cpp/ranges/sized range

@1@ The concept specifies the requirements of a  type that knows its size in constant time with the  function.

@2@ exists to allow use of range types that provide a  function (either as a member or as a non-member) but do not in fact model. Users may specialize for cv-unqualified program-defined types. Such specializations shall be usable in constant expressions and have type.

Semantic requirements
@1@ Given an lvalue of type,  models  only if
 * has amortized constant-time complexity,
 * does not alter the value of in a manner observable to equality-preserving expressions, and
 * is equal to, and
 * is equal to, and


 * if models,  is well-defined regardless of the evaluation of  (in other words, a single-pass sized range may support a call to size only before the first call to begin, but a forward range must support size at all times).