CODEWITHSUNDEEP
c++gccoverloadingrvalue-reference
jxh
jxh

Reputation: 70472

Cannot match rvalue reference function with GCC

When attempting to use an rvalue reference, I get an error from GCC (clang does not generate an error).

Why is the compiler generating an error? Is it a bug in GCC?

The "simplified" error is:

|In file included from /usr/include/c++/8/sstream:38,
| from .code.tio.cpp:1:
|/usr/include/c++/8/istream: In instantiation of 'struct std::__is_extractable<std::istream&, std::istream& (*&)(std::istream&, std::string&), void>':
|/usr/include/c++/8/type_traits:131:12: required from 'struct std::__and_<std::__is_convertible_to_basic_istream<std::istringstream > >, std::__is_extractable<std::istream&, std::istream& (*&)(std::istream&, std::string&), void> >'
|/usr/include/c++/8/type_traits:136:12: required from 'struct std::__and_<std::__not_<std::is_lvalue_reference<std::istringstream > > >, std::__is_convertible_to_basic_istream<std::istringstream > >, std::__is_extractable<std::istream&, std::istream& (*&)(std::istream&, std::string&), void> >'
|/usr/include/c++/8/istream:980:5: required by substitution of 'template<class _Istream, class _Tp> typename std::enable_if<std::__and_<std::__not_<std::is_lvalue_reference<_Tp> >, std::__is_convertible_to_basic_istream<_Istream>, std::__is_extractable<typename std::__is_convertible_to_basic_istream<_Tp>::__istream_type, _Tp&&, void> >::value, typename std::__is_convertible_to_basic_istream<_Tp>::__istream_type>::type std::operator>>(_Istream&&, _Tp&&) [with _Istream = std::istringstream; _Tp = std::istream& (*&)(std::istream&, std::string&)]'
|.code.tio.cpp:12:32: required from here
|/usr/include/c++/8/istream:951:12: error: no match for 'operator>>' (operand types are 'std::istream' and 'std::istream& (*)(std::istream&, std::string&)')
| __void_t<decltype(declval<_Istream&>()
| ~~~~~~~~~~~~~~~~~~~~
| >> declval<_Tp>())>>
| ^~~~~~~~~~~~~~~~~

The code I am trying to compile is:

typedef std::istream &getline_type(std::istream &, std::string &);

void operator>>(std::istream &, getline_type) {}   // 1
void operator>>(std::istream &&, getline_type) {}

int main()
{
    std::string s;
    getline_type *getline = std::getline;          // 2
    //using std::getline;                          // 3
    std::istringstream("a") >> getline;
}

If I comment out the line marked // 1, it works. If I use the line marked // 3 instead of // 2, it also works.

I believe clang is correctly assessing that the code is well-formed, but if there is a valid reason to reject the code, it might indicate a bug in clang.

Try it online!

Upvotes: 3

Views: 143

Answers (1)

Davis Herring
Davis Herring

Reputation: 40013

This does appear to be a GCC bug: the line at which the overload resolution fails is part of the declaration of a partial specialization for a trait, so a substitution failure should just have the usual effect of eliminating the “overload” (the partial specialization) and thus producing false_type for the trait. Then the operator>> template specialization would itself be eliminated for the same reason, leaving your overloads. I assume that GCC is letting your functions bleed into the lookup set (but only partially, since it then rejects for want of an istream& overload after all).

Upvotes: 1

Related Questions