Generic type inference doesn't work when extending nullable type

[HosseinYousefi]

This works fine:

class Foo {}

class Bar<T extends Foo?> {
  final T foo;
  Bar(this.foo);
  
  static Bar<T> bar<T extends Foo>(T foo) {
    return Bar(foo); // <-- No need to write Bar<T>(foo)
  }
}

but if T extends Foo? instead, it doesn't:

class Foo {}

class Bar<T extends Foo?> {
  final T foo;
  Bar(this.foo);
  
  static Bar<T> bar<T extends Foo?>(T foo) {
    return Bar(foo);
    //         ^^^
    // The argument type 'T' can't be assigned to the parameter type 'Never'.
  }
}

Is this expected behavior?

[dart-github-bot]

Summary: The issue reports that generic type inference fails when a type parameter T extends a nullable type (Foo?) in a static method. The code compiles when T extends a non-nullable type (Foo), but throws an error when T extends a nullable type.

[eernstg]

Related to #56998?

[johnniwinther]

I don't think it's related. @chloestefantsova can you take a look?

[lrhn]

Looks like a bug, or a very undeseriable property of inference.

The return Bar(foo); should infer return Bar<T>(foo); from the context type (return type of function is Bar<T>), and then foo is a valid argument.
If you write return Bar<T>(foo); it works. For some reason inference fails to do the obvious thing with a context type of Bar<T>, and infers Bar<Never>(foo) instead.