From d06d53694ab30b48865c8f502374243517a768f3 Mon Sep 17 00:00:00 2001
From: "Ralf W. Grosse-Kunstleve" <rwgk@google.com>
Date: Thu, 14 Sep 2023 09:47:34 -0700
Subject: [PATCH] Fix small bug introduced with PR #4735 (#4845)

* Bug fix: `result[0]` called if `result.empty()`

* Add unit test that fails without the fix.
---
 include/pybind11/pybind11.h        | 12 +++++++-----
 tests/test_kwargs_and_defaults.cpp |  2 ++
 tests/test_kwargs_and_defaults.py  |  4 ++++
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/include/pybind11/pybind11.h b/include/pybind11/pybind11.h
index 5e3c5c657..25ca508ab 100644
--- a/include/pybind11/pybind11.h
+++ b/include/pybind11/pybind11.h
@@ -57,11 +57,13 @@ inline std::string replace_newlines_and_squash(const char *text) {
     std::string result(text);
     bool previous_is_whitespace = false;
 
-    // Do not modify string representations
-    char first_char = result[0];
-    char last_char = result[result.size() - 1];
-    if (first_char == last_char && first_char == '\'') {
-        return result;
+    if (result.size() >= 2) {
+        // Do not modify string representations
+        char first_char = result[0];
+        char last_char = result[result.size() - 1];
+        if (first_char == last_char && first_char == '\'') {
+            return result;
+        }
     }
     result.clear();
 
diff --git a/tests/test_kwargs_and_defaults.cpp b/tests/test_kwargs_and_defaults.cpp
index 614c826da..9a12c42af 100644
--- a/tests/test_kwargs_and_defaults.cpp
+++ b/tests/test_kwargs_and_defaults.cpp
@@ -85,6 +85,8 @@ TEST_SUBMODULE(kwargs_and_defaults, m) {
         "kw_lb_func7",
         [](const std::string &) {},
         py::arg("str_arg") = "First line.\n  Second line.");
+    m.def(
+        "kw_lb_func8", [](const CustomRepr &) {}, py::arg("custom") = CustomRepr(""));
 
     // test_args_and_kwargs
     m.def("args_function", [](py::args args) -> py::tuple {
diff --git a/tests/test_kwargs_and_defaults.py b/tests/test_kwargs_and_defaults.py
index 9d9738de7..0e3bbdb76 100644
--- a/tests/test_kwargs_and_defaults.py
+++ b/tests/test_kwargs_and_defaults.py
@@ -55,6 +55,10 @@ def test_function_signatures(doc):
         doc(m.kw_lb_func7)
         == "kw_lb_func7(str_arg: str = 'First line.\\n  Second line.') -> None"
     )
+    assert (
+        doc(m.kw_lb_func8)
+        == "kw_lb_func8(custom: m.kwargs_and_defaults.CustomRepr = ) -> None"
+    )
 
 
 def test_named_arguments():