https://llvm.org/bugs/show_bug.cgi?id=31481
Bug ID: 31481
Summary: 3+ parameter common_type isn't sufficiently
SFINAE-friendly
Product: libc++
Version: unspecified
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: rs2...@gmail.com
CC: llvm-bugs@lists.llvm.org, mclow.li...@gmail.com
Classification: Unclassified
#include <type_traits>
struct A {};
template class std::common_type<int, int, A>;
In file included from prog.cc:1:
/usr/local/llvm-head/include/c++/v1/type_traits:2029:8: error: no type named
'type' in 'std::__1::common_type<int, A>'
>::type type;
~~~^~~~
/usr/local/llvm-head/include/c++/v1/type_traits:2034:7: note: in instantiation
of template class 'std::__1::__common_type_impl<std::__1::__common_types<int,
int, A>, void>' requested here
: __common_type_impl<__common_types<_Tp, _Up, _Vp...> > {};
^
prog.cc:4:22: note: in instantiation of template class
'std::__1::common_type<int, int, A>' requested here
template struct std::common_type<int, int, A>;
^
The __void_t check for this case only verified the existence of
common_type<_Tp, _Up>::type, not common_type<typename common_type<_Tp,
_Up>::type, _Vp...>::type. The simplest fix is to inherit from
common_type<typename common_type<_Tp, _Up>::type, _Vp...> instead.
Of course, P0435R1 is touching this area again, but the principal change is to
the two-parameter version and not >2, so figured I'd file this anyway.
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