[llvm-bugs] [Bug 28386] New: Zero-sized array not Sfinae-failure in non-type template parameter

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28386

            Bug ID: 28386
           Summary: Zero-sized array not Sfinae-failure in non-type
                    template parameter
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lf-bt at thax.hardliners.org
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

#include <type_traits>

template <unsigned int... Is>
struct index_sequence {};

template <bool B> using Sfinae=char (*)[B]; // does not work with msvc+clang;
works with g++
//template <bool B> using Sfinae=char (*)[2*B-1]; // works with g++ and
clang;(msvc does strange things)
//template <bool B> using Sfinae=typename std::enable_if<B,int>::type; //
works.

template <unsigned int N,unsigned int... Is,typename =Sfinae<N==0>>
constexpr index_sequence<Is...> make_index_sequence(...)
{ return {}; }

template <unsigned int N,unsigned int... Is,typename =Sfinae<(N>0)>>
constexpr auto make_index_sequence(...)
  -> decltype(make_index_sequence<N-1,N-1,Is...>(index_sequence<>())) //
argument forces ADL
{ return {}; }

int main()
{
  index_sequence<0,1> t=make_index_sequence<2>(); 
}

----
clang++ will happily go from I=0 to I=4294967295 ... until it hits the template
instantiation limit.

According to my and g++'s understanding, using the first version should work,
as zero-size array should lead to an SFINAE failure, just like negative sized
arrays do.

Using the Sfinae-type as function argument instead of as template parameter,
all three versions compile fine, i.e. the zero sized array is treated as SFINAE
failure, e.g.:

  template <unsigned int N,unsigned int... Is>
  constexpr index_sequence<Is...> make_index_sequence(index_sequence<>
x={},Sfinae<N==0> =0)
  ...

  template <unsigned int N,unsigned int... Is>
  constexpr auto make_index_sequence(index_sequence<> x={},Sfinae<(N>0)> =0)
  ...


(MSVC's problem with the second version has been reported here:
https://connect.microsoft.com/VisualStudio/feedback/details/2881697 )

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