Make PYBIND11_OBJECT_CVT only convert if the type check fails

Currently types that are capable of conversion always call their convert
function when invoked with a `py::object` which is actually the correct
type.  This means that code such as `py::cast<py::list>(obj)` and
`py::list l(obj.attr("list"))` make copies, which was an oversight
rather than an intentional feature.

While at first glance there might be something behind having
`py::list(obj)` make a copy (as it would in Python), this would be
inconsistent when you dig a little deeper because `py::list(l)`
*doesn't* make a copy for an existing `py::list l`, and having an
inconsistency within C++ would be worse than a C++ <-> Python
inconsistency.

It is possible to get around the copying using a
`reinterpret_borrow<list>(o)` (and this commit fixes one place, in
`embed.h`, that does so), but that seems a misuse of
`reinterpret_borrow`, which is really supposed to be just for dealing
with raw python-returned values, not `py::object`-derived wrappers which
are supposed to be higher level.

This changes the constructor of such converting types (i.e. anything
using PYBIND11_OBJECT_CVT -- `str`, `bool_`, `int_`, `float_`, `tuple`,
`dict`, `list`, `set`, `memoryview`) to reference rather than copy when
the check function passes.

It also adds an `object &&` constructor that is slightly more efficient
by avoiding an inc_ref when the check function passes.

include/pybind11/embed.h

@@ -99,8 +99,7 @@ inline void initialize_interpreter(bool init_signal_handlers = true) {
     Py_InitializeEx(init_signal_handlers ? 1 : 0);
 
     // Make .py files in the working directory available by default
-    auto sys_path = reinterpret_borrow<list>(module::import("sys").attr("path"));
-    sys_path.append(".");
+    module::import("sys").attr("path").cast<list>().append(".");
 }
 
 /** \rst

include/pybind11/pytypes.h

@@ -731,7 +731,12 @@ NAMESPACE_END(detail)
 #define PYBIND11_OBJECT_CVT(Name, Parent, CheckFun, ConvertFun) \
     PYBIND11_OBJECT_COMMON(Name, Parent, CheckFun) \
     /* This is deliberately not 'explicit' to allow implicit conversion from object: */ \
-    Name(const object &o) : Parent(ConvertFun(o.ptr()), stolen_t{}) { if (!m_ptr) throw error_already_set(); }
+    Name(const object &o) \
+    : Parent(check_(o) ? o.inc_ref().ptr() : ConvertFun(o.ptr()), stolen_t{}) \
+    { if (!m_ptr) throw error_already_set(); } \
+    Name(object &&o) \
+    : Parent(check_(o) ? o.release().ptr() : ConvertFun(o.ptr()), stolen_t{}) \
+    { if (!m_ptr) throw error_already_set(); }
 
 #define PYBIND11_OBJECT(Name, Parent, CheckFun) \
     PYBIND11_OBJECT_COMMON(Name, Parent, CheckFun) \

tests/test_pytypes.py

@@ -174,9 +174,20 @@ def test_constructors():
     }
     inputs = {k.__name__: v for k, v in data.items()}
     expected = {k.__name__: k(v) for k, v in data.items()}
+
     assert m.converting_constructors(inputs) == expected
     assert m.cast_functions(inputs) == expected
 
+    # Converting constructors and cast functions should just reference rather
+    # than copy when no conversion is needed:
+    noconv1 = m.converting_constructors(expected)
+    for k in noconv1:
+        assert noconv1[k] is expected[k]
+
+    noconv2 = m.cast_functions(expected)
+    for k in noconv2:
+        assert noconv2[k] is expected[k]
+
 
 def test_implicit_casting():
     """Tests implicit casting when assigning or appending to dicts and lists."""