AST: Relax an assertion

Author: slavapestov

lib/AST/Type.cpp

@@ -2920,7 +2920,8 @@ Optional<ProtocolConformanceRef>
 MakeAbstractConformanceForGenericType::operator()(CanType dependentType,
                                        Type conformingReplacementType,
                                        ProtocolType *conformedProtocol) const {
-  assert((conformingReplacementType->is<SubstitutableType>()
+  assert((conformingReplacementType->is<ErrorType>()
+          || conformingReplacementType->is<SubstitutableType>()
           || conformingReplacementType->is<DependentMemberType>())
          && "replacement requires looking up a concrete conformance");
   return ProtocolConformanceRef(conformedProtocol->getDecl());