Fix debugging output for nameless py::arg_v annotations

include/pybind11/attr.h

@@ -303,19 +303,21 @@ template <> struct process_attribute<arg_v> : process_attribute_default<arg_v> {
 
         if (!a.value) {
 #if !defined(NDEBUG)
-            auto descr = "'" + std::string(a.name) + ": " + a.type + "'";
+            std::string descr("'");
+            if (a.name) descr += std::string(a.name) + ": ";
+            descr += a.type + "'";
             if (r->is_method) {
                 if (r->name)
                     descr += " in method '" + (std::string) str(r->scope) + "." + (std::string) r->name + "'";
                 else
                     descr += " in method of '" + (std::string) str(r->scope) + "'";
             } else if (r->name) {
-                descr += " in function named '" + (std::string) r->name + "'";
+                descr += " in function '" + (std::string) r->name + "'";
             }
-            pybind11_fail("arg(): could not convert default keyword argument "
+            pybind11_fail("arg(): could not convert default argument "
                           + descr + " into a Python object (type not registered yet?)");
 #else
-            pybind11_fail("arg(): could not convert default keyword argument "
+            pybind11_fail("arg(): could not convert default argument "
                           "into a Python object (type not registered yet?). "
                           "Compile in debug mode for more information.");
 #endif

tests/test_issues.cpp

@@ -74,7 +74,6 @@ namespace std {
 template <> struct hash<TplConstrClass> { size_t operator()(const TplConstrClass &t) const { return std::hash<std::string>()(t.str); } };
 }
 
-
 void init_issues(py::module &m) {
     py::module m2 = m.def_submodule("issues");
 
@@ -397,5 +396,5 @@ void init_issues(py::module &m) {
 #endif
 }
 
-// MSVC workaround: trying to use a lambda here crashes MSCV
+// MSVC workaround: trying to use a lambda here crashes MSVC
 test_initializer issues(&init_issues);

tests/test_methods_and_attributes.cpp

@@ -150,6 +150,9 @@ template <> struct type_caster<ArgAlwaysConverts> {
 };
 }}
 
+/// Issue/PR #648: bad arg default debugging output
+class NotRegistered {};
+
 test_initializer methods_and_attributes([](py::module &m) {
     py::class_<ExampleMandA>(m, "ExampleMandA")
         .def(py::init<>())
@@ -270,4 +273,18 @@ test_initializer methods_and_attributes([](py::module &m) {
     m.def("floats_preferred", [](double f) { return 0.5 * f; }, py::arg("f"));
     m.def("floats_only", [](double f) { return 0.5 * f; }, py::arg("f").noconvert());
 
+    /// Issue/PR #648: bad arg default debugging output
+#if !defined(NDEBUG)
+    m.attr("debug_enabled") = true;
+#else
+    m.attr("debug_enabled") = false;
+#endif
+    m.def("bad_arg_def_named", []{
+        auto m = py::module::import("pybind11_tests.issues");
+        m.def("should_fail", [](int, NotRegistered) {}, py::arg(), py::arg("a") = NotRegistered());
+    });
+    m.def("bad_arg_def_unnamed", []{
+        auto m = py::module::import("pybind11_tests.issues");
+        m.def("should_fail", [](int, NotRegistered) {}, py::arg(), py::arg() = NotRegistered());
+    });
 });

tests/test_methods_and_attributes.py

@@ -255,3 +255,27 @@ def test_noconvert_args(msg):
 
         Invoked with: 4
     """
+
+
+def test_bad_arg_default(msg):
+    from pybind11_tests import debug_enabled, bad_arg_def_named, bad_arg_def_unnamed
+
+    with pytest.raises(RuntimeError) as excinfo:
+        bad_arg_def_named()
+    assert msg(excinfo.value) == (
+        "arg(): could not convert default argument 'a: NotRegistered' in function 'should_fail' "
+        "into a Python object (type not registered yet?)"
+        if debug_enabled else
+        "arg(): could not convert default argument into a Python object (type not registered "
+        "yet?). Compile in debug mode for more information."
+    )
+
+    with pytest.raises(RuntimeError) as excinfo:
+        bad_arg_def_unnamed()
+    assert msg(excinfo.value) == (
+        "arg(): could not convert default argument 'NotRegistered' in function 'should_fail' "
+        "into a Python object (type not registered yet?)"
+        if debug_enabled else
+        "arg(): could not convert default argument into a Python object (type not registered "
+        "yet?). Compile in debug mode for more information."
+    )