template non-type with decltype(auto) placeholder does not work with template specialization when a reference is passed

[narutse]

Bugzilla Link	39373
Version	7.0
OS	Linux

Extended Description

Code:

#include <type_traits>
#include

// definition
template <bool, decltype(auto)> struct X : std::false_type {};
// sepcialization
template <decltype(auto) val>
struct X<true, val> : std::true_type {};

using namespace std;

const double d = 10.0;

int main()
{
cout << X<true, 1>::value << '\n';
cout << X<true, (d)>::value << '\n';
}

Will print out 1 and 0, which means that it fails to specialize the (d) reference version. I tried it on gcc and it prints 1 and 1. Alternatively, if I left struct X undefined in the declaration (instead of inheriting from false_type), it will give me a compiler error "implicit instantiation of undefined template" only for the second instantiation, whereas in gcc it will succeed instantiating both.

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#40983