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].
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 [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.