From dc0db358abe19481e475e10c32149b53370f1a1c Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Thu, 30 May 2024 05:57:31 +0200 Subject: Merging upstream version 1.72.1+dfsg1. Signed-off-by: Daniel Baumann --- .../hrtb-exists-forall-trait-covariant.rs | 37 ++++++++++++++++++++++ 1 file changed, 37 insertions(+) create mode 100644 tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs (limited to 'tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs') diff --git a/tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs b/tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs new file mode 100644 index 000000000..f95496a6c --- /dev/null +++ b/tests/ui/higher-ranked/trait-bounds/hrtb-exists-forall-trait-covariant.rs @@ -0,0 +1,37 @@ +// 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 {} + +fn foo() +where + T: Trait fn(fn(&'b u32))>, +{ +} + +impl<'a> Trait for () {} + +fn main() { + // Here, proving that `(): Trait fn(&'b u32)>` uses the impl: + // + // - The impl provides the clause `forall<'a> { (): Trait }` + // - We instantiate `'a` existentially to get `(): Trait` + // - 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::<()>(); +} -- cgit v1.2.3