add known-bug test for unsound issue 25860

Author: whtahy

tests/ui/implied-bounds/implied-bounds-on-nested-references-plus-variance.rs

@@ -0,0 +1,16 @@
+// check-pass
+// known-bug: #25860
+
+// Should fail. The combination of variance and implied bounds for nested
+// references allows us to infer a longer lifetime than we can prove.
+
+static UNIT: &'static &'static () = &&();
+
+fn foo<'a, 'b, T>(_: &'a &'b (), v: &'b T) -> &'a T { v }
+
+fn bad<'a, T>(x: &'a T) -> &'static T {
+    let f: fn(_, &'a T) -> &'static T = foo;
+    f(UNIT, x)
+}
+
+fn main() {}