<tuple>: Confusing diagnostic for get<T>() when the type doesn't occur exactly once #4575
Description
The MSVC STL diagnostic for invalid get<T>() is very confusing (regardless of whether the compiler is MSVC or Clang), whereas libstdc++ and libc++ emit better diagnostics: https://godbolt.org/z/n77dfWr9G
We should verify that both the "not found" and "found more than once" scenarios generate improved diagnostics.
C:\Temp>type meow.cpp
#include <tuple>
using namespace std;
int main() {
tuple<int, long, double> t{17, 29L, 3.14};
(void) get<float>(t);
}C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /c meow.cpp
meow.cpp
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): error C2039: '_Ttype': is not a member of 'std::_Tuple_element<_Ty,std::tuple<>>'
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(918): note: see declaration of 'std::_Tuple_element<_Ty,std::tuple<>>'
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): note: the template instantiation context (the oldest one first) is
meow.cpp(5): note: see reference to function template instantiation '_Ty &std::get<float,int,long,double>(std::tuple<int,long,double> &) noexcept' being compiled
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(967): note: see reference to class template instantiation 'std::_Tuple_element<float,std::tuple<int,long,double>>' being compiled
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): note: see reference to class template instantiation 'std::_Tuple_element<_Ty,std::tuple<long,double>>' being compiled
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): note: see reference to class template instantiation 'std::_Tuple_element<_Ty,std::tuple<double>>' being compiled
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): error C2061: syntax error: identifier '_Ttype'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): error C2039: '_Ttype': is not a member of 'std::_Tuple_element<_Ty,std::tuple<double>>'
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(927): note: see declaration of 'std::_Tuple_element<_Ty,std::tuple<double>>'
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928): error C2039: '_Ttype': is not a member of 'std::_Tuple_element<_Ty,std::tuple<long,double>>'
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(927): note: see declaration of 'std::_Tuple_element<_Ty,std::tuple<long,double>>'
with
[
_Ty=float
]
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(967): error C2039: '_Ttype': is not a member of 'std::_Tuple_element<float,std::tuple<int,long,double>>'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(927): note: see declaration of 'std::_Tuple_element<float,std::tuple<int,long,double>>'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(967): error C2061: syntax error: identifier '_Ttype'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(968): error C2061: syntax error: identifier '_Ttype'
C:\Temp>clang-cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /c meow.cpp
In file included from meow.cpp:1:
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928,67): error: no type
named '_Ttype' in 'std::_Tuple_element<float, std::tuple<>>'
928 | using _Ttype = typename _Tuple_element<_Ty, tuple<_Rest...>>::_Ttype;
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928,29): note: in
instantiation of template class 'std::_Tuple_element<float, std::tuple<double>>' requested here
928 | using _Ttype = typename _Tuple_element<_Ty, tuple<_Rest...>>::_Ttype;
| ^
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(928,29): note: in
instantiation of template class 'std::_Tuple_element<float, std::tuple<long, double>>' requested here
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(967,29): note: in
instantiation of template class 'std::_Tuple_element<float, std::tuple<int, long, double>>' requested here
967 | using _Ttype = typename _Tuple_element<_Ty, tuple<_Types...>>::_Ttype;
| ^
meow.cpp(5,12): note: in instantiation of function template specialization 'std::get<float, int, long, double>'
requested here
5 | (void) get<float>(t);
| ^
In file included from meow.cpp:1:
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.40.33721\include\tuple(968,12): error: non-const
lvalue reference to type '_Ttype' (aka 'int') cannot bind to a value of unrelated type 'tuple<int, long, double>'
968 | return static_cast<_Ttype&>(_Tuple)._Myfirst._Val;
| ^ ~~~~~~
meow.cpp(5,12): note: in instantiation of function template specialization 'std::get<float, int, long, double>'
requested here
5 | (void) get<float>(t);
| ^
2 errors generated.