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Description
I don't have enough practice with standard-ese to express myself any better than that, unfortunately. Define EVEN_WORSE or ALTERNATIVE_EVEN_WORSE for a different, but seemingly related, inconsistency. Define FIXED to fix the original issue, and "fix" the "even worse" inconsistencies (successful compile, unexpectedly).
I'm happy to provide the motivating example, it's just larger. A whole load of transitional layers from one set of types to another.
Reduced Example
template<typename>
struct Wrapper;
template<template<typename> typename, typename T>
using ComposedWrapper = Wrapper<T>;
template<typename...>
struct mp_list;
template<typename>
struct converter_impl;
template<typename T>
using converter = converter_impl<T>::type;
template<
#if !defined(EVEN_WORSE)
template<typename> typename Wrapper,
#endif
typename T
>
struct converter_impl<mp_list<Wrapper<T>>> {
using type = void;
};
template<typename T>
struct converter_impl<ComposedWrapper<Wrapper, T>> {
using type = converter<T>;
};
#if defined(__clang__) && defined(FIXED)
template<typename T>
struct converter_impl<ComposedWrapper<Wrapper, mp_list<T>>> {
using type = int;
};
#endif
template<typename>
concept valid = true;
static_assert(valid<converter<ComposedWrapper<Wrapper,
#if !defined(ALTERNATIVE_EVEN_WORSE)
mp_list<
#endif
mp_list<int>
#if !defined(ALTERNATIVE_EVEN_WORSE)
>
#endif
>>>);