22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.15 Function object binders [func.bind]

22.10.15.5 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 are declared as: extern unspecified _1;