[libc] Rename cpp::function to cpp::function_ref (#236108)
The class does not own the callable, so the old name (suggesting
std::function) was misleading. While renaming it, I also made it behave
more like the C++26 std::function_ref:
- remove the default and nullptr constructors and operator bool. A
std::function_ref is never empty, and nobody was using these anyway.
- add a constructor for function pointers, which stores the pointer
itself. Previously, `function<void()> f = &foo;` would store the address
of the temporary pointer object and dangle as soon as the statement
ended.
- delete assignment from anything other than function_ref and function
pointers, to prevent `f = [&] {...};`, which dangles for the same
reason. Like std::function_ref, this also rejects lvalue callables
(which would not dangle). These can still be assigned by wrapping them
explicitly: `f = function_ref<void()>(callable);`.
I also fixed a bug where binding a non-void callable to a function_ref
with a void return type failed to compile, even though the constructor
explicitly allowed it.
There was no test for this class, so I'm adding one.
Some differences from std::function_ref remain (e.g. no support for
const and noexcept signatures, nontype_t constructors or constexpr). I'm
not addressing those here.
Assisted-by: Gemini
