[clang] [ExprConstant] Handle shift overflow the same way as other kinds of overflow (PR #99579)

[Richard Smith via cfe-commits]

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@@ -2849,19 +2849,23 @@ static bool handleIntIntBinOp(EvalInfo &Info, const BinaryOperator *E,
     if (SA != RHS) {
       Info.CCEDiag(E, diag::note_constexpr_large_shift)
         << RHS << E->getType() << LHS.getBitWidth();
+      if (!Info.noteUndefinedBehavior())
+        return false;
     } else if (LHS.isSigned() && !Info.getLangOpts().CPlusPlus20) {
       // C++11 [expr.shift]p2: A signed left shift must have a non-negative
       // operand, and must not overflow the corresponding unsigned type.
       // C++2a [expr.shift]p2: E1 << E2 is the unique value congruent to
       // E1 x 2^E2 module 2^N.
-      if (LHS.isNegative())
+      if (LHS.isNegative()) {
         Info.CCEDiag(E, diag::note_constexpr_lshift_of_negative) << LHS;
-      else if (LHS.countl_zero() < SA)
+        if (!Info.noteUndefinedBehavior())
+          return false;
+      } else if (LHS.countl_zero() < SA) {
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zygoloid wrote:

(In passing, not related to this PR:) it's weird that the comment mentions the C++2a rule but the code doesn't implement it. This final overflow check should not be performed in C++20 onwards.

https://github.com/llvm/llvm-project/pull/99579