38 lines
1.3 KiB
Rust
38 lines
1.3 KiB
Rust
// Test a case where variance and higher-ranked types interact in surprising ways.
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//
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// In particular, we test this pattern in trait solving, where it is not connected
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// to any part of the source code.
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//
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// check-pass
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trait Trait<T> {}
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fn foo<T>()
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where
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T: Trait<for<'b> fn(fn(&'b u32))>,
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{
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}
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impl<'a> Trait<fn(fn(&'a u32))> for () {}
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fn main() {
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// Here, proving that `(): Trait<for<'b> fn(&'b u32)>` uses the impl:
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//
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// - The impl provides the clause `forall<'a> { (): Trait<fn(fn(&'a u32))> }`
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// - We instantiate `'a` existentially to get `(): Trait<fn(fn(&?a u32))>`
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// - We unify `fn(fn(&?a u32))` with `for<'b> fn(fn(&'b u32))` -- this does a
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// "bidirectional" subtyping check, so we wind up with:
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// - `fn(fn(&?a u32)) <: for<'b> fn(fn(&'b u32))` :-
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// - `fn(&!b u32) <: fn(&?a u32)`
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// - `&?a u32 <: &!b u32`
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// - `?a: !'b` -- solveable if `?a` is inferred to `'static`
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// - `for<'b> fn(fn(&'b u32)) <: fn(fn(&?a u32))` :-
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// - `fn(&?a u32) <: fn(&?b u32)`
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// - `&?b u32 <: &?a u32`
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// - `?b: ?a` -- solveable if `?b` is inferred to `'static`
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// - So the subtyping check succeeds, somewhat surprisingly.
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// This is because we can use `'static`.
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foo::<()>();
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}
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