Fix #11125 FP unreadVariable with loop over array (cppcheck-opensource#8357)

Author: chrchr-github

lib/fwdanalysis.cpp

@@ -410,6 +410,16 @@ FwdAnalysis::Result FwdAnalysis::checkRecursive(const Token *expr, const Token *
     return Result(Result::Type::NONE);
 }
 
+static bool isSimpleIndexExpression(const Token* tok)
+{
+    const Token* idx = tok->astOperand2();
+    if (!idx)
+        return false;
+    if (idx->isIncDecOp())
+        idx = idx->astOperand1();
+    return idx->variable() && idx->variable()->scope() == tok->scope();
+}
+
 std::set<nonneg int> FwdAnalysis::getExprVarIds(const Token* expr, bool* localOut, bool* unknownVarIdOut) const
 {
     // all variable ids in expr.
@@ -418,7 +428,7 @@ std::set<nonneg int> FwdAnalysis::getExprVarIds(const Token* expr, bool* localOu
     bool unknownVarId = false;
     visitAstNodes(expr,
                   [&](const Token *tok) {
-        if (tok->str() == "[" && mWhat == What::UnusedValue)
+        if (tok->str() == "[" && mWhat == What::UnusedValue && isSimpleIndexExpression(tok))
             return ChildrenToVisit::op1;
         if (tok->varId() == 0 && tok->isName() && tok->strAt(-1) != ".") {
             // unknown variable

test/testunusedvar.cpp

@@ -5813,6 +5813,22 @@ class TestUnusedVar : public TestFixture {
                               "    {}\n"
                               "}");
         ASSERT_EQUALS("", errout_str());
+
+        functionVariableUsage("void f(const int* b, int x) {\n" // #11125
+                              "    int a[6];\n"
+                              "    int i = 0;\n"
+                              "    for (int j = 0; j < 6; ++j) {\n"
+                              "        if (b[j] != 0) {\n"
+                              "            a[i] = j;\n"
+                              "            ++i;\n"
+                              "        }\n"
+                              "    }\n"
+                              "    if (i > 1) {\n"
+                              "        a[i] = a[0];\n"
+                              "        (void)a[x];\n"
+                              "    }\n"
+                              "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     void localvarForEach() { // #4155 - foreach