AST: Add ExistentialLayout::isErrorExistential()

Author: slavapestov

include/swift/AST/ExistentialLayout.h

@@ -58,8 +58,13 @@ struct ExistentialLayout {
     return requiresClass && !containsNonObjCProtocol;
   }
 
+  // Does this existential contain the Error protocol?
   bool isExistentialWithError(ASTContext &ctx) const;
 
+  // Does this existential consist of an Error protocol only with no other
+  // constraints?
+  bool isErrorExistential() const;
+
   ArrayRef<ProtocolType *> getProtocols() const {
     if (singleProtocol)
       return ArrayRef<ProtocolType *>{&singleProtocol, 1};

lib/AST/Type.cpp

@@ -602,6 +602,13 @@ bool TypeBase::isAnyObject() {
   return canTy.getExistentialLayout().isAnyObject();
 }
 
+bool ExistentialLayout::isErrorExistential() const {
+  auto protocols = getProtocols();
+  return (!requiresClass &&
+          protocols.size() == 1 &&
+          protocols[0]->getDecl()->isSpecificProtocol(KnownProtocolKind::Error));
+}
+
 bool ExistentialLayout::isExistentialWithError(ASTContext &ctx) const {
   auto errorProto = ctx.getProtocol(KnownProtocolKind::Error);
   if (!errorProto) return false;