Don't warn about unreachable code in @available

rdar://137999979

Author: ahatanaka

clang/lib/Analysis/ReachableCode.cpp

@@ -308,6 +308,13 @@ static bool shouldTreatSuccessorsAsReachable(const CFGBlock *B,
     if (const auto *IS = dyn_cast<IfStmt>(Term);
         IS != nullptr && IS->isConstexpr())
       return true;
+
+    // Do not treat successors of `if (@available(domain:))` as unreachable.
+    if (const auto *IS = dyn_cast<IfStmt>(Term))
+      if (const auto *AC = dyn_cast<ObjCAvailabilityCheckExpr>(
+              IS->getCond()->IgnoreParenImpCasts());
+          AC && AC->hasDomainName())
+        return true;
   }
 
   const Stmt *Cond = B->getTerminatorCondition(/* stripParens */ false);

clang/test/Sema/feature-availability.c

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -triple arm64-apple-macosx15 -fblocks -ffeature-availability=feature1:1 -ffeature-availability=feature2:0 -ffeature-availability=feature3:on -fsyntax-only -verify %s
-// RUN: %clang_cc1 -triple arm64-apple-macosx15 -fblocks -fsyntax-only -verify -DUSE_DOMAIN %s
+// RUN: %clang_cc1 -triple arm64-apple-macosx15 -fblocks -ffeature-availability=feature1:1 -ffeature-availability=feature2:0 -ffeature-availability=feature3:on -fsyntax-only -Wunreachable-code -verify %s
+// RUN: %clang_cc1 -triple arm64-apple-macosx15 -fblocks -fsyntax-only -Wunreachable-code -verify -DUSE_DOMAIN %s
 
 #include <feature-availability.h>
 
@@ -33,6 +33,14 @@ __attribute__((availability(domain:feature4, AVAIL))) int g4;
 __attribute__((availability(domain:feature4, UNAVAIL))) int g5;
 #endif
 
+void test_unreachable_code(void) {
+  if (__builtin_available(domain:feature1)) {
+  } else {
+    // Warning -Wunreachable-code isn't emitted.
+    (void)2;
+  }
+}
+
 __attribute__((availability(domain:2, AVAIL))) void func5(void); // expected-error {{expected a domain name}}
 __attribute__((availability(domain, AVAIL))) void func4(void); // expected-error {{expected ':'}}
 __attribute__((availability(domain:, AVAIL))) void func0(void); // expected-error {{expected a domain name}}