[Bug libstdc++/67537] [6 Regression] r225189 breaks building boost hana
redi at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Thu Sep 10 13:51:00 GMT 2015
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67537
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:
template<typename T, typename U>
struct pair { };
template<typename... _Elements>
void sink(_Elements&&...);
template<bool, typename... _Elements>
struct _TC
{
template<typename... _UElements>
static constexpr bool _MoveConstructibleTuple()
{
sink( pair<_Elements, _UElements>{} ... );
return true;
}
};
template<typename... _Elements>
struct _TC<false, _Elements...>
{
template<typename... _UElements>
static constexpr bool _MoveConstructibleTuple()
{
return false;
}
};
template<bool>
struct enable_if { using type = bool; };
template<>
struct enable_if<false> { };
template<typename... _Elements>
struct tuple
{
template<typename... _UElements> using _TMC =
_TC<(sizeof...(_Elements) == sizeof...(_UElements)), _Elements...>;
template<typename... _UElements, typename
enable_if<_TMC<_UElements...>::template
_MoveConstructibleTuple<_UElements...>()>::type=true>
constexpr tuple(_UElements&&...) { }
constexpr tuple() { }
};
struct X { };
constexpr tuple<X> t{};
This compiles with GCC but Clang gives:
c.cc:13:41: error: pack expansion contains parameter packs '_Elements' and
'_UElements' that have different lengths (1 vs. 0)
sink( pair<_Elements, _UElements>{} ... );
~~~~~~~~~ ~~~~~~~~~~ ^
c.cc:41:52: note: in instantiation of function template specialization
'_TC<true, X>::_MoveConstructibleTuple<>' requested here
enable_if<_TMC<_UElements...>::template
_MoveConstructibleTuple<_UElements...>()>::type=true>
^
c.cc:42:17: note: while substituting prior template arguments into non-type
template parameter [with _UElements = <>]
constexpr tuple(_UElements&&...) { }
^~~~~
c.cc:35:8: note: while substituting deduced template arguments into function
template 'tuple' [with _UElements = <>, $1 = (no value)]
struct tuple
^
1 error generated.
Here we get an error inside _TC<true, X> even though the true specialization
should be unreachable.
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