[Bug c++/109888] New: GCC 13 Fails to Compile Code with Explicit Constructor for std::array in Template Class

[Bug c++/109888] New: GCC 13 Fails to Compile Code with Explicit Constructor for std::array in Template Class

[vincent.lebourlot at starqube dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109888

            Bug ID: 109888
           Summary: GCC 13 Fails to Compile Code with Explicit Constructor
                    for std::array in Template Class
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent.lebourlot at starqube dot com
  Target Milestone: ---

In a C++ codebase, a class String is defined with an explicit constructor that
takes a variable number of arguments and constructs a std::array from them.
This constructor is being called when creating a std::pair<String, String> in a
function call.

While this code compiles successfully with GCC 12, it fails to compile with GCC
13. The error messages indicate that the std::array must be initialized with a
brace-enclosed initializer, which is not what's happening when forwarding the
arguments to the std::array's constructor.

This issue seems to be specific to GCC 13 and does not occur in GCC 12. It's
unclear whether this is due to changes in the C++ standard or in GCC's
implementation of the standard. The code snippet that reproduces this issue is
provided below:

#include <string>
#include <cstring>
#include <array>
#include <vector>
#include <utility>

class String {
public:
    template<size_t n>String(const char(&s)[n])noexcept:value(){
        if constexpr(n<=16){
            std::memcpy(value.data(),s,n);}
        else{value=std::array<unsigned char,16>{};}}
    template<typename... Args,
std::enable_if_t<std::conjunction_v<std::is_constructible<std::string,
Args>...>, int> = 0>
    explicit String(Args&&... args) : value(std::forward<Args>(args)...) {}
private:
    std::array<unsigned char,16>value;
};

int main() {
    auto check=[&](std::vector<std::pair<String,String>>textInputs){};
    check({{{"Hello", "World"}, {"Foo", "Bar"}}});
    return 0;
}


And here's the error message:

test.cpp: In function ‘int main()’:
test.cpp:20:10: error: converting to ‘const String’ from initializer list would
use explicit constructor ‘String::String(Args&& ...) [with Args = {const char
(&)[6], const char (&)[6]}; typename
std::enable_if<conjunction_v<std::is_constructible<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >, Args>...>, int>::type
<anonymous> = 0]’
   20 |     check({{{"Hello", "World"}, {"Foo", "Bar"}}});
      |     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:20:10: error: converting to ‘const String’ from initializer list would
use explicit constructor ‘String::String(Args&& ...) [with Args = {const char
(&)[4], const char (&)[4]}; typename
std::enable_if<conjunction_v<std::is_constructible<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >, Args>...>, int>::type
<anonymous> = 0]’
test.cpp: In instantiation of ‘String::String(Args&& ...) [with Args = {const
char (&)[6], const char (&)[6]}; typename
std::enable_if<conjunction_v<std::is_constructible<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >, Args>...>, int>::type
<anonymous> = 0]’:
test.cpp:20:10:   required from here
test.cpp:12:39: error: no matching function for call to ‘std::array<unsigned
char, 16>::array(const char [6], const char [6])’
   12 |     explicit String(Args&&... args) :
value(std::forward<Args>(args)...) {}
      |                                      
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from test.cpp:3:
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note: candidate:
‘std::array<unsigned char, 16>::array()’
   94 |     struct array
      |            ^~~~~
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note:   candidate expects
0 arguments, 2 provided
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note: candidate:
‘constexpr std::array<unsigned char, 16>::array(const std::array<unsigned char,
16>&)’
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note:   candidate expects
1 argument, 2 provided
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note: candidate:
‘constexpr std::array<unsigned char, 16>::array(std::array<unsigned char,
16>&&)’
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note:   candidate expects
1 argument, 2 provided
test.cpp: In instantiation of ‘String::String(Args&& ...) [with Args = {const
char (&)[4], const char (&)[4]}; typename
std::enable_if<conjunction_v<std::is_constructible<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >, Args>...>, int>::type
<anonymous> = 0]’:
test.cpp:20:10:   required from here
test.cpp:12:39: error: no matching function for call to ‘std::array<unsigned
char, 16>::array(const char [4], const char [4])’
   12 |     explicit String(Args&&... args) :
value(std::forward<Args>(args)...) {}
      |                                      
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note: candidate:
‘std::array<unsigned char, 16>::array()’
   94 |     struct array
      |            ^~~~~
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note:   candidate expects
0 arguments, 2 provided
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note: candidate:
‘constexpr std::array<unsigned char, 16>::array(const std::array<unsigned char,
16>&)’
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note:   candidate expects
1 argument, 2 provided
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note: candidate:
‘constexpr std::array<unsigned char, 16>::array(std::array<unsigned char,
16>&&)’
/usr/local/gcc/gcc-13/include/c++/13.1.0/array:94:12: note:   candidate expects
1 argument, 2 provided

[Bug c++/109888] GCC 13 Fails to Compile Code with Explicit Constructor for std::array in Template Class

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109888

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Another dup of Bug 109247

*** This bug has been marked as a duplicate of bug 109247 ***