Fix generalizer bug

Author: compiler-errors

compiler/rustc_infer/src/infer/relate/generalize.rs

@@ -108,16 +108,11 @@ impl<'tcx> InferCtxt<'tcx> {
         } else {
             // HACK: make sure that we `a_is_expected` continues to be
             // correct when relating the generalized type with the source.
-            if target_is_expected == relation.a_is_expected() {
-                relation.relate_with_variance(
-                    ambient_variance,
-                    ty::VarianceDiagInfo::default(),
-                    generalized_ty,
-                    source_ty,
-                )?;
+            if target_is_expected {
+                relation.relate(generalized_ty, source_ty)?;
             } else {
                 relation.relate_with_variance(
-                    ambient_variance.xform(ty::Contravariant),
+                    ty::Contravariant,
                     ty::VarianceDiagInfo::default(),
                     source_ty,
                     generalized_ty,

compiler/rustc_infer/src/infer/relate/type_relating.rs

@@ -125,18 +125,12 @@ impl<'tcx> TypeRelation<'tcx> for TypeRelating<'_, '_, 'tcx> {
             }
 
             (&ty::Infer(TyVar(a_vid)), _) => {
-                infcx.instantiate_ty_var(
-                    self,
-                    self.a_is_expected,
-                    a_vid,
-                    self.ambient_variance,
-                    b,
-                )?;
+                infcx.instantiate_ty_var(self, true, a_vid, self.ambient_variance, b)?;
             }
             (_, &ty::Infer(TyVar(b_vid))) => {
                 infcx.instantiate_ty_var(
                     self,
-                    !self.a_is_expected,
+                    false,
                     b_vid,
                     self.ambient_variance.xform(ty::Contravariant),
                     a,