C++ new specification syntax problem

Hello guys,
I have some trouble understanding the following syntax.

This is supposed to be compiled under VS2019. Now, I want to compile it under 2015. How to make the changes to make them compatible?
C++ has been evolving to a more and more cryptic standard. Really.

Code:

#define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \ ABSL_INTERNAL_EXTERN_DECL(type, name) \ inline constexpr ::absl::internal::identity_t<type> name = init #else // See above comment at top of file for details. // // Note: // identity_t is used here so that the const and name are in the // appropriate place for pointer types, reference types, function pointer // types, etc.. #define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init) \ template <class /*AbslInternalDummy*/ = void> \ struct AbslInternalInlineVariableHolder##name { \ static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \ }; \ \ template <class AbslInternalDummy> \ constexpr ::absl::internal::identity_t<var_type> \ AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance; \ \ static constexpr const ::absl::internal::identity_t<var_type>& \ name = /* NOLINT */ \ AbslInternalInlineVariableHolder##name<>::kInstance; \ static_assert(sizeof(void (*)(decltype(name))) != 0, \ "Silence unused variable warnings.") #endif // __cpp_inline_variables

Code:

// Problem A // error C2131: expression did not evaluate to a constant ABSL_INTERNAL_INLINE_CONSTEXPR(in_place_t, in_place, {});

Code:

// Problem B // error C4579: 'absl::AbslInternalInlineVariableHoldernullopt<void>::kInstance': // in-class initialization for type 'const absl::nullopt_t' is not yet implemented; static member will remain uninitialized at runtime but use in constant-expressions is supported ABSL_INTERNAL_INLINE_CONSTEXPR(nullopt_t, nullopt, nullopt_t(optional_internal::init_t()));