20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.14 Function object binders [func.bind]

20.14.14.4 Placeholders [func.bind.place]

namespace std::placeholders {
  // M is the implementation-defined number of placeholders
  see below _1;
  see below _2;
              .
              .
              .
  see below _M;
}
All placeholder types meet the Cpp17DefaultConstructible and Cpp17CopyConstructible requirements, and their default constructors and copy/move constructors are constexpr functions that do not throw exceptions.
It is implementation-defined whether placeholder types meet the Cpp17CopyAssignable requirements, but if so, their copy assignment operators are constexpr functions that do not throw exceptions.
Placeholders should be defined as:
inline constexpr unspecified _1{};
If they are not, they shall be declared as:
extern unspecified _1;