Only generate a warning in swift 3

Author: Brian King

include/swift/AST/DiagnosticsSema.def

@@ -1706,6 +1706,9 @@ ERROR(override_ownership_mismatch,none,
 ERROR(override_class_declaration_in_extension,none,
       "cannot override a non-dynamic class declaration from an extension.", 
       ())
+WARNING(override_class_declaration_in_extension_warning,none,
+      "cannot override a non-dynamic class declaration from an extension.",
+      ())
 ERROR(override_throws,none,
       "cannot override non-throwing %select{method|initializer}0 with "
       "throwing %select{method|initializer}0", (bool))

lib/Sema/TypeCheckDecl.cpp

@@ -6082,7 +6082,10 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       if (baseDecl->hasKnownSwiftImplementation() && 
           !base->isDynamic() &&
           override->getDeclContext()->isExtensionContext()) {
-        TC.diagnose(override, diag::override_class_declaration_in_extension);
+        // For compatibility, only generate a warning in Swift 3
+        TC.diagnose(override, (TC.Context.isSwiftVersion3()
+          ? diag::override_class_declaration_in_extension_warning
+          : diag::override_class_declaration_in_extension));
         TC.diagnose(base, diag::overridden_here);
       }
     }