Correct swift_bridge duplicate attribute warning logic

[aaron.ballman]

Summary

The swift_bridge attribute warns when the attribute is applied multiple times to the same declaration. However, it warns about the arguments being different to the attribute without ever checking if the arguments actually are different. If the arguments are different, diagnose, otherwise silently accept the code. Either way, drop the duplicated attribute.

[compnerd]

Thanks!

[aaron.ballman]

Thanks for the review! I've committed in 81bc1365d8f85a125a6db6a5a1acff3ceddcbe9e

clang/lib/Sema/SemaDeclAttr.cpp

 
 static void handleSwiftBridge(Sema &S, Decl *D, const ParsedAttr &AL) {
   // Make sure that there is a string literal as the annotation's single
   // argument.
   StringRef BT;
   if (!S.checkStringLiteralArgumentAttr(AL, 0, BT))
     return;
 
-  // Don't duplicate annotations that are already set.
-  if (D->hasAttr<SwiftBridgeAttr>()) {
-    S.Diag(AL.getLoc(), diag::warn_duplicate_attribute) << AL;
+  // Warn about duplicate attributes if they have different arguments, but drop
+  // any duplicate attributes regardless.
+  if (const auto *Other = D->getAttr<SwiftBridgeAttr>()) {
+    if (Other->getSwiftType() != BT)
+      S.Diag(AL.getLoc(), diag::warn_duplicate_attribute) << AL;
     return;
   }
 
   D->addAttr(::new (S.Context) SwiftBridgeAttr(S.Context, AL, BT));
 }
 
 static bool isErrorParameter(Sema &S, QualType QT) {
   const auto *PT = QT->getAs<PointerType>();

clang/test/SemaObjC/attr-swift_bridge.m

 
 __attribute__((__swift_bridge__("ProtocolP")))
 @protocol P
 @end
 
 typedef NSArray *NSArrayAlias __attribute__((__swift_bridge__("ArrayAlias")));
 
 struct __attribute__((__swift_bridge__("StructT"))) T {};
+
+// Duplicate attributes with the same arguments are fine.
+struct __attribute__((swift_bridge("foo"), swift_bridge("foo"))) S;
+// Duplicate attributes with different arguments are not.
+struct __attribute__((swift_bridge("foo"), swift_bridge("bar"))) S; // expected-warning {{attribute 'swift_bridge' is already applied with different arguments}}