Restrict the ability to specify "protocol P : class, AnyObject" to Swift >= 5.

Author: DougGregor

lib/Sema/TypeCheckDecl.cpp

@@ -477,8 +477,9 @@ void TypeChecker::checkInheritanceClause(Decl *decl,
     if (knownType != inheritedTypes.end()) {
       // If the duplicated type is 'AnyObject', check whether the first was
       // written as 'class'. Downgrade the error to a warning in such cases
-      // for backward compatibility.
-      if (inheritedTy->isAnyObject() &&
+      // for backward compatibility with Swift <= 4.
+      if (!Context.LangOpts.isSwiftVersionAtLeast(5) &&
+          inheritedTy->isAnyObject() &&
           (isa<ProtocolDecl>(decl) || isa<AbstractTypeParamDecl>(decl)) &&
           Lexer::getTokenAtLocation(Context.SourceMgr,
                                     knownType->second.second.Start)

test/Compatibility/anyobject_class.swift

@@ -1,5 +1,6 @@
 // RUN: %target-typecheck-verify-swift -swift-version 3
 // RUN: %target-typecheck-verify-swift -swift-version 4
+// RUN: not %target-swift-frontend -tyepcheck -swift-version 5
 
 protocol P : class, AnyObject { } // expected-warning{{redundant inheritance from 'AnyObject' and Swift 3 'class' keyword}}{{14-21=}}
 // expected-warning@-1{{redundant layout constraint 'Self' : 'AnyObject'}}