Generic type defaults should not be allowed if they don’t satisfy type constraints

[nagisa]

As spotted in this internals thread:

use std::fmt::Debug;

struct Foo;

fn show<T: Debug = Foo>(t: T) {
    println!("{:?}", t)
}

compiles. Lack of early error/warning/lint here might be hiding bugs in API design if the default never actually kicks in inside the library.

[alexcrichton]

triage: I-nominated

[nikomatsakis]

triage: P-high

Seems like a straightforward oversight in the WF checker.

[nikomatsakis]

Hmm, thinking more about this, it's perhaps not as straightforward as I originally thought. It gets into problems much like higher-ranked regions etc.

For example:

fn foo<T,U = T>()
    where U: Clone
{
}

Here, we can't fully check the value of the default ahead of time, because it references another type parameter (T), and we don't know yet whether T: Clone holds or not. I think for maximal consistency with rust-lang/rfcs#1214, though, we would strive to check where possible, and not otherwise.

cc @rust-lang/lang

[nikomatsakis]

Nominating for discussion.

[nikomatsakis]

I'm also going to lower the triage. It seems unlikely that anyone would intentionally abuse this, and I think we'll be able to fix this without undue anger or breakage out there in the wild.

triage: P-medium