Clang warns me about trace_io_transform's parameter fmt:

io_base.hpp:42:32: warning: unused parameter 'fmt'

which is because my build, having DEBUG_IO_TRANSFORM disabled, renders the parameter useless. The simplest solution should be to make the parameter unnamed. E.g. replace

static void
trace_io_transform(char const* fmt, ...)
{
#ifdef DEBUG_IO_TRANSFORM
    va_list va;
    va_start(va, fmt);
    fprintf(stderr, "TRANSFORM_IO: ");
    vfprintf(stderr, fmt, va);
    va_end(va);
#endif
}

with

static void
#ifdef DEBUG_IO_TRANSFORM
trace_io_transform(char const* fmt, ...)
{
    va_list va;
    va_start(va, fmt);
    fprintf(stderr, "TRANSFORM_IO: ");
    vfprintf(stderr, fmt, va);
    va_end(va);
}
#else
trace_io_transform(char const*, ...)
{
}
#endif