1 files changed, 29 insertions, 0 deletions
diff --git a/src/test/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-invariant.rs b/src/test/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-invariant.rs
new file mode 100644
index 000000000..9b9e4496a
--- /dev/null
+++ b/src/test/ui/higher-rank-trait-bounds/hrtb-exists-forall-trait-invariant.rs
@@ -0,0 +1,29 @@
+// Test an `exists<'a> { forall<'b> { 'a = 'b } }` pattern -- which should not compile!
+//
+// In particular, we test this pattern in trait solving, where it is not connected
+// to any part of the source code.
+
+use std::cell::Cell;
+
+trait Trait<T> {}
+
+fn foo<T>()
+where
+    T: Trait<for<'b> fn(Cell<&'b u32>)>,
+{
+}
+
+impl<'a> Trait<fn(Cell<&'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 requires (among other things) instantiating `'b` universally,
+    //     yielding `fn(&!b u32)`, in a fresh universe U1
+    //   - So we get `?a = !b` but the universe U0 assigned to `?a` cannot name `!b`.
+
+    foo::<()>(); //~ ERROR implementation of `Trait` is not general enough
+}