1 files changed, 37 insertions, 0 deletions
diff --git a/src/test/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-covariant.rs b/src/test/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-covariant.rs
new file mode 100644
index 000000000..f95496a6c
--- /dev/null
+++ b/src/test/ui/higher-rank-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<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::<()>();
+}