rust/tests/ui/coherence/coherence-fn-inputs.rs

// Test that we consider these two types completely equal:
//
// * `for<'a, 'b> fn(&'a u32, &'b u32)`
// * `for<'c> fn(&'c u32, &'c u32)`
//
// For a long time we considered these to be distinct types. But in fact they
// are equivalent, if you work through the implications of subtyping -- this is
// because:
//
// * `'c` can be the intersection of `'a` and `'b` (and there is always an intersection)
// * `'a` and `'b` can both be equal to `'c`

trait Trait {}
impl Trait for for<'a, 'b> fn(&'a u32, &'b u32) {}
impl Trait for for<'c> fn(&'c u32, &'c u32) {
    //~^ ERROR conflicting implementations
    //
    // Note in particular that we do NOT get a future-compatibility warning
    // here. This is because the new leak-check proposed in [MCP 295] does not
    // "error" when these two types are equated.
    //
    // [MCP 295]: https://github.com/rust-lang/compiler-team/issues/295
}

fn main() {}