[feature availability] Don't disallow annotating ObjC interfaces and

protocols with features when there are unannotated forward declarations
of them

clang shouldn't reject this as forward delarations of ObjC interfaces
and protocols cannot be annotated with attributes.

rdar://154522966

Author: ahatanaka

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3958,11 +3958,27 @@ void Sema::copyFeatureAvailability(Decl *Dst, Decl *Src) {
   }
 }
 
+static bool isForwardDeclaration(Decl *Prev, Decl *D) {
+  if (Prev->getCanonicalDecl() != D->getCanonicalDecl())
+    return false;
+  if (auto *ID = dyn_cast<ObjCInterfaceDecl>(Prev))
+    return !ID->getPreviousDecl();
+  if (auto *PD = dyn_cast<ObjCProtocolDecl>(Prev))
+    return !PD->getPreviousDecl();
+  return false;
+}
+
 void Sema::copyFeatureAvailabilityCheck(Decl *Dst, NamedDecl *Src,
                                         bool Redeclaration) {
   if (Dst->isInvalidDecl())
     return;
 
+  // Don't check whether a new feature is being added if Src is a
+  // forward declaration of classes and protocols as they cannnot be
+  // annotated with attributes.
+  if (isForwardDeclaration(Src, Dst))
+    return;
+
   llvm::SmallDenseMap<StringRef, DomainAvailabilityAttr *> DstToAttr;
 
   for (auto *AA : Dst->specific_attrs<DomainAvailabilityAttr>())

clang/test/SemaObjC/feature-availability.m

@@ -34,6 +34,8 @@ -(struct S1)m2 __attribute__((availability(domain:feature1, AVAIL)));
 -(struct S1)m3 __attribute__((availability(domain:feature1, UNAVAIL))); // expected-error {{use of 'S1' requires feature 'feature1' to be available}}
 @end
 
+@class Base0;
+
 __attribute__((availability(domain:feature1, AVAIL))) // expected-note 2 {{is incompatible with __attribute__((availability(domain:feature1, 0)))}}
 @interface Base0 {
   struct S0 ivar0; // expected-error {{use of 'S0' requires feature 'feature1' to be unavailable}}
@@ -134,6 +136,8 @@ @interface Derived2(Cat1)
 @implementation Derived2(Cat1)
 @end
 
+@protocol P1;
+
 __attribute__((availability(domain:feature1, UNAVAIL)))
 @protocol P1
 @end