Re: [boost] C++11 decltype/SFINAE puzzler

Subject: Re: [boost] C++11 decltype/SFINAE puzzler
From: Doug Gregor (doug.gregor_at_[hidden])
Date: 2012-07-09 11:42:28

On Fri, Jul 6, 2012 at 6:17 PM, Eric Niebler <eric_at_[hidden]> wrote:
> On 7/6/2012 5:43 PM, Nathan Ridge wrote:
>>
>>> From: doug.gregor_at_[hidden]
>>> On Fri, Jul 6, 2012 at 1:23 PM, Eric Niebler <eric_at_[hidden]> wrote:
>>>> On 7/6/2012 12:54 PM, Marc Glisse wrote:
>>>>> On Fri, 6 Jul 2012, Eric Niebler wrote:
>>>>>> The end result is that the user is presented with an error like: "no
>>>>>> viable function A, template substitution failed." The user is given no
>>>>>> information about which function in the chain failed to compile, or why.
>>>>>> This is a serious usability problem!
>>>>>
>>>>> ... in your compiler.
>>>>>
>>>>>> All suggestions welcome,
>>>>>
>>>>> Use g++.
>>>> <snip informative backtrace>
>>>>
>>>> Ah! Thanks. I'm using clang. I agree, it's a compiler QoI issue. I'll
>>>> follow up with them.
>>>
>>> FWIW, top-of-tree Clang produces:
>>>
>>> t.cpp:12:3: error: no matching function for call to 'f'
>>> f(0);
>>> ^
>>> t.cpp:8:8: note: candidate template ignored: substitution failure
>>> [with T = int]: no matching function for call to 'g'
>>> auto f(T x) -> decltype(g(x)) { return g(x); };
>>> ^ ~
>>>
>>> Which could probably still be improved, although I can't convince
>>> myself that GCC 4.8 is doing us favors by producing the complete
>>> SFINAE backtrace.
>>
>> I think that as a matter of principle, more information is better than
>> less. The raw error message can relatively easily be parsed by a tool
>> that filters out undesired detail, but if the detail is not emitted in
>> the first place, it's gone.
>
> I'd like to second what Nate said. My experience is that not providing
> the backtrace is a usability nightmare. To me, this is *exactly*
> analogous to a template instantiation backtrace and should, IMO, be
> handled exactly the same way. If you want to snip the middle of the
> backtrace, fine, but show the start and the end (the deepest point, I
> guess), and perhaps provide an option for dumping the complete thing.

The typical complaints about template error messages concern excess
verbosity rather than a lack of detail, so simply providing the full
backtrace in all cases is not necessarily helpful. A more nuanced
approach that provides this detail when it is interesting (e.g., only
one candidate needs it) would be ideal; failing that, some
command-line options to dial up the verbosity would allow experts to
get the information they need while not flooding the terminals of the
majority of C++ programmers.

This is certainly worth of a Clang bug: http://llvm.org/bugs/

  - Doug