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

@@ -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();
 

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 {

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():