Add a FIXME comment

Author: JohnTitor

src/test/ui/higher-rank-trait-bounds/issue-59311.rs

@@ -2,6 +2,10 @@
 // rust-lang/rust/issues/71546#issuecomment-620638437
 // as they seem to have the same cause.
 
+// FIXME: It's not clear that this code ought to report
+// an error, but the regression test is here to ensure
+// that it does not ICE. See discussion on #74889 for details.
+
 pub trait T {
     fn t<F: Fn()>(&self, _: F) {}
 }

src/test/ui/higher-rank-trait-bounds/issue-59311.stderr

@@ -1,5 +1,5 @@
 error: higher-ranked subtype error
-  --> $DIR/issue-59311.rs:13:9
+  --> $DIR/issue-59311.rs:17:9
    |
 LL |     v.t(|| {});
    |         ^^^^^