Sentinel default for a specializable variable-template customization point with no usable default.
Declared in <mp-units/utility/unspecified.h>
struct unspecified_t;A variable template cannot be = deleted the way a function can, so a customization point that must be specialized before use defaults to unspecified, and a specialization replaces it with a real value. specified reports whether that has happened. This is public authoring vocabulary shared by the library's own customization points and by higher-level tools, hence it lives in the mp_units::utility namespace.