fix(types): remove newlines from docstring signature

include/pybind11/pybind11.h

@@ -52,6 +52,43 @@ PYBIND11_WARNING_DISABLE_MSVC(4127)
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
+inline std::string replaceNewlinesAndSquash(const char *text) {
+    std::string result;
+
+    // Replace newlines with spaces and squash consecutive spaces
+    while (*text != '\0') {
+        if (*text == '\n') {
+            result += ' ';
+            while (*(text + 1) == ' ') {
+                ++text;
+            }
+        } else {
+            result += *text;
+        }
+        ++text;
+    }
+
+    // Strip leading spaces
+    size_t firstNonSpace = result.find_first_not_of(" \t\n\r\f\v");
+
+    if (firstNonSpace != std::string::npos) {
+        result.erase(0, firstNonSpace);
+    } else {
+        result.clear();
+    }
+
+    // Strip trailing spaces
+    size_t lastNonSpace = result.find_last_not_of(" \t\n\r\f\v");
+
+    if (lastNonSpace != std::string::npos) {
+        result.erase(lastNonSpace + 1);
+    } else {
+        result.clear();
+    }
+
+    return result;
+}
+
 // Apply all the extensions translators from a list
 // Return true if one of the translators completed without raising an exception
 // itself. Return of false indicates that if there are other translators
@@ -424,7 +461,7 @@ class cpp_function : public function {
                 // Write default value if available.
                 if (!is_starred && arg_index < rec->args.size() && rec->args[arg_index].descr) {
                     signature += " = ";
-                    signature += rec->args[arg_index].descr;
+                    signature += detail::replaceNewlinesAndSquash(rec->args[arg_index].descr);
                 }
                 // Separator for positional-only arguments (placed after the
                 // argument, rather than before like *