[WIP]

Author: fmease

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -781,6 +781,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             rustc_ast::BoundPolarity::Positive => ty::PredicatePolarity::Positive,
             rustc_ast::BoundPolarity::Negative(_) => ty::PredicatePolarity::Negative,
             rustc_ast::BoundPolarity::Maybe(_) => {
+                // FIXME(fmease): Validate *here* that `?` can only be applied to
+                //                `Sized` (or `X` where `X` is a default trait under
+                //                `more_maybe_bounds`).
+
                 // Validate associated type at least. We may want to reject these
                 // outright in the future...
                 for constraint in trait_segment.args().constraints {