Add a regression test for rust-lang#85921

Author: JohnTitor

src/test/ui/generic-associated-types/issue-85921.rs

@@ -0,0 +1,19 @@
+// check-pass
+
+#![feature(generic_associated_types)]
+
+trait Trait {
+    type Assoc<'a>;
+
+    fn with_assoc(f: impl FnOnce(Self::Assoc<'_>));
+}
+
+impl Trait for () {
+    type Assoc<'a> = i32;
+
+    fn with_assoc(f: impl FnOnce(Self::Assoc<'_>)) {
+        f(5i32)
+    }
+}
+
+fn main() {}