https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80987
Bug ID: 80987
Summary: Existence of a std::initializer_list constructor
breaks deduction guides
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
This valid C++17 program is rejected:
namespace std
{
using size_t = decltype(sizeof(0));
template<typename T> struct initializer_list {
constexpr initializer_list(const T*, size_t) { }
T* _M_array;
size_t _M_len;
};
template<typename C, typename A> struct basic_string {
template<typename Iter> basic_string(Iter, const A& = A()) { }
basic_string(std::initializer_list<C>) { }
};
template<typename T> struct allocator { };
template<typename CharT, typename Allocator = allocator<CharT>>
basic_string(CharT, Allocator = Allocator())
-> basic_string<CharT, Allocator>;
}
int main()
{
const char* it = "";
std::allocator<char> a;
std::basic_string s1(it, a); // OK
std::basic_string s2(it); // OK
std::basic_string s3{it, a}; // ERROR
}
prog.cc: In function 'int main()':
prog.cc:29:31: error: class template argument deduction failed:
std::basic_string s3{it, a};
^
prog.cc:29:31: error: no matching function for call to
'basic_string(<brace-enclosed initializer list>)'
prog.cc:19:9: note: candidate: 'template<class CharT, class Allocator>
std::basic_string(CharT, Allocator)-> std::basic_string<CharT, Allocator>'
basic_string(CharT, Allocator = Allocator())
^~~~~~~~~~~~
prog.cc:19:9: note: template argument deduction/substitution failed:
prog.cc:29:31: note: couldn't deduce template parameter 'CharT'
std::basic_string s3{it, a};
^
prog.cc:13:9: note: candidate: 'template<class C, class A>
basic_string(std::initializer_list<C>)-> std::basic_string<C, A>'
basic_string(std::initializer_list<C>) { }
^~~~~~~~~~~~
prog.cc:13:9: note: template argument deduction/substitution failed:
prog.cc:29:31: note: deduced conflicting types for parameter 'C' ('const
char*' and 'std::allocator<char>')
std::basic_string s3{it, a};
^
prog.cc:12:33: note: candidate: 'template<class C, class A, class Iter>
basic_string(Iter, const A&)-> std::basic_string<C, A>'
template<typename Iter> basic_string(Iter, const A& = A()) { }
^~~~~~~~~~~~
prog.cc:12:33: note: template argument deduction/substitution failed:
prog.cc:29:31: note: couldn't deduce template parameter 'C'
std::basic_string s3{it, a};
^
prog.cc:11:45: note: candidate: 'template<class C, class A>
basic_string(std::basic_string<C, A>)-> std::basic_string<C, A>'
template<typename C, typename A> struct basic_string {
^~~~~~~~~~~~
prog.cc:11:45: note: template argument deduction/substitution failed:
prog.cc:29:31: note: couldn't deduce template parameter 'C'
std::basic_string s3{it, a};
^
