Issue a clearer warning about negative modulus operands

src/compiler/translator/IntermNode.cpp

@@ -1926,11 +1926,11 @@ const TConstantUnion *TIntermConstantUnion::FoldBinary(TOperator op,
                                 if (lhs < 0 || divisor < 0)
                                 {
                                     // ESSL 3.00.6 section 5.9: Results of modulus are undefined
-                                    // when
-                                    // either one of the operands is negative.
+                                    // when either one of the operands is negative.
                                     diagnostics->warning(line,
                                                          "Negative modulus operator operand "
-                                                         "encountered during constant folding",
+                                                         "encountered during constant folding. "
+                                                         "Results are undefined.",
                                                          "%");
                                     resultArray[i].setIConst(0);
                                 }

src/tests/compiler_tests/ConstantFolding_test.cpp

@@ -1645,3 +1645,19 @@ TEST_F(ConstantFoldingExpressionTest, LargeFloatToUint)
     ASSERT_TRUE(constantFoundInAST(3221225472u));
     ASSERT_FALSE(hasWarning());
 }
+
+// Test that folding % with a negative dividend generates a warning.
+TEST_F(ConstantFoldingExpressionTest, IntegerModulusNegativeDividend)
+{
+    const std::string &intString = "(-5) % 3";
+    evaluateInt(intString);
+    ASSERT_TRUE(hasWarning());
+}
+
+// Test that folding % with a negative divisor generates a warning.
+TEST_F(ConstantFoldingExpressionTest, IntegerModulusNegativeDivisor)
+{
+    const std::string &intString = "5 % (-3)";
+    evaluateInt(intString);
+    ASSERT_TRUE(hasWarning());
+}