From 015da40e49262ea950980c01c980d377a3c1ec6b Mon Sep 17 00:00:00 2001
From: chrchr-github <78114321+chrchr-github@users.noreply.github.com>
Date: Tue, 15 Sep 2026 14:34:44 +0200
Subject: [PATCH 1/3] Update checkstl.cpp

---
 lib/checkstl.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/checkstl.cpp b/lib/checkstl.cpp
index 637f5e1e8b8..97ca1c853d1 100644
--- a/lib/checkstl.cpp
+++ b/lib/checkstl.cpp
@@ -1986,9 +1986,11 @@ static bool isc_strCall(const Token* tok, const Library::Container* container)
 
 static bool isc_strConcat(const Token* tok)
 {
-    if (!tok->isBinaryOp() || !Token::simpleMatch(tok, "+"))
+     if (!tok->isBinaryOp() || !Token::Match(tok, "+|+="))
         return false;
     for (const Token* op : { tok->astOperand1(), tok->astOperand2() }) { // NOLINT(readability-use-anyofallof)
+        if (tok->isAssignmentOp() && astIsLHS(op))
+            continue;
         const Token* sibling = op->astSibling();
         if (!sibling->valueType())
             continue;

From f2a4eb84c57aa8d1f10ac1e1a353e0b1def9fb66 Mon Sep 17 00:00:00 2001
From: chrchr-github <78114321+chrchr-github@users.noreply.github.com>
Date: Tue, 15 Sep 2026 14:35:56 +0200
Subject: [PATCH 2/3] Update teststl.cpp

---
 test/teststl.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/test/teststl.cpp b/test/teststl.cpp
index 197cc3fafa7..dad71b5aec9 100644
--- a/test/teststl.cpp
+++ b/test/teststl.cpp
@@ -5161,6 +5161,12 @@ class TestStl : public TestFixture {
               "}\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());
+
+        check("void f(std::string& a, const std::string& b) {\n"
+              "    a += b.c_str();\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:2:7]: (performance) Concatenating the result of c_str() and a std::string is slow and redundant. [stlcstrConcat]\n",
+                      errout_str());
     }
 
     void uselessCalls() {

From 5e51acf1a2cd13e7835f54945a4d291414e794ec Mon Sep 17 00:00:00 2001
From: chrchr-github <78114321+chrchr-github@users.noreply.github.com>
Date: Tue, 15 Sep 2026 14:37:46 +0200
Subject: [PATCH 3/3] Update checkstl.cpp

---
 lib/checkstl.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/checkstl.cpp b/lib/checkstl.cpp
index 97ca1c853d1..45c6ab59bf8 100644
--- a/lib/checkstl.cpp
+++ b/lib/checkstl.cpp
@@ -1986,7 +1986,7 @@ static bool isc_strCall(const Token* tok, const Library::Container* container)
 
 static bool isc_strConcat(const Token* tok)
 {
-     if (!tok->isBinaryOp() || !Token::Match(tok, "+|+="))
+    if (!tok->isBinaryOp() || !Token::Match(tok, "+|+="))
         return false;
     for (const Token* op : { tok->astOperand1(), tok->astOperand2() }) { // NOLINT(readability-use-anyofallof)
         if (tok->isAssignmentOp() && astIsLHS(op))