Type parameters as constraints dosen't work for methods

[hayeah]

PR #5949 implements F-bounded polymorphism. It doesn't seem to be working correctly for methods.

interface Table<T> {
  // For this method `T` is apparently a brand new type parameter.
  insert<T extends U, U>(obj: U): T;
  // insert(obj: any)
}

interface Foo {
  a: string;
  b: string;
}

let Foos: Table<Foo>;

// `foo1`'s type is inferred to be {a: 1}
let foo1 = Foos.insert({a: 1}) // => ;

// Should produce error
let foo2 = Foos.insert({c: 1});

[ahejlsberg]

This is by design. As you mention in your comment, your insert method introduces a new (and unused) type parameter called T that is constrained to extend U, but there is no constraint on U itself. I'm guessing you're trying to express that U must be a supertype of T. That's not currently possible, but there is some discussion of that issue in #6613.