[Bug c++/93870] New: User-defined conversion function not working in evaluation of template argument

[o_kniemeyer at maxon dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93870

            Bug ID: 93870
           Summary: User-defined conversion function not working in
                    evaluation of template argument
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: o_kniemeyer at maxon dot net
  Target Milestone: ---

According to gcc.godbolt.org GCC 7.1 up to 9.2 fail to compile this piece of
code:


template <typename ENUM> struct EnumWrapper
{
        ENUM value;

        constexpr operator ENUM() const
        {
                return value;
        }
};

enum E : int
{
    V
};

constexpr EnumWrapper<E> operator ~(E a)
{
    return {E(~int(a))};
}

template <E X> struct R
{
    static void Func();
};

template <E X> struct S : R<~X>
{
};

void Test()
{
    S<E::V>::Func();
}


The can't evaluate the template argument ~X for R:

<source>: In instantiation of 'struct S<(E)0>':
<source>:32:12:   required from here
<source>:26:31: error: taking address of temporary [-fpermissive]
 template <E X> struct S : R<~X>
                               ^
<source>:26:31: error: no matching function for call to
'EnumWrapper<E>::operator E(EnumWrapper<E>*)'
<source>:5:12: note: candidate: constexpr EnumWrapper<ENUM>::operator ENUM()
const [with ENUM = E]
  constexpr operator ENUM() const
            ^~~~~~~~
<source>:5:12: note:   candidate expects 0 arguments, 1 provided
<source>: In function 'void Test()':
<source>:32:14: error: 'Func' is not a member of 'S<(E)0>'
     S<E::V>::Func();
              ^~~~

All other compiler are fine. A workaround is to convert to E explicitly by
writing R<E(~X)>.