[Clang] Diagnose undefined behavior in a constant expression while ev…

…aluating a compound assignment with remainder as operand

Currently we don't diagnose overflow in a constant expression for the case of
compound assignment with remainder as a operand.

In handleIntIntBinOp the arguments LHS and Result can be the same source but in
the check for remainder in this function we assigned to Result before checking
for overflow. In all the other operations the check is done before Result is
assigned to.

Differential Revision: https://reviews.llvm.org/D140455

clang/docs/ReleaseNotes.rst

@@ -447,6 +447,8 @@ Improvements to Clang's diagnostics
   ``#pragma clang __debug sloc_usage`` can also be used to request this report.
 - Clang no longer permits the keyword 'bool' in a concept declaration as a
   concepts-ts compatibility extension.
+- Clang now diagnoses overflow undefined behavior in a constant expression while
+  evaluating a compound assignment with remainder as operand.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/lib/AST/ExprConstant.cpp

@@ -2751,6 +2751,7 @@ static bool CheckedIntArithmetic(EvalInfo &Info, const Expr *E,
 static bool handleIntIntBinOp(EvalInfo &Info, const Expr *E, const APSInt &LHS,
                               BinaryOperatorKind Opcode, APSInt RHS,
                               APSInt &Result) {
+  bool HandleOverflowResult = true;
   switch (Opcode) {
   default:
     Info.FFDiag(E);
@@ -2773,14 +2774,14 @@ static bool handleIntIntBinOp(EvalInfo &Info, const Expr *E, const APSInt &LHS,
       Info.FFDiag(E, diag::note_expr_divide_by_zero);
       return false;
     }
-    Result = (Opcode == BO_Rem ? LHS % RHS : LHS / RHS);
     // Check for overflow case: INT_MIN / -1 or INT_MIN % -1. APSInt supports
     // this operation and gives the two's complement result.
     if (RHS.isNegative() && RHS.isAllOnes() && LHS.isSigned() &&
         LHS.isMinSignedValue())
-      return HandleOverflow(Info, E, -LHS.extend(LHS.getBitWidth() + 1),
-                            E->getType());
-    return true;
+      HandleOverflowResult = HandleOverflow(
+          Info, E, -LHS.extend(LHS.getBitWidth() + 1), E->getType());
+    Result = (Opcode == BO_Rem ? LHS % RHS : LHS / RHS);
+    return HandleOverflowResult;
   case BO_Shl: {
     if (Info.getLangOpts().OpenCL)
       // OpenCL 6.3j: shift values are effectively % word size of LHS.

clang/test/CXX/expr/expr.const/p2-0x.cpp

@@ -298,6 +298,15 @@ constexpr float negpi = -pi; // expect no error on unary operator
     static_assert(isinf(f6), "");
     static_assert(isinf(f9), "");
   }
+
+#if __cplusplus >= 201703L
+namespace CompoundAssignment {
+constexpr int rem() { // expected-error {{constexpr function never produces a constant expression}}
+    int x = ~__INT_MAX__;
+    return x%=-1; // cxx20-note {{value 2147483648 is outside the range of representable values of type 'int'}}
+}
+}
+#endif
 }
 
 // - a lambda-expression (5.1.2);