Regression in interaction between lifetime parameter and defaulted type parameter

[bsteinb]

The mpi crate contains (approximately) the following code that used to work before nightly-2018-03-02:

use std::cell::Cell;
use std::marker::PhantomData;

pub struct Request<'a, S: Scope<'a> = StaticScope> {
    scope: S,
    phantom: PhantomData<Cell<&'a ()>>,
}

pub unsafe trait Scope<'a> {
    /// Registers a request with the scope.
    fn register(&self);

    /// Unregisters a request from the scope.
    unsafe fn unregister(&self);
}

pub struct StaticScope;

unsafe impl Scope<'static> for StaticScope {
    fn register(&self) {}

    unsafe fn unregister(&self) {}
}

The newest nightly compiler that successfully compiles this is:

rustc 1.26.0-nightly (0ff9872b2 2018-02-28)
binary: rustc
commit-hash: 0ff9872b2280009f094af0df3dcdc542cc46a5fd
commit-date: 2018-02-28
host: x86_64-apple-darwin
release: 1.26.0-nightly
LLVM version: 6.0

It no longer compiles on:

rustc 1.26.0-nightly (3eeb5a665 2018-03-01)
binary: rustc
commit-hash: 3eeb5a665e313c5b281820099e04d4c6c8188b46
commit-date: 2018-03-01
host: x86_64-apple-darwin
release: 1.26.0-nightly
LLVM version: 6.0

Was this not supposed to ever work or is this a regression?

[bsteinb]

Oops, it seems I completely forgot to paste the resulting error message:

error[E0308]: mismatched types
 --> src/lib.rs:4:1
  |
4 | / pub struct Request<'a, S: Scope<'a> = StaticScope> {
5 | |     scope: S,
6 | |     phantom: PhantomData<Cell<&'a ()>>,
7 | | }
  | |_^ lifetime mismatch
  |
  = note: expected type `Scope<'a>`
             found type `Scope<'static>`
note: the lifetime 'a as defined on the struct at 4:1...
 --> src/lib.rs:4:1
  |
4 | pub struct Request<'a, S: Scope<'a> = StaticScope> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: ...does not necessarily outlive the static lifetime

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `rsmpi-request-lifetime-issue`.

To learn more, run the command again with --verbose.

Sorry about that. As mentioned in the original issue the problem can be worked around by not having the type parameter S default to StaticScope and instead always specify both parameters explicitly, e.g. Request<'static, StaticScope>

[nikomatsakis]

I'm guessing this is caused by #46785 but I'm not sure yet

[nikomatsakis]

cc @leodasvacas -- I think was something of an unexpected interaction, do you agree? I think we are probably checking that the default value (StaticScope) meets the bounds Scope<'a>, but we shouldn't be, because it talks about 'a.

[nikomatsakis]

triage: P-high

[leoyvens]

Yes it's a bug in that PR, we should not be doing this check in predicates with lifetimes, I'll work on a fix.