[BUG]: default value for Custom type casters not taken into count

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What version (or hash if on master) of pybind11 are you using?

2.10.4

Problem description

When creating a Custom type caster for a C++ defined type and then passing it as a function parameter, the default value is not taken into count.

Reproducible example code

I have this function:

void my_functions(int a, some_type b);

And I want to bind it using only the default argument for b:

 m.def("my_functions", &my_functions,  pb::arg("a"),  pb::arg("b") = function_that_returns_my_type()); // just default for b

some_type has an automatic custom type caster declared in PYBIND11_NAMESPACE (!)

This compiles, but when I call in python

my_functions(5)
I get:

TypeError: my_functions(): incompatible function arguments. The following argument types are supported:
    1. (a: int, b: some_type = python_object_custom_casted)

Is this a regression? Put the last known working version here if it is.

Not a regression

Start by reproducing the reported binding with a custom type caster and the call my_functions(5). Inspect pybind11's handling of custom type casters and default arguments, including the PYBIND11_NAMESPACE declaration. Done means the default value for b is accepted when the function is called with only a.