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()));