Determines if the given template argument is a flushable stream type, which
is any output stream that defines a Optional!StreamError flushStream()
method, which should cause any data buffered by the stream or its resources
to be flushed. The exact nature of how a flush operates is implementation-
dependent.
Determines if the given template argument is a flushable stream type, which is any output stream that defines a Optional!StreamError flushStream() method, which should cause any data buffered by the stream or its resources to be flushed. The exact nature of how a flush operates is implementation- dependent.