Fix cppcheck-opensource#8192: FN knownConditionTrueFalse for always-false for loop condition

When a for loop's condition is impossible given the initial value (e.g.
`for (int i = 0; i > 10; i++)`), cppcheck was not emitting a
knownConditionTrueFalse warning.

Fix by populating memory1, memory2 and memoryAfter with the init state
when the condition is immediately false (and no error occured). We can
then set the value for the condition token.

Signed-off-by: Francois Berder <fberder@outlook.fr>

Author: francois-berder

lib/valueflow.cpp

@@ -5180,8 +5180,21 @@ static bool valueFlowForLoop2(const Token *tok,
     if (error)
         return false;
     execute(secondExpression, programMemory, &result, &error, settings);
-    if (result == 0) // 2nd expression is false => no looping
+    if (result == 0) {
+        if (!error) { // 2nd expression is definitely false => no looping
+            ProgramMemory startMemory(programMemory);
+            ProgramMemory endMemory(programMemory);
+
+            if (memory1)
+                memory1->swap(startMemory);
+            if (memory2)
+                memory2->swap(endMemory);
+            if (memoryAfter)
+                memoryAfter->swap(programMemory);
+            return true;
+        }
         return false;
+    }
     if (error) {
         // If a variable is reassigned in second expression, return false
         bool reassign = false;
@@ -5390,23 +5403,35 @@ static void valueFlowForLoop(const TokenList &tokenlist, const SymbolDatabase& s
         } else {
             ProgramMemory mem1, mem2, memAfter;
             if (valueFlowForLoop2(tok, &mem1, &mem2, &memAfter, settings)) {
-                for (const auto& p : mem1) {
-                    if (!p.second.isIntValue())
-                        continue;
-                    if (p.second.isImpossible())
-                        continue;
-                    if (p.first.tok->varId() == 0)
-                        continue;
-                    valueFlowForLoopSimplify(bodyStart, p.first.tok, false, p.second.intvalue, tokenlist, errorLogger, settings);
-                }
-                for (const auto& p : mem2) {
-                    if (!p.second.isIntValue())
-                        continue;
-                    if (p.second.isImpossible())
-                        continue;
-                    if (p.first.tok->varId() == 0)
-                        continue;
-                    valueFlowForLoopSimplify(bodyStart, p.first.tok, false, p.second.intvalue, tokenlist, errorLogger, settings);
+                if (mem1 == memAfter) { // #8192 check if loop never runs
+                    Token* condTok = getCondTok(tok);
+                    if (condTok && !condTok->hasKnownIntValue()) {
+                        ValueFlow::Value v(0);
+                        v.setKnown();
+                        v.errorPath.emplace_back(tok, "for loop condition is always false");
+                        ValueFlow::setTokenValue(condTok, std::move(v), settings);
+                    }
+                } else {
+                    for (const auto& p : mem1) {
+                        if (!p.second.isIntValue())
+                            continue;
+                        if (p.second.isImpossible())
+                            continue;
+                        if (p.first.tok->varId() == 0)
+                            continue;
+
+                        valueFlowForLoopSimplify(bodyStart, p.first.tok, false, p.second.intvalue, tokenlist, errorLogger, settings);
+                    }
+                    for (const auto& p : mem2) {
+                        if (!p.second.isIntValue())
+                            continue;
+                        if (p.second.isImpossible())
+                            continue;
+                        if (p.first.tok->varId() == 0)
+                            continue;
+
+                        valueFlowForLoopSimplify(bodyStart, p.first.tok, false, p.second.intvalue, tokenlist, errorLogger, settings);
+                    }
                 }
                 for (const auto& p : memAfter) {
                     if (!p.second.isIntValue())

test/testcondition.cpp

@@ -5581,7 +5581,7 @@ class TestCondition : public TestFixture {
         check("void f() {\n" // #8192
               "    for (int i = 0; i > 10; ++i) {}\n"
               "}\n");
-        TODO_ASSERT_EQUALS("[test.cpp:2]: (style) Condition 'i>10' is always false\n", "", errout_str());
+        ASSERT_EQUALS("[test.cpp:2:23]: (style) Condition 'i>10' is always false [knownConditionTrueFalse]\n", errout_str());
 
         check("void f() {\n"
               "    for (int i = 1000; i < 20; ++i) {}\n"