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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c++
Subject: Re: C++20 concepts rocks
Date: Sun, 06 Feb 2022 02:16:35 -0800
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"Alf P. Steinbach" <alf.p.steinbach@gmail.com> writes:

> On 5 Feb 2022 08:37, Tim Rentsch wrote:
>
>> "Alf P. Steinbach" <alf.p.steinbach@gmail.com> writes:
>>
>>> On 4 Feb 2022 17:15, Ben Bacarisse wrote:
>>
>> [...]
>>
>>>> template<typename StringType>
>>>> requires is_same_v<StringType,
>>>>                      basic_string<typename StringType::value_type,
>>>>                                   typename StringType::traits_type,
>>>>                                   typename StringType::allocator_type>>
>>>> StringType formatClockCycles(uint64_t clockCycles)
>>>> {
>>>>      char result[28], *ep = (&result)[1];
>>>>      do {
>>>>         sprintf(ep - 4, "%03lu", clockCycles % 1000);
>>>>         if (ep != (&result)[1]) ep[-1] = '.';
>>>>         ep -= 4;
>>>>      } while (clockCycles /= 1000);
>>>>      while (*ep == '0' && ep[1]) ep++;
>>>>      return ep;
>>>> }
>>>>
>>>> (the appropriate comment on the 28 is left as an exercise to the reader!)
>>>
>>> Nice, but I believe it's formal UB to move a pointer to `char` from
>>> beyond an array into the array, and dereference it.
>>>
>>> The C committee wrote a rationale for their earlier decision to make
>>> it so, instead of admitting that it was bludner.  Uh, blunder.  I
>>> don't recall who provided that information, but someone in this
>>> group.
>>
>> I suspect that you either are misremembering or have misunderstood.
>> Usage patterns like that seen above are defined behavior in C,
>> because they were in common use for at least 10 years before the
>> first C standard.  It would be unthinkable for WG14 to have turned
>> it into undefined behavior.
>
> The "unthinkable" is why I maintain that it was originally a blunder,
> not intentional.

After reading some of your other comments elsethread, I now see
the difficulty:  your earlier comments didn't clearly explain
what the issue is that you wanted to point out.