Improve make_tuple error message under debugging

When make_tuple fails (for example, when print() is called with a
non-convertible argument, as in #778) the error message a less helpful
than it could be:

    make_tuple(): unable to convert arguments of types 'std::tuple<type1, type2>' to Python object

There is no actual std::tuple involved (only a parameter pack and a
Python tuple), but it also doesn't immediately reveal which type caused
the problem.

This commit changes the debugging mode output to show just the
problematic type:

    make_tuple(): unable to convert argument of type 'type2' to Python object

include/pybind11/cast.h

@@ -1246,18 +1246,19 @@ NAMESPACE_END(detail)
 
 template <return_value_policy policy = return_value_policy::automatic_reference,
           typename... Args> tuple make_tuple(Args&&... args_) {
-    const size_t size = sizeof...(Args);
+    constexpr size_t size = sizeof...(Args);
     std::array<object, size> args {
         { reinterpret_steal<object>(detail::make_caster<Args>::cast(
             std::forward<Args>(args_), policy, nullptr))... }
     };
-    for (auto &arg_value : args) {
-        if (!arg_value) {
+    for (size_t i = 0; i < args.size(); i++) {
+        if (!args[i]) {
 #if defined(NDEBUG)
             throw cast_error("make_tuple(): unable to convert arguments to Python object (compile in debug mode for details)");
 #else
-            throw cast_error("make_tuple(): unable to convert arguments of types '" +
-                (std::string) type_id<std::tuple<Args...>>() + "' to Python object");
+            std::array<std::string, size> argtypes { type_id<Args>()... };
+            throw cast_error("make_tuple(): unable to convert argument of type '" +
+                argtypes[i] + "' to Python object");
 #endif
         }
     }

tests/test_python_types.cpp

@@ -186,6 +186,7 @@ struct MoveOutContainer {
     std::list<Value> move_list() const { return {{0}, {1}, {2}}; }
 };
 
+struct UnregisteredType { };
 
 test_initializer python_types([](py::module &m) {
     /* No constructor is explicitly defined below. An exception is raised when
@@ -235,6 +236,13 @@ test_initializer python_types([](py::module &m) {
         py::print("{a} + {b} = {c}"_s.format("a"_a="py::print", "b"_a="str.format", "c"_a="this"));
     });
 
+    m.def("test_print_failure", []() { py::print(42, UnregisteredType()); });
+#if !defined(NDEBUG)
+    m.attr("debug_enabled") = true;
+#else
+    m.attr("debug_enabled") = false;
+#endif
+
     m.def("test_str_format", []() {
         auto s1 = "{} + {} = {}"_s.format(1, 2, 3);
         auto s2 = "{a} + {b} = {c}"_s.format("a"_a=1, "b"_a=2, "c"_a=3);

tests/test_python_types.py

@@ -258,7 +258,7 @@ def test_module():
 
 
 def test_print(capture):
-    from pybind11_tests import test_print_function
+    from pybind11_tests import test_print_function, test_print_failure, debug_enabled
 
     with capture:
         test_print_function()
@@ -272,6 +272,14 @@ def test_print(capture):
     """
     assert capture.stderr == "this goes to stderr"
 
+    with pytest.raises(RuntimeError) as excinfo:
+        test_print_failure()
+    assert str(excinfo.value) == "make_tuple(): unable to convert " + (
+        "argument of type 'UnregisteredType' to Python object"
+        if debug_enabled else
+        "arguments to Python object (compile in debug mode for details)"
+    )
+
 
 def test_str_api():
     from pybind11_tests import test_str_format