add known-bug test for unsound issue 104005

tests/ui/wf/wf-in-fn-type-implicit.rs

@@ -0,0 +1,37 @@
+// check-pass
+// known-bug: #104005
+
+// Should fail. Function type parameters with implicit type annotations are not
+// checked for well-formedness, which allows incorrect borrowing.
+
+// In contrast, user annotations are always checked for well-formedness, and the
+// commented code below is correctly rejected by the borrow checker.
+
+use std::fmt::Display;
+
+trait Displayable {
+    fn display(self) -> Box<dyn Display>;
+}
+
+impl<T: Display> Displayable for (T, Option<&'static T>) {
+    fn display(self) -> Box<dyn Display> {
+        Box::new(self.0)
+    }
+}
+
+fn extend_lt<T, U>(val: T) -> Box<dyn Display>
+where
+    (T, Option<U>): Displayable,
+{
+    Displayable::display((val, None))
+}
+
+fn main() {
+    // *incorrectly* compiles
+    let val = extend_lt(&String::from("blah blah blah"));
+    println!("{}", val);
+
+    // *correctly* fails to compile
+    // let val = extend_lt::<_, &_>(&String::from("blah blah blah"));
+    // println!("{}", val);
+}