[domain availability] Don't diagnose unguarded uses of enumerators

inside case label expressions

rdar://160337621

Author: ahatanaka

clang/lib/Sema/SemaFeatureAvailability.cpp

@@ -67,6 +67,11 @@ class DiagnoseUnguardedFeatureAvailability
 
   bool TraverseIfStmt(IfStmt *If);
 
+  // Ignore unguarded uses of enumerators inside case label expressions.
+  bool TraverseCaseStmt(CaseStmt *Case) {
+    return TraverseStmt(Case->getSubStmt());
+  }
+
   bool VisitDeclRefExpr(DeclRefExpr *DRE) {
     diagnoseDeclFeatureAvailability(DRE->getDecl(), DRE->getBeginLoc());
     return true;

clang/test/Sema/feature-availability.c

@@ -235,6 +235,17 @@ void test7(void) {
     enum E e;
     e = EA;
     e = EB; // expected-error {{cannot use 'EB' because feature 'feature2' is unavailable in this context}}
+
+    switch (e) {
+    case EA: {
+      if (__builtin_available(domain:feature2))
+        e = EB;
+      break;
+    }
+    case EB: // no diagnostic
+      e = EB; // expected-error {{cannot use 'EB' because feature 'feature2' is unavailable in this context}}
+      break;
+    }
   }
 
   if (__builtin_available(domain:feature2)) {