Fix cppcheck-opensource#6492 valueflow: for loop condition not used for conditional analysis (cppcheck-opensource#8472)

To be merged after cppcheck-opensource#8471.
Ignoring the debug messages since the line numbers were flaky.

---------

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

Author: chrchr-github

lib/valueflow.cpp

@@ -4961,6 +4961,24 @@ static void valueFlowCondition(const ValuePtr<ConditionHandler>& handler,
     handler->afterCondition(tokenlist, symboldatabase, errorLogger, settings, skippedFunctions);
 }
 
+static const Token* getConditionVariable(const Token* tok)
+{
+    if (tok->str() == "!")
+        return tok->astOperand1();
+
+    if (const Token* parent = tok->astParent()) {
+        if (Token::Match(parent, "%oror%|&&|?") ||
+            Token::Match(parent->previous(), "if|while (") ||
+            (parent->str() == ";" && astIsLHS(tok) && Token::simpleMatch(parent->astParent(), ";"))) { // for loop condition
+            if (Token::simpleMatch(tok, "="))
+                return tok->astOperand1();
+            if (!Token::Match(tok, "%comp%|%assign%"))
+                return tok;
+        }
+    }
+    return nullptr;
+}
+
 struct SimpleConditionHandler : ConditionHandler {
     std::vector<Condition> parse(const Token* tok, const Settings& /*settings*/) const override {
 
@@ -4979,19 +4997,7 @@ struct SimpleConditionHandler : ConditionHandler {
         if (!conds.empty())
             return conds;
 
-        const Token* vartok = nullptr;
-
-        if (tok->str() == "!") {
-            vartok = tok->astOperand1();
-
-        } else if (tok->astParent() && (Token::Match(tok->astParent(), "%oror%|&&|?") ||
-                                        Token::Match(tok->astParent()->previous(), "if|while ("))) {
-            if (Token::simpleMatch(tok, "="))
-                vartok = tok->astOperand1();
-            else if (!Token::Match(tok, "%comp%|%assign%"))
-                vartok = tok;
-        }
-
+        const Token* vartok = getConditionVariable(tok);
         if (!vartok)
             return {};
         Condition cond;

test/testnullpointer.cpp

@@ -3723,6 +3723,13 @@ class TestNullPointer : public TestFixture {
         ASSERT_EQUALS("[test.cpp:3:9] -> [test.cpp:2:19]: (warning) Either the condition 'p' is redundant or there is possible null pointer dereference: p. [nullPointerRedundantCheck]\n"
                       "[test.cpp:8:9] -> [test.cpp:7:18]: (warning) Either the condition 's' is redundant or there is possible null pointer dereference: s. [nullPointerRedundantCheck]\n",
                       errout_str());
+
+        check("struct S { int a; };\n" // #6492
+              "void h(const S* s) {\n"
+              "    for (int i = s->a; s; ++i) {}\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:3:24] -> [test.cpp:3:18]: (warning) Either the condition 's' is redundant or there is possible null pointer dereference: s. [nullPointerRedundantCheck]\n",
+                      errout_str());
     }
 
     void nullpointerDeadCode() {

test/testsimplifytokens.cpp

@@ -2207,6 +2207,7 @@ class TestSimplifyTokens : public TestFixture {
                                 "for ( i = 0 ; ( i < sz ) && ( sz > 3 ) ; ++ i ) { }\n"
                                 "}";
         ASSERT_EQUALS(expected, tokenizeAndStringify(code, dinit(TokenizeAndStringifyOptions, $.cpp = false)));
+        ignore_errout();
     }
 
     void simplifyKnownVariables49() { // #3691