Fix FN stlcstrAssignment (#4764)

Author: chrchr-github

lib/checkstl.cpp

@@ -1976,8 +1976,9 @@ void CheckStl::string_c_str()
                     const Variable* var = tok->variable();
                     if (var->isPointer())
                         string_c_strError(tok);
-                } else if (printPerformance && Token::Match(tok->tokAt(2), "%var% . c_str|data ( ) ;")) {
-                    if (tok->variable() && tok->variable()->isStlStringType() && tok->tokAt(2)->variable() && tok->tokAt(2)->variable()->isStlStringType())
+                } else if (printPerformance && tok->tokAt(1)->astOperand2() && Token::Match(tok->tokAt(1)->astOperand2()->tokAt(-3), "%var% . c_str|data ( ) ;")) {
+                    const Token* vartok = tok->tokAt(1)->astOperand2()->tokAt(-3);
+                    if (tok->variable() && tok->variable()->isStlStringType() && vartok->variable() && vartok->variable()->isStlStringType())
                         string_c_strAssignment(tok);
                 }
             } else if (printPerformance && tok->function() && Token::Match(tok, "%name% ( !!)") && tok->str() != scope.className) {

test/teststl.cpp

@@ -4142,6 +4142,19 @@ class TestStl : public TestFixture {
                       "[test.cpp:12]: (performance) Passing the result of c_str() to a stream is slow and redundant.\n"
                       "[test.cpp:14]: (performance) Passing the result of c_str() to a stream is slow and redundant.\n",
                       errout.str());
+
+        check("struct S { std::string str; };\n"
+              "struct T { S s; };\n"
+              "struct U { T t[1]; };\n"
+              "void f(const T& t, const U& u, std::string& str) {\n"
+              "    if (str.empty())\n"
+              "        str = t.s.str.c_str();\n"
+              "    else\n"
+              "        str = u.t[0].s.str.c_str();\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:6]: (performance) Assigning the result of c_str() to a std::string is slow and redundant.\n"
+                      "[test.cpp:8]: (performance) Assigning the result of c_str() to a std::string is slow and redundant.\n",
+                      errout.str());
     }
 
     void uselessCalls() {