pessimistically treat all function items as containing an opaque type

compiler/rustc_middle/src/ty/flags.rs

@@ -207,6 +207,11 @@ impl FlagComputation {
 
             &ty::FnDef(_, substs) => {
                 self.add_substs(substs);
+                // HACK(#98608, oli-obk): Function items with opaque types in their signature will
+                // end up not having the HAS_TY_OPAQUE flag set, causing `evaluate_obligation` to
+                // optimistically assume the function item matches any signature. See documentation
+                // on `HAS_FREE_LOCAL_NAMES` for details.
+                self.add_flags(TypeFlags::HAS_TY_OPAQUE);
             }
 
             &ty::FnPtr(fn_sig) => self.bound_computation(fn_sig, |computation, fn_sig| {