writable::list() creates a NULL

[hadley]

cpp11::cpp_eval("cpp11::writable::list()")
#> NULL

^{Created on 2021-04-08 by the reprex package (v2.0.0)}

[jimhester]

This is really the crux of it, the error is coming from the cpp11::list constructor.

cpp11::list foo(cpp11::writable::list())

We could have a more informative error message when a cpp11::writable::list() is passed and the SEXP is R_NilValue, this indicates trying to create a view on a default constructed list.

We also should mark the default constructors as noexcept