[clang] [clang][bytecode] Handle corner condition for sign negation (PR #176390)

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https://github.com/hax0kartik updated https://github.com/llvm/llvm-project/pull/176390

>From d293e4acd557a0361a5d02ce88d81e42283c694a Mon Sep 17 00:00:00 2001
From: hax0kartik <agarwala.kartik at gmail.com>
Date: Fri, 16 Jan 2026 19:37:54 +0530
Subject: [PATCH] [clang][bytecode] Handle corner condition for sign negation

RHS = -RHS works for most cases, however, the behaviour
when RHS is INTXX_MIN is undefined. In these particular
case(s), we should use INTXX_MAX instead.
---
 clang/lib/AST/ByteCode/Interp.h    | 4 +++-
 clang/test/AST/ByteCode/shifts.cpp | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/clang/lib/AST/ByteCode/Interp.h b/clang/lib/AST/ByteCode/Interp.h
index 7700b6bc5f2dd..0eae7848b4fe0 100644
--- a/clang/lib/AST/ByteCode/Interp.h
+++ b/clang/lib/AST/ByteCode/Interp.h
@@ -2888,7 +2888,9 @@ inline bool DoShift(InterpState &S, CodePtr OpPC, LT &LHS, RT &RHS,
     S.CCEDiag(Loc, diag::note_constexpr_negative_shift) << RHS.toAPSInt();
     if (!S.noteUndefinedBehavior())
       return false;
-    RHS = -RHS;
+
+    RHS = RHS.isMin() ? RT(APSInt::getMaxValue(RHS.bitWidth(), false)) : -RHS;
+
     return DoShift<LT, RT,
                    Dir == ShiftDir::Left ? ShiftDir::Right : ShiftDir::Left>(
         S, OpPC, LHS, RHS, Result);
diff --git a/clang/test/AST/ByteCode/shifts.cpp b/clang/test/AST/ByteCode/shifts.cpp
index f02aaf004cbd6..58118bbea7941 100644
--- a/clang/test/AST/ByteCode/shifts.cpp
+++ b/clang/test/AST/ByteCode/shifts.cpp
@@ -75,6 +75,8 @@ namespace shifts {
                           // ref-cxx17-warning {{shift count >= width of type}}
     (void)((long)c << __CHAR_BIT__);
 
+    c = 1 << (int)INT_MIN; // all-warning {{shift count is negative}}
+
     int i; // cxx17-warning {{uninitialized variable}} \
            // ref-cxx17-warning {{uninitialized variable}}
     i = 1 << (__INT_WIDTH__ - 2);