add known-bug test for unsoundness issue #105787

Part of the resolution to #105107

tests/ui/specialization/issue-105787.rs

@@ -0,0 +1,39 @@
+// check-pass
+// known-bug: #105787
+trait ToUnit<'a> {
+    type Unit;
+}
+
+struct LocalTy;
+impl<'a> ToUnit<'a> for *const LocalTy {
+    type Unit = ();
+}
+
+impl<'a, T: Copy + ?Sized> ToUnit<'a> for *const T {
+    type Unit = ();
+}
+
+trait Overlap<T> {
+    type Assoc;
+}
+
+type Assoc<'a, T> = <*const T as ToUnit<'a>>::Unit;
+
+impl<T> Overlap<T> for T {
+    type Assoc = usize;
+}
+
+impl<T> Overlap<for<'a> fn(&'a (), Assoc<'a, T>)> for T
+where
+    for<'a> *const T: ToUnit<'a>,
+{
+    type Assoc = Box<usize>;
+}
+
+fn foo<T: Overlap<U>, U>(x: T::Assoc) -> T::Assoc {
+    x
+}
+
+fn main() {
+    foo::<for<'a> fn(&'a (), ()), for<'a> fn(&'a (), ())>(3usize);
+}