Emit error in Swift 4 mode for final protocol extensions

lib/Sema/TypeCheckAttr.cpp

@@ -186,7 +186,8 @@ 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 (D->getDeclContext()->getAsProtocolExtensionContext())
+      if (TC.Context.isSwiftVersion3() && 
+          D->getDeclContext()->getAsProtocolExtensionContext())
         TC.diagnose(attr->getLocation(), 
           diag::protocol_extension_cannot_be_final)
           .fixItRemove(attr->getRange());

test/Compatibility/attr_final_protocol_extension.swift

@@ -0,0 +1,8 @@
+// RUN: %target-typecheck-verify-swift -swift-version 3
+// Generate a swift 3 compatible warning if final is specified in an extension
+
+protocol P {}
+
+extension P {
+  final func inExtension() {} // expected-warning{{functions in a protocol extension do not need to be marked with 'final'}} {{3-9=}}
+}

test/attr/attr_final_protocol_extension.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -swift-version 4
+
+protocol P {}
+
+extension P {
+  final func inExtension() {} // expected-error {{only classes and class members may be marked with 'final'}} {{3-9=}}
+}