Document that type_caster requires default-constructible types
[skip ci]
diff --git a/docs/advanced/cast/custom.rst b/docs/advanced/cast/custom.rst
index c854e7f..e4f99ac 100644
--- a/docs/advanced/cast/custom.rst
+++ b/docs/advanced/cast/custom.rst
@@ -78,6 +78,12 @@
};
}} // namespace pybind11::detail
+.. note::
+
+ A ``type_caster<T>`` defined with ``PYBIND11_TYPE_CASTER(T, ...)`` requires
+ that ``T`` is default-constructible (``value`` is first default constructed
+ and then ``load()`` assigns to it).
+
.. warning::
When using custom type casters, it's important to declare them consistently