Compilation error with clang/libc++ and std::equal() on CharacterVectors

[lionel-]

This pattern recently started to cause CRAN failures in purrrlyr on the fedora/clang machine. I can reproduce it with clang on my macOS machine:

#include <algorithm>
#include <Rcpp.h>

// [[Rcpp::export]]
bool test(Rcpp::CharacterVector x, Rcpp::CharacterVector y) {
  if (x.length() != y.length()) {
    return 0;
  }

  return std::equal(x.begin(), x.end(), y.begin());
}


/*** R
  test("foo", "bar")
*/

In file included from clangCharacterVector.cpp:1:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:682:71: error: use of overloaded operator '==' is ambiguous (with operand types 'const Rcpp::internal::string_proxy<16, PreserveStorage>' and 'const Rcpp::internal::string_proxy<16, PreserveStorage>')
    bool operator()(const _T1& __x, const _T1& __y) const {return __x == __y;}
                                                                  ~~~ ^  ~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:1325:14: note: in instantiation of member function 'std::__1::__equal_to<Rcpp::internal::string_proxy<16, PreserveStorage>, Rcpp::internal::string_proxy<16, PreserveStorage> >::operator()' requested here
        if (!__pred(*__first1, *__first2))
             ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:1337:19: note: in instantiation of function template specialization 'std::__1::equal<Rcpp::internal::Proxy_Iterator<Rcpp::internal::string_proxy<16, PreserveStorage> >, Rcpp::internal::Proxy_Iterator<Rcpp::internal::string_proxy<16, PreserveStorage> >, std::__1::__equal_to<Rcpp::internal::string_proxy<16, PreserveStorage>, Rcpp::internal::string_proxy<16, PreserveStorage> > >' requested here
    return _VSTD::equal(__first1, __last1, __first2, __equal_to<__v1, __v2>());
                  ^
clangCharacterVector.cpp:10:15: note: in instantiation of function template specialization 'std::__1::equal<Rcpp::internal::Proxy_Iterator<Rcpp::internal::string_proxy<16, PreserveStorage> >, Rcpp::internal::Proxy_Iterator<Rcpp::internal::string_proxy<16, PreserveStorage> > >' requested here
  return std::equal(x.begin(), x.end(), y.begin());
              ^
/Users/lionel/R/library/3.5/Rcpp/include/Rcpp/vector/string_proxy.h:204:22: note: candidate function
                bool operator==( SEXP other ) const {
                     ^
/Users/lionel/R/library/3.5/Rcpp/include/Rcpp/String.h:525:17: note: candidate function
    inline bool operator==(const String::StringProxy& lhs, const String& rhs) {
                ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:682:71: note: built-in candidate operator==(struct SEXPREC *, struct SEXPREC *)
    bool operator()(const _T1& __x, const _T1& __y) const {return __x == __y;}
                                                                      ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:682:71: note: built-in candidate operator==(const struct SEXPREC *, const struct SEXPREC *)
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:682:71: note: built-in candidate operator==(char *, char *)
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/algorithm:682:71: note: built-in candidate operator==(const char *, const char *)
1 error generated.
make: *** [clangCharacterVector.o] Error 1
ccache clang++ -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I"/Users/lionel/R/library/3.5/Rcpp/include" -I"/Users/lionel/Dropbox/Projects/R/mock/clangCharacterVector/.metadata" -I"/Users/lionel/Dropbox/Projects/R/mock/clangCharacterVector/.metadata/../inst/include" -I/usr/local/include   -fPIC  -arch x86_64  -ftemplate-depth-256  -Wall  -pedantic  -O0 -g -c clangCharacterVector.cpp -o clangCharacterVector.o
Error in Rcpp::sourceCpp("~/Dropbox/Projects/R/mock/clangCharacterVector/.metadata/clangCharacterVector.cpp") : 
  Error 1 occurred building shared library.

[eddelbuettel]

Hm. Help would be welcome, maybe between you and @romainfrancois who recently touched / updated StringProxy to add a storage policy. Maybe this is a regression?

[romainfrancois]

Sure. I’ll pick it up.