add known-bug test for unsound issue 100051

tests/ui/fn/implied-bounds-impl-header-projections.rs

@@ -0,0 +1,31 @@
+// check-pass
+// known-bug: #100051
+
+// Should fail. Implied bounds from projections in impl headers can create
+// improper lifetimes.  Variant of issue #98543 which was fixed by #99217.
+
+trait Trait {
+    type Type;
+}
+
+impl<T> Trait for T {
+    type Type = ();
+}
+
+trait Extend<'a, 'b> {
+    fn extend(self, s: &'a str) -> &'b str;
+}
+
+impl<'a, 'b> Extend<'a, 'b> for <&'b &'a () as Trait>::Type
+where
+    for<'what, 'ever> &'what &'ever (): Trait,
+{
+    fn extend(self, s: &'a str) -> &'b str {
+        s
+    }
+}
+
+fn main() {
+    let y = <() as Extend<'_, '_>>::extend((), &String::from("Hello World"));
+    println!("{}", y);
+}