[Bug c++/85695] New: if constexpr misevaluates typedefed type value

[msharov at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85695

            Bug ID: 85695
           Summary: if constexpr misevaluates typedefed type value
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msharov at users dot sourceforge.net
  Target Milestone: ---

template <typename T, T v>
struct integral_constant {
    using value_type = T;
    static constexpr const value_type value = v;
    constexpr operator value_type (void) const { return value; }
};
template <typename T> struct is_trivial
    : public integral_constant<bool, __is_trivial(T)> {};

template <typename T>
T clone_object (const T& p)
{
    if constexpr (is_trivial<T>::value)
        return p;
    else
        return p.clone();
}
int main (void) { return clone_object(0); }

This fails to compile: "error: request for member ‘clone’ in ‘p’". The strange
part is that changing the type of integral_constant::value to T makes it work,
as  does using is_trivial<T>() in the conditional, invoking the cast operator.
For some reason, value_type is evaluated differently if it is a variable or
return value, and differently from T.