Infer all inference variables via InferCx

The previous algorithm was correct for the example given in its
documentation, but when the TAIT was declared as a free item
instead of an associated item, the generic parameters were the
wrong ones.

compiler/rustc_middle/src/ty/mod.rs

@@ -856,7 +856,7 @@ impl<'tcx> InstantiatedPredicates<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable, TypeFoldable)]
 pub struct OpaqueTypeKey<'tcx> {
     pub def_id: DefId,
     pub substs: SubstsRef<'tcx>,