std::codecvt::out() returns a no-op in marginal situations

[Sam Varshavchik]

Paolo Carlini writes:

> Sam Varshavchik wrote:
>> So, if out() returns with:
>>
>> 1) from_next < from_end, and
>>
>> 2) to_next < to_end
>>
>> then can I assume that out() wants a larger output buffer?
> In short, I think so. I cannot imagine how things could be different:
> one internal element is enough for a meaningful conversion, and you know
> you have got it. And you have a partial return value. The only possible
> explanation is that the output buffer is too small.

Well, does the same apply to inp(), then? Anyone know if there exists some 
encoding where a multibyte sequence produces more than one wchar_t?  I want 
to see what happens when you feed it to inp(), but give it a one element 
wchar_t outbuf buffer. Reading iconv(3) closely:

       4. The output buffer has no more room for the next converted character.
       In this case it sets errno to E2BIG and returns (size_t)(-1).

A brief experiment:

	iconv_t i=iconv_open("UTF-8", "WCHAR_T");
	wchar_t iarr[]={0x00e8,0x00e8};
	char carr[3];

	char *inbuf=(char *)&iarr[0];
	char *outbuf=carr;

	size_t inbytesleft=sizeof(iarr);
	size_t outbytesleft=sizeof(carr);

	size_t n=iconv(i, &inbuf, &inbytesleft, &outbuf, &outbytesleft);

	printf("Return code %d, errno=%s\n", (int)n, strerror(errno));
	printf("inbytesleft=%d, outbytesleft=%d\n",
	       (int)inbytesleft, (int)outbytesleft);

Result:

Return code -1, errno=Argument list too long
inbytesleft=4, outbytesleft=1

iconv() converted the first wchar_t, but then stopped and terminated with 
room in both input and outbuf buffers. Since iconv(), via mbr..(), is 
presumably used for inp() as well, I should expect the same behavior for 
encodings where multibyte sequences can produce more than one wchar_t, if 
such encoding exists. I'm wondering if the current logic in basic_filebuf 
handles this situation -- haven't yet deciphered it.



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