Fix 12116: FP negativeContainerIndex with redundant assignment (regression) (#5602)

Author: pfultz2

lib/valueflow.cpp

@@ -5643,12 +5643,15 @@ static void valueFlowForwardConst(Token* start,
                     if (v.tokvalue->varId() != var->declarationId())
                         continue;
                     for (ValueFlow::Value value : values) {
+                        if (!v.isKnown() && value.isImpossible())
+                            continue;
                         if (v.intvalue != 0) {
                             if (!value.isIntValue())
                                 continue;
                             value.intvalue += v.intvalue;
                         }
-                        value.valueKind = v.valueKind;
+                        if (!value.isImpossible())
+                            value.valueKind = v.valueKind;
                         value.bound = v.bound;
                         value.errorPath.insert(value.errorPath.end(), v.errorPath.cbegin(), v.errorPath.cend());
                         setTokenValue(tok, std::move(value), settings);

test/testcondition.cpp

@@ -4722,6 +4722,16 @@ class TestCondition : public TestFixture {
               "           (it != end) && *it;\n"
               "}\n");
         ASSERT_EQUALS("", errout.str());
+
+        // #12116
+        check("void f(int n) {\n"
+              "    for (int i = 0; i < N; ++i) {\n"
+              "        if (i < n) {}\n"
+              "        else if (i > n) {}\n"
+              "        else {}\n"
+              "    }\n"
+              "}\n");
+        ASSERT_EQUALS("", errout.str());
     }
 
     void alwaysTrueInfer() {

test/testvalueflow.cpp

@@ -8174,6 +8174,18 @@ class TestValueFlow : public TestFixture {
                "    }\n"
                "}\n";
         ASSERT_EQUALS(true, testValueOfXImpossible(code, 4U, -1));
+
+        code = "void f(int N, int z) {\n"
+               "  std::vector<int> a(N);\n"
+               "  int m = -1;\n"
+               "  m = 0;\n"
+               "  for (int k = 0; k < N; k++) {\n"
+               "    int x = m + k;\n"
+               "    if (z == a[x]) {}\n"
+               "  }\n"
+               "}\n";
+        ASSERT_EQUALS(true, testValueOfXImpossible(code, 7U, -1));
+        ASSERT_EQUALS(false, testValueOfXKnown(code, 7U, -1));
     }
 
     void valueFlowImpossibleUnknownConstant()