Merging upstream version 1.68.2+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 36 insertions, 0 deletions

diff --git a/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-contravariant.rs b/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-contravariant.rs
new file mode 100644
index 000000000..921061916
--- /dev/null
+++ b/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-contravariant.rs
@@ -0,0 +1,36 @@
+// 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
+}