False positive error: pack expansion contains parameter pack '_UTypes' that has a different length (1 vs. 2) from outer parameter packs

[Romain-Geissler-1A]

Hi,

This is an issue similar #58452, actually the snippet I posted in this issue is the same. Using libstc++'s 13 tuple implementation is still not possible when using -std=c++2b:

#include <tuple>

void g(std::tuple<std::tuple<int, int>>);

void f() 
{
    g(std::forward_as_tuple<std::tuple<int, int>>({0, 0}));
}

Godbolt output (https://godbolt.org/z/Meca376M6):

In file included from <source>:1:
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:691:2: error: pack expansion contains parameter pack '_UTypes' that has a different length (1 vs. 2) from outer parameter packs
        using __convertible = __and_<is_convertible<_UTypes, _Types>...>;
        ^~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:853:27: note: in instantiation of template type alias '__convertible' requested here
          = _TCC<true>::template __convertible<_Args...>::value;
                                 ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:948:12: note: in instantiation of static data member 'std::tuple<std::tuple<int, int> &&>::__convertible<int &, int &>' requested here
        explicit(!__convertible<_UElements&...>)
                  ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:2000:36: note: while substituting deduced template arguments into function template 'tuple' [with _UElements = <int, int>]
    { return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }
                                   ^
<source>:7:12: note: in instantiation of function template specialization 'std::forward_as_tuple<std::tuple<int, int>>' requested here
    g(std::forward_as_tuple<std::tuple<int, int>>({0, 0}));
           ^
In file included from <source>:1:
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:691:2: error: pack expansion contains parameter pack '_UTypes' that has a different length (1 vs. 2) from outer parameter packs
        using __convertible = __and_<is_convertible<_UTypes, _Types>...>;
        ^~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:853:27: note: in instantiation of template type alias '__convertible' requested here
          = _TCC<true>::template __convertible<_Args...>::value;
                                 ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:959:12: note: in instantiation of static data member 'std::tuple<std::tuple<int, int> &&>::__convertible<const int, const int>' requested here
        explicit(!__convertible<const _UElements...>)
                  ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/13.0.1/../../../../include/c++/13.0.1/tuple:2000:36: note: while substituting deduced template arguments into function template 'tuple' [with _UElements = <int, int>]
    { return tuple<_Elements&&...>(std::forward<_Elements>(__args)...); }
                                   ^
<source>:7:12: note: in instantiation of function template specialization 'std::forward_as_tuple<std::tuple<int, int>>' requested here
    g(std::forward_as_tuple<std::tuple<int, int>>({0, 0}));
           ^
2 errors generated.
Compiler returned: 1

[shafik]

CC @erichkeane

[llvmbot]

@llvm/issue-subscribers-c-2b

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[stbergmann]

(might be a duplicate of #17042)

[erichkeane]

I'll need a minimal example here, I got lost on the initial one trying to figure out what is going on inside of Tuple. Also, would be nice to know if perhaps this is just a libc++ bug instead :)

[Romain-Geissler-1A]

Ok I will try to reduce it today, so we stop relying on libstc++ headers.

[stbergmann]

I think a reduced example exhibiting the same underlying issue is still as discussed at Pack expansion bug?,

$ cat test.cc
template<typename, typename> struct Pair;
template<typename...> struct Tuple;
template<typename... Ts> struct A {
    template<typename... Us> using B = Tuple<Pair<Ts, Us>...>;
};
template<typename... Ts> struct C {
    template<typename... Us> using D = typename A<Ts...>::template B<Us...>;
};
using E = C<int, int>::D<int, int>;

$ clang++ -fsyntax-only test.cc
test.cc:4:30: error: pack expansion contains parameter pack 'Us' that has a different length (2 vs. 1) from outer parameter packs
    template<typename... Us> using B = Tuple<Pair<Ts, Us>...>;
                             ^~~~~
test.cc:7:68: note: in instantiation of template type alias 'B' requested here
    template<typename... Us> using D = typename A<Ts...>::template B<Us...>;
                                                                   ^
test.cc:9:11: note: in instantiation of template class 'C<int, int>' requested here
using E = C<int, int>::D<int, int>;
          ^
1 error generated.

[Romain-Geissler-1A]

Note, I didn't try to reduce my case since @stbergmann posted his own reduced C++ snippet. Apparently it's not a regression in clang, as it seems clang never accepted this code in any version (checking on Compiler Explorer). In practice, given the updates made on libstdc++ side, it makes the just released clang 16 error when using libstdc++ tuples with C++23.