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// 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.
//
// check-pass
trait Trait<T> {}
fn foo<T>()
where
T: Trait<for<'b> fn(fn(&'b u32))>,
{
}
impl<'a> Trait<fn(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(fn(&'a u32))> }`
// - We instantiate `'a` existentially to get `(): Trait<fn(fn(&?a u32))>`
// - We unify `fn(fn(&?a u32))` with `for<'b> fn(fn(&'b u32))` -- this does a
// "bidirectional" subtyping check, so we wind up with:
// - `fn(fn(&?a u32)) <: for<'b> fn(fn(&'b u32))` :-
// - `fn(&!b u32) <: fn(&?a u32)`
// - `&?a u32 <: &!b u32`
// - `?a: !'b` -- solveable if `?a` is inferred to `'static`
// - `for<'b> fn(fn(&'b u32)) <: fn(fn(&?a u32))` :-
// - `fn(&?a u32) <: fn(&?b u32)`
// - `&?b u32 <: &?a u32`
// - `?b: ?a` -- solveable if `?b` is inferred to `'static`
// - So the subtyping check succeeds, somewhat surprisingly.
// This is because we can use `'static`.
foo::<()>();
}
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