Sema: Protocol conformance checking fixes for variadic generics

Author: slavapestov

lib/AST/RequirementEnvironment.cpp

@@ -108,8 +108,12 @@ RequirementEnvironment::RequirementEnvironment(
       // invalid code.
       if (genericParam->getDepth() != 1)
         return Type();
-      auto substGenericParam = GenericTypeParamType::get(
+      Type substGenericParam = GenericTypeParamType::get(
           genericParam->isParameterPack(), depth, genericParam->getIndex(), ctx);
+      if (genericParam->isParameterPack()) {
+        substGenericParam = PackType::getSingletonPackExpansion(
+            substGenericParam);
+      }
       return substGenericParam;
     },
     [selfType, substConcreteType, conformance, conformanceDC, &ctx](

lib/Sema/TypeCheckProtocol.cpp

@@ -1118,6 +1118,11 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
         replacedInReq = syntheticEnv->mapTypeIntoContext(replacedInReq);
       }
 
+      if (auto packType = replacedInReq->getAs<PackType>()) {
+        if (auto unwrapped = packType->unwrapSingletonPackExpansion())
+          replacedInReq = unwrapped->getPatternType();
+      }
+
       cs->addConstraint(ConstraintKind::Bind, replacement.second, replacedInReq,
                         reqLocator);
     }