C++ named requirements:
RangeAdaptorClosureObject (od C++20)

Range adaptor closure objects are FunctionObjects that are callable via the pipe operator: if C is a range adaptor closure object and R is a range, these two expressions are equivalent:

C(R)
R | C

Two range adaptor closure objects can be chained by operator| to produce another range adaptor closure object: if C and D are range adaptor closure objects, then C | D produces a range adaptor closure object E with the following properties:

• E stores a copy of C and D, direct-non-list-initialized from std::forward<decltype((C))>(C) and std::forward<decltype((D))>(D) respectively. If such initialization is invalid, C | D is also invalid.
• Let c and d be the stored copies (with the same constness and value category as E), and R be a range object, the following expressions are equivalent:

d(c(R))
R | c | d
E(R)
R | E // R | (C | D)

Notes

operator() is unsupported for volatile-qualified or const-volatile-qualified version of range adaptor object closure types.

Objects whose type is the same as one of the following objects (ignoring cv-qualification) are range adaptor closure objects:

• unary range adaptor objects,
• objects of user-defined types that meet the requirements of implementing a range adaptor closure object (od C++23),
• the results of binding trailing arguments by range adaptor objects, and
• the results of chaining two range adaptor closure objects by operator|.