Support nullable `external_ptr<>`

[krlmlr]

Currently, a nullptr but not an R_NilValue can be assigned to an external_ptr<> object. This is inconsistent and surprising, in particular because the roundtrip works correctly the other way. Should we support this?

CC @hannes @Tmonster.

library(cpp11)

# Works
cpp_source(
 code = '
 #include "cpp11/external_pointer.hpp"
 #include "cpp11/strings.hpp"

 [[cpp11::register]]
 cpp11::external_pointer<int> test() {
 return cpp11::external_pointer<int>(nullptr);
 }
 '
)

test()
#> NULL

# Fails, but perhaps shouldn't
cpp_source(
 code = '
 #include "cpp11/external_pointer.hpp"
 #include "cpp11/strings.hpp"

 [[cpp11::register]]
 cpp11::external_pointer<int> test() {
 return cpp11::external_pointer<int>(R_NilValue);
 }
 '
)

test()
#> Error: Invalid input type, expected 'externalptr' actual 'NULL'

^{Created on 2023-04-02 with reprex v2.0.2}

[stephematician]

I can't see a good reason why it shouldn't accept R_NilValue (given that that is what the default constructor assigns). A simple change to external_pointer::valid_type() should fix this, i.e.

 static SEXP valid_type(SEXP data) {
 if (data == nullptr) {
 throw type_error(EXTPTRSXP, NILSXP);
 }
 if (TYPEOF(data) != EXTPTRSXP && data != R_NilValue) {
 throw type_error(EXTPTRSXP, TYPEOF(data));
 }

 return data;
 }