Add a regression test for issue-51446

Author: JohnTitor

src/test/ui/traits/associated_type_bound/issue-51446.rs

@@ -0,0 +1,34 @@
+// Regression test for #51446.
+// check-pass
+
+trait Foo {
+    type Item;
+    fn get(&self) -> Self::Item;
+}
+
+fn blah<T, F>(x: T, f: F) -> B<T::Item, impl Fn(T::Item)>
+where
+    T: Foo,
+    F: Fn(T::Item),
+{
+    B { x: x.get(), f }
+}
+
+pub struct B<T, F>
+where
+    F: Fn(T),
+{
+    pub x: T,
+    pub f: F,
+}
+
+impl Foo for i32 {
+    type Item = i32;
+    fn get(&self) -> i32 {
+        *self
+    }
+}
+
+fn main() {
+    let _ = blah(0, |_| ());
+}