1 files changed, 18 insertions, 0 deletions
diff --git a/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-fn.rs b/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-fn.rs
new file mode 100644
index 000000000..567802376
--- /dev/null
+++ b/tests/ui/higher-rank-trait-bounds/hrtb-exists-forall-fn.rs
@@ -0,0 +1,18 @@
+// 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.
+
+fn foo<'a>() -> fn(&'a u32) {
+    panic!()
+}
+
+fn main() {
+    // Here, proving that `fn(&'a u32) <: for<'b> fn(&'b u32)`:
+    //
+    // - instantiates `'b` with a placeholder `!b`,
+    // - requires that `&!b u32 <: &'a u32` and hence that `!b: 'a`,
+    // - but we can never know this.
+
+    let _: for<'b> fn(&'b u32) = foo(); //~ ERROR mismatched types
+}