType<T>() => print(T);

void substitute<T>() => printType<FutureOr<T>?>();

main() {
  substitute<FutureOr<String>?>();
}

prints

FutureOr<FutureOr<String>?>?

whereas it should print

FutureOr<FutureOr<String>?>

The outer FutureOr should normalize to declared non-nullable, since the type argument is nullable.

The bug is also triggered by the test added in https://dart-review.googlesource.com/c/sdk/+/320821

[lrhn]

I'm not sure we guarantee normalization of toString, only of ==.
Is the resulting type unequal to a normalized type?

(It likely is unequal, otherwise it has to do runtime normalization anyway. Also to ensure having the same hash code.)

[askeksa]

They are indeed unequal.

import 'dart:async';

bool compare<T, U>() => T == U;

bool substitute<T>() => compare<FutureOr<T>?, FutureOr<T>>();

main() {
  print(substitute<FutureOr<String>?>());
}

prints false on the VM, but it should print true.