mirror of
https://github.com/rust-lang/rust.git
synced 2024-11-01 06:51:58 +00:00
37 lines
1.3 KiB
Rust
37 lines
1.3 KiB
Rust
// Test a case where variance and higher-ranked types interact in surprising ways.
|
|
//
|
|
// In particular, we test this pattern in trait solving, where it is not connected
|
|
// to any part of the source code.
|
|
|
|
trait Trait<T> {}
|
|
|
|
fn foo<T>()
|
|
where
|
|
T: Trait<for<'b> fn(&'b u32)>,
|
|
{
|
|
}
|
|
|
|
impl<'a> Trait<fn(&'a u32)> for () {}
|
|
|
|
fn main() {
|
|
// Here, proving that `(): Trait<for<'b> fn(&'b u32)>` uses the impl:
|
|
//
|
|
// - The impl provides the clause `forall<'a> { (): Trait<fn(&'a u32)> }`
|
|
// - We instantiate `'a` existentially to get `(): Trait<fn(&?a u32)>`
|
|
// - We unify `fn(&?a u32)` with `for<'b> fn(&'b u32)` -- this does a
|
|
// "bidirectional" subtyping check, so we wind up with:
|
|
// - `fn(&?a u32) <: for<'b> fn(&'b u32)` :-
|
|
// - `&'!b u32 <: &?a u32`
|
|
// - `!'b: ?a` -- solveable if `?a` is inferred to `'empty`
|
|
// - `for<'b> fn(&'b u32) <: fn(&?a u32)` :-
|
|
// - `&?a u32 u32 <: &?b u32`
|
|
// - `?a: ?b` -- solveable if `?b` is also inferred to `'empty`
|
|
// - So the subtyping check succeeds, somewhat surprisingly.
|
|
// This is because we can use `'empty`.
|
|
//
|
|
// NB. *However*, the reinstated leak-check gives an error here.
|
|
|
|
foo::<()>();
|
|
//~^ ERROR implementation of `Trait` is not general enough
|
|
}
|