Fix #14600 FN stlcstrConstructor with offset (regression)

Author: web-flow

lib/checkstl.cpp

@@ -2003,7 +2003,20 @@ static bool isc_strConstructor(const Token* tok)
 {
     if (!tok->valueType() || !Token::Match(tok, "%var% (|{"))
         return false;
-    return isc_strCall(tok->tokAt(1)->astOperand2(), tok->valueType()->container);
+
+    const Token* callTok = tok->tokAt(1)->astOperand2();
+    while (Token::simpleMatch(callTok, "+") && callTok->isBinaryOp()) {
+        if (callTok->astOperand1()->str() == "+") {
+            callTok = callTok->astOperand1();
+            continue;
+        }
+        if (callTok->astOperand1()->str() == "(")
+            callTok = callTok->astOperand1();
+        else if (tok->astOperand2()->str() == "(")
+            callTok = callTok->astOperand2();
+        break;
+    }
+    return isc_strCall(callTok, tok->valueType()->container);
 }
 
 namespace {

test/teststl.cpp

@@ -4719,6 +4719,13 @@ class TestStl : public TestFixture {
               "    auto a = + s.c_str();\n"
               "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        check("std::string f(const std::string& a) {\n" // #14600
+              "    std::string b(a.c_str() + 1 + 2);\n"
+              "    return b;\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:2:17]: (performance) Constructing a std::string from the result of c_str() is slow and redundant. [stlcstrConstructor]\n",
+                      errout_str());
     }
 
     void uselessCalls() {