Add more regression tests

[JohnTitor]

Closes#39618
Closes#51798
Closes#62894
Closes#63952
Closes#68653

r? @Centril

[Centril]

This looks suspicions... There's no default anywhere, yet logically, we have u64: Foo, hence where T: Foo is discharged, hence u64: Bar, hence T: Bar is discharged, and thus impl<T> Foo for T applies (and so we have overlap).

cc @jonas-schievink (I thought this had been fixed?)
cc @matthewjasper & @nikomatsakis

[Centril]

Oops; didn't mean to approve this. Everything except the specialization looks fine modulo nits.

[Centril]

@JohnTitor If you want to get this merged sooner we could also drop the specialization bit and revisit once the discussion has settled. :) If so, r=me with that test removed or a FIXME noting the question added to the specialization test, either at your option.

[JohnTitor]

If so, r=me with that test removed or a FIXME noting the question added to the specialization test, either at your option.

On my local, the test shows SIGSEGV with old nightly mentioned in that issue so I feel it's worth to add this test. I'll add a FIXME note.

[Centril]

@bors r+

[bors]

📌 Commit 2f39ec2 has been approved by Centril