31 Input/output library [input.output]

31.9 Span-based streams [span.streams]

31.9.1 Overview [span.streams.overview]

The header <spanstream> defines class templates and types that associate stream buffers with objects whose types are specializations of span as described in [views.span].
[Note 1: 
A user of these classes is responsible for ensuring that the character sequence represented by the given span outlives the use of the sequence by objects of the classes in subclause [span.streams].
Using multiple basic_spanbuf objects referring to overlapping underlying sequences from different threads, where at least one basic_spanbuf object is used for writing to the sequence, results in a data race.
— end note]