"when" function syntax for traits

Currently, you can have this:

struct X<T> {
  x: T
}

impl<T> X<T> {
  fn a(self) -> T {
    self.x
  }
}

impl X<String> {
  fn b(&self) -> String { // doesn't exist in X<T> unless T: String
    self.x.clone()
  }
}

But if you have a trait, you can't have this:

trait X<T> {
  fn a(self) -> T;
}

trait X<String> {
  fn b(&self) -> String;
}

So I propose the following syntax, for methods that are only available if certain generic type constraints are met:

trait X<T> {
  fn a(self) -> T;

  fn b(&self) -> String when T = String; // or T: Traits

  fn c<U>(&self) -> &[U] when T = Vec<U>;
}

Then an impl would have to provide b or c if and only if T can be one of String or Vec, respectively.

Start with the generic impl and trait examples in the issue, then examine the proposed when constraints and the requirement that implementations provide methods conditionally. There are no files or tests named; done would require a settled language design and an accepted resolution for the proposed trait syntax.