Cannot assign generic type aliases that should be equivalent.

Cannot assign generic type aliases that should be equivalent.

Simple test case:

export type Example<TData>
    = (data: TData | null | undefined) => string | null;

function foo<T>()
{
    let x: Example<T> = undefined!;
    let y: Example<T | null | undefined> = undefined!;

    y = x;

    let x1: (data: T | null | undefined) => string | null = undefined!;
    let y1: (data: (T | null | undefined) | null | undefined) => string | null = undefined!;

    y1 = x1
}

Note that 'x' is not assignable to 'y' with strictNullChecks on. It results in the error:

Type 'Example<T>' is not assignable to type 'Example<T | null | undefined>'.
  Type 'T | null | undefined' is not assignable to type 'T'.
    'T' could be instantiated with an arbitrary type which could be unrelated to 'T | null | undefined'.
      Type 'undefined' is not assignable to type 'T'.
        'T' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.(2322)

This seems very strange though. Example is just a type alias, which shouldn't have an impact on assignability. If we simply expand the type alias, we get the x1/y1 case. In the expanded form the compiler allows this just fine.

Note: this was working in 3.3. It does not seem to work on 4.0. I'm not sure at what point it may have broken.