Fix -Wunsequenced false-positives in code controlled by a branch on

__builtin_constant_p.

If the operand of __builtin_constant_p is not constant and has
side-effects, then code controlled by a branch on it is unreachable and
we should not emit runtime behavior warnings in such code.

llvm-svn: 359844

Author: zygoloid

clang/lib/AST/ExprConstant.cpp

@@ -8269,11 +8269,14 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
 
   case Builtin::BI__builtin_constant_p: {
     const Expr *Arg = E->getArg(0);
-    if (EvaluateBuiltinConstantP(Info, Arg))
+    if (EvaluateBuiltinConstantP(Info, Arg)) {
       return Success(true, E);
-    else if (Info.InConstantContext)
+    } else if (Info.InConstantContext || Arg->HasSideEffects(Info.Ctx)) {
+      // Outside a constant context, eagerly evaluate to false in the presence
+      // of side-effects in order to avoid -Wunsequenced false-positives in
+      // a branch on __builtin_constant_p(expr).
       return Success(false, E);
-    else {
+    } else {
       Info.FFDiag(E, diag::note_invalid_subexpr_in_const_expr);
       return false;
     }

clang/test/Sema/warn-unsequenced.c

@@ -93,4 +93,6 @@ void test() {
   _Generic(++a, default: 0) + ++a; // ok
   sizeof(++a) + ++a; // ok
   _Alignof(++a) + ++a; // expected-warning {{extension}}
+
+  __builtin_constant_p(f(++a, 0)) ? f(f(++a, 0), f(++a, 0)) : 0;
 }