Sema / Test: Allow fallback to CSDiag still using the new diagnostic

Author: sl

lib/Sema/CSDiag.cpp

@@ -754,8 +754,15 @@ void FailureDiagnosis::diagnoseUnviableLookupResults(
     case MemberLookupResult::UR_WritableKeyPathOnReadOnlyMember:
     case MemberLookupResult::UR_ReferenceWritableKeyPathOnMutatingMember:
     case MemberLookupResult::UR_KeyPathWithAnyObjectRootType:
-    case MemberLookupResult::UR_UnavailableInExistential:
       break;
+
+    case MemberLookupResult::UR_UnavailableInExistential: {
+      AllowProtocolTypeMemberFailure failure(baseExpr, CS, instanceTy, memberName,
+                                             CS.getConstraintLocator(E));
+      failure.diagnoseAsError();
+      return;
+    }
+
     case MemberLookupResult::UR_InstanceMemberOnType:
     case MemberLookupResult::UR_TypeMemberOnInstance: {
       auto locatorKind = isa<SubscriptExpr>(E)

test/decl/protocol/protocols.swift

@@ -267,7 +267,7 @@ struct WrongIsEqual : IsEqualComparable { // expected-error{{type 'WrongIsEqual'
 //===----------------------------------------------------------------------===//
 
 func existentialSequence(_ e: Sequence) { // expected-error{{has Self or associated type requirements}}
-  var x = e.makeIterator() // expected-error{{value of type 'Sequence' has no member 'makeIterator'}}
+  var x = e.makeIterator() // expected-error{{member 'makeIterator' cannot be used on value of protocol type 'Sequence'; use a generic constraint instead}}
   x.next()
   x.nonexistent()
 }