Using trait bounds in default methods fails

[msullivan]

If a trait has parameters with trait bounds, and default methods use them, the compiler ICEs.

This depends on #7266 being fixed and is part of #2794.

[huonw]

@msullivan #7266 is closed, is this fixed?

[msullivan]

This is "half" fixed. It does not work in certain cross crate cases because the information about vtable resolutions for impls isn't stored properly in cross crate cases.