[cxx2a] P1236R1: the validity of a left shift does not depend on the

value of the LHS operand.

llvm-svn: 364265

1 files changed, 8 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 99508b1cc35..94ee9e185ea 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -9645,9 +9645,11 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &LHS, ExprResult &RHS,
     return;
 
   // When left shifting an ICE which is signed, we can check for overflow which
-  // according to C++ has undefined behavior ([expr.shift] 5.8/2). Unsigned
-  // integers have defined behavior modulo one more than the maximum value
-  // representable in the result type, so never warn for those.
+  // according to C++ standards prior to C++2a has undefined behavior
+  // ([expr.shift] 5.8/2). Unsigned integers have defined behavior modulo one
+  // more than the maximum value representable in the result type, so never
+  // warn for those. (FIXME: Unsigned left-shift overflow in a constant
+  // expression is still probably a bug.)
   Expr::EvalResult LHSResult;
   if (LHS.get()->isValueDependent() ||
       LHSType->hasUnsignedIntegerRepresentation() ||
@@ -9656,8 +9658,9 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &LHS, ExprResult &RHS,
   llvm::APSInt Left = LHSResult.Val.getInt();
 
   // If LHS does not have a signed type and non-negative value
-  // then, the behavior is undefined. Warn about it.
-  if (Left.isNegative() && !S.getLangOpts().isSignedOverflowDefined()) {
+  // then, the behavior is undefined before C++2a. Warn about it.
+  if (Left.isNegative() && !S.getLangOpts().isSignedOverflowDefined() &&
+      !S.getLangOpts().CPlusPlus2a) {
     S.DiagRuntimeBehavior(Loc, LHS.get(),
                           S.PDiag(diag::warn_shift_lhs_negative)
                             << LHS.get()->getSourceRange());