Fix #12900 FP useStlAlgorithm with assignment (#7247)

Author: chrchr-github

Diff for: lib/checkstl.cpp

@@ -2905,6 +2905,13 @@ void CheckStl::useStlAlgorithm()
         return isConstExpression(tok->linkAt(-1)->astOperand2(), mSettings->library);
     };
 
+    auto isAccumulation = [](const Token* tok, int varId) {
+        if (tok->str() != "=")
+            return true;
+        const Token* end = Token::findmatch(tok, "%varid%|;", varId); // TODO: lambdas?
+        return end && end->varId() != 0;
+    };
+
     for (const Scope *function : mTokenizer->getSymbolDatabase()->functionScopes) {
         for (const Token *tok = function->bodyStart; tok != function->bodyEnd; tok = tok->next()) {
             // Parse range-based for loop
@@ -2970,8 +2977,10 @@ void CheckStl::useStlAlgorithm()
                         algo = "std::any_of, std::all_of, std::none_of, or std::accumulate";
                     else if (Token::Match(assignTok, "= %var% <|<=|>=|> %var% ? %var% : %var%") && hasVarIds(assignTok->tokAt(6), loopVar->varId(), assignVarId))
                         algo = minmaxCompare(assignTok->tokAt(2), loopVar->varId(), assignVarId, assignTok->tokAt(5)->varId() == assignVarId);
-                    else
+                    else if (isAccumulation(assignTok, assignVarId))
                         algo = "std::accumulate";
+                    else
+                        continue;
                 }
                 useStlAlgorithmError(assignTok, algo);
                 continue;

Diff for: test/teststl.cpp

@@ -5430,6 +5430,14 @@ class TestStl : public TestFixture {
               "    return sum;\n"
               "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        check("int f(const std::vector<int>& v) {\n" // #12900
+              "    int x{};\n"
+              "    for (const auto i : v)\n"
+              "        x = dostuff(i);\n"
+              "    return x;\n"
+              "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     void loopAlgoContainerInsert() {