Do not require to implement a trait method if a constraint is not satisfied

[IvanUkhov]

Consider the following example:

trait Foo {
    fn foo(&self);
    fn bar(&self) where Self: Bar;
}

trait Bar {
}

struct Baz;

impl Foo for Baz {
    fn foo(&self) {}
}

The example produces the following error:

<anon>:11:1: 13:2 error: not all trait items implemented, missing: `bar` [E0046]
<anon>:11 impl Foo for Baz {
<anon>:12     fn foo(&self) {}
<anon>:13 }

However, Baz doesn’t implement Bar, and presumably it shouldn’t be required to implement bar when implementing Foo.

Regards,
Ivan

[steveklabnik]

The Foo trait here has a bar method that's not implemented.

[apasel422]

This appears to be a duplicate of #20021.