Only generate a warning for Swift 3 and 4 mode.

Author: Brian King

lib/Sema/TypeCheckAttr.cpp

@@ -185,8 +185,7 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
     // 'final' only makes sense in the context of a class declaration.
     // Reject it on global functions, protocols, structs, enums, etc.
     if (!D->getDeclContext()->getAsClassOrClassExtensionContext()) {
-      if (TC.Context.isSwiftVersion3() &&
-          D->getDeclContext()->getAsProtocolExtensionContext())
+      if (D->getDeclContext()->getAsProtocolExtensionContext())
         TC.diagnose(attr->getLocation(), 
           diag::protocol_extension_cannot_be_final)
           .fixItRemove(attr->getRange());

test/Compatibility/final_protocol_extension.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -swift-version 4
+// RUN: %target-typecheck-verify-swift
 
 class Super {
   final var i: Int { get { return 5 } } // expected-note{{overridden declaration is here}}
@@ -40,8 +40,8 @@ protocol SomeProtocol {
 }
 
 extension SomeProtocol {
-  final var i: Int { return 1 } // expected-error {{only classes and class members may be marked with 'final'}}
-  final func protoExtensionFunc() {}  // expected-error {{only classes and class members may be marked with 'final'}} {{3-9=}}
+  final var i: Int { return 1 } // expected-warning {{functions in a protocol extension do not need to be marked with 'final'}}
+  final func protoExtensionFunc() {}  // expected-warning {{functions in a protocol extension do not need to be marked with 'final'}} {{3-9=}}
 }
 
 extension SomeStruct {