Fix #14762 FN knownConditionTrueFalse (default constructed container) (#8575)

Co-authored-by: chrchr-github <noreply@github.com>

Author: chrchr-github

lib/vf_settokenvalue.cpp

@@ -291,9 +291,9 @@ namespace ValueFlow
 
                         combineValueProperties(value1, value2, result);
 
-                        if (Token::simpleMatch(parent, "==") && result.intvalue)
+                        if (Token::simpleMatch(parent, "==") && result.intvalue && !(value1.intvalue == 0 && value2.intvalue == 0))
                             continue;
-                        if (Token::simpleMatch(parent, "!=") && !result.intvalue)
+                        if (Token::simpleMatch(parent, "!=") && !result.intvalue && !(value1.intvalue == 0 && value2.intvalue == 0))
                             continue;
 
                         setTokenValue(parent, std::move(result), settings);

test/testvalueflow.cpp

@@ -9249,6 +9249,19 @@ class TestValueFlow : public TestFixture {
                "}\n";
         ASSERT_EQUALS(false, testValueOfX(code, 5U, 1));
         ASSERT_EQUALS(false, testValueOfX(code, 5U, 0));
+
+        code = "bool f() {\n"
+               "    std::string s1, s2;\n"
+               "    bool x = (s1 == s2);\n"
+               "    return x;\n"
+               "}\n"
+               "bool g() {\n"
+               "    std::string s1, s2;\n"
+               "    bool x = (s1 != s2);\n"
+               "    return x;\n"
+               "}\n";
+        ASSERT_EQUALS(true, testValueOfXKnown(code, 4U, 1));
+        ASSERT_EQUALS(true, testValueOfXKnown(code, 9U, 0));
     }
 
     void valueFlowBailoutIncompleteVar() {