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When using `method_adaptor` (usually implicitly via a `cl.def("f",
&D::f)`) a compilation failure results if `f` is actually a method of
an inaccessible base class made public via `using`, such as:
class B { public: void f() {} };
class D : private B { public: using B::f; };
pybind deduces `&D::f` as a `B` member function pointer. Since the base
class is inaccessible, the cast in `method_adaptor` from a base class
member function pointer to derived class member function pointer isn't
valid, and a cast failure results.
This was sort of a regression in 2.2, which introduced `method_adaptor`
to do the expected thing when the base class *is* accessible. It wasn't
actually something that *worked* in 2.1, though: you wouldn't get a
compile-time failure, but the method was not callable (because the `D *`
couldn't be cast to a `B *` because of the access restriction). As a
result, you'd simply get a run-time failure if you ever tried to call
the function (this is what #855 fixed).
Thus the change in 2.2 essentially promoted a run-time failure to a
compile-time failure, so isn't really a regression.
This commit simply adds a `static_assert` with an accessible-base-class
check so that, rather than just a cryptic cast failure, you get
something more informative (along with a suggestion for a workaround).
The workaround is to use a lambda, e.g.:
class Derived : private Base {
public:
using Base::f;
};
// In binding code:
//cl.def("f", &Derived::f); // fails: &Derived::f is actually a base
// class member function pointer
cl.def("f", [](Derived &self) { return self.f(); });
This is a bit of a nuissance (especially if there are a bunch of
arguments to forward), but I don't really see another solution.
Fixes #1124
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