[cfe-dev] Explicit instantiation with body?
Larisse Voufo
lvoufo at google.com
Sat Jun 22 07:13:03 PDT 2013
On Fri, Jun 21, 2013 at 11:24 PM, Serge Pavlov <sepavloff at gmail.com> wrote:
> As Richard pointed out, I used wrong wording, so let me give long
> explanation, just to be understood correctly.
>
> The most common kind of error is typos. The next is something omitted,
> these errors are typical for experienced programmers ("brain is faster than
> fingers"). Misplaced and extra things are much rarer, the latter are more
> often for novices. So if we have wrong piece of code, we should try to
> recover assuming that something is missed. The original case:
>
> template struct pr15466a { int a; };
>
> can be recovered by:
> - Adding parameter list to 'template', making it a template definition, or
> - Removal of 'template' turning it into non-template class definition.
> According the above rules, missed things are more probable, than spurious
> 'template'. People are lazy, if a user typed 'template', there must a
> template somewhere. So this code should be recovered as template definition.
>
> Now let's consider the next example:
>
> template <class T> struct pr15466a;
> template struct pr15466a { int a; };
>
> In this case we know that 'pr15466a' is a template, so almost definitely
> the second line must be recovered as a specialization by putting '<>' after
> 'template' and
>
> template parameters after 'pr15466'.
>
Not to be too picky, but I think you meant "template arguments" here? :)
> Chances that a user made two different errors (typed extra 'template' and
> made a typo in 'pr15466') are pretty low.
>
> Bottom line: in both cases I would complain about missed template
> arguments.
>
I believe there have been long and substantial discussions before this bug
fix about the proper way to recover from errors. This fix does not address
what is the proper way, but rather applies what seems to be the currently
agreed-upon rules. Therefore, I'd say that the issue you raise here is
orthogonal to this bug fix.
>
> Another problem with your fix is too long messages. Short messages (no
> more that 5-6 significant words) are perceived as a whole, longer require
> reading which is substantially (several times) slower and annoy users. The
> message
> class cannot be defined in an explicit instantiation; if this
> declaration is meant to be a class definition, remove the 'template' keyword
> I would make something like:
> class cannot be defined in an explicit instantiation; spurious
> 'template'?
>
You may be right, but this bug fix did not make this message. So, I'd
suggest filing a separate bug report addressing the length of error
messages.
I hope this helps.
Thanks,
-- Larisse.
> Thanks,
> --Serge
>
>
> 2013/6/22 Larisse Voufo <lvoufo at google.com>
>
>>
>>
>>
>> On Fri, Jun 21, 2013 at 1:46 PM, Richard Smith <richard at metafoo.co.uk>wrote:
>>
>>> On Fri, Jun 21, 2013 at 12:31 PM, Serge Pavlov <sepavloff at gmail.com>
>>> wrote:
>>> > With this fix diagnostics in some cases look unclear. For instance, in
>>> these
>>> > declarations:
>>> >
>>> > template <class T> struct pr15466a;
>>> > template struct pr15466a { int a; };
>>> >
>>> > the second is obviously an instantiation with template arguments
>>> missing.
>>>
>>> I think you mean, it's obviously meant to be a definition of the
>>> primary template.
>>>
>>> > Compiler messages however may be confusing:
>>> >
>>> > t2.cpp:2:26: error: class cannot be defined in an explicit
>>> instantiation;
>>> > if this declaration is meant to be a class definition,
>>> > remove the 'template' keyword
>>> > template struct pr15466a { int a; };
>>> > ~~~~~~~~~ ^
>>> > t2.cpp:2:17: error: redefinition of 'pr15466a' as different kind of
>>> symbol
>>> > template struct pr15466a { int a; };
>>> > ^
>>> > t2.cpp:1:27: note: previous definition is here
>>> > template <class T> struct pr15466a;
>>> > ^
>>> >
>>> > As pr15466a in this example is already known as a template, maybe a
>>> message
>>> > like "missing argumet list" is more appropriate?
>>>
>>> That makes sense to me, but I think it's somewhat orthogonal to this
>>> bug fix. We should also produce a "missing template parameter list"
>>> diagnostic for a case like:
>>>
>>> template <class T> struct pr15466b;
>>> struct pr15466b { int a; };
>>>
>>
>> Interesting. The general logic of the bug fix is that if a declaration
>> starts as if a template instantiation, but provides a body, then it is
>> likely meant to be either a non-template definition or a template
>> specialization.
>> We decided to select one of these two alternatives based on whether
>> template arguments are explicitly provided.
>>
>> It sounds like you are suggesting to also consider the fact that the
>> existing template may not have been defined as an alternative, in which
>> case we should treat cases lacking explicit arguments as mis-entered
>> template definitions (?).
>>
>> In this latter case, with our recovering from failure by removing the
>> template keyword (if no argument is provided), the issue becomes the same
>> as the case that Richard brings up (without the 'template' keyword).
>>
>> The only adjustment that this bug fix could do for the case you presented
>> is to not produce the "cannot be defined in an explicit instantiation"
>> diagnosis when the existing template does not have a definition.
>>
>> In other words, would you prefer the following diagnosis to the current
>> one (above)?
>>
>> > t2.cpp:2:17: error: redefinition of 'pr15466a' as different kind of
>> symbol
>> > template struct pr15466a { int a; };
>> > ^
>> > t2.cpp:1:27: note: previous definition is here
>> > template <class T> struct pr15466a;
>> > ^
>>
>> If so, I can quickly update the bug fix appropriately. If not, then the
>> issue is probably whether and how the "redefinition" diagnosis should be
>> issued...
>>
>> Thanks,
>> -- Larisse.
>>
>>
>>
>>> > 2013/6/22 Larisse Voufo <lvoufo at google.com>
>>> >>
>>> >> Problem solved. r184577.
>>> >> I hope I got everything right.
>>> >> Thanks,
>>> >> -- Larisse.
>>> >>
>>> >>
>>> >> On Wed, Jun 19, 2013 at 2:17 PM, John McCall <rjmccall at apple.com>
>>> wrote:
>>> >>>
>>> >>> On Jun 19, 2013, at 2:12 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>> >>> > On Wed, Jun 19, 2013 at 11:42 AM, John McCall <rjmccall at apple.com>
>>> >>> > wrote:
>>> >>> >> On Jun 19, 2013, at 10:59 AM, Larisse Voufo <lvoufo at google.com>
>>> wrote:
>>> >>> >>
>>> >>> >> On Wed, Jun 19, 2013 at 10:43 AM, John McCall <rjmccall at apple.com
>>> >
>>> >>> >> wrote:
>>> >>> >>>
>>> >>> >>> On Jun 19, 2013, at 9:15 AM, Larisse Voufo <lvoufo at google.com>
>>> wrote:
>>> >>> >>>> Just out of curiosity, I have noticed that Clang currently
>>> allows
>>> >>> >>>> the
>>> >>> >>>> following program:
>>> >>> >>>>
>>> >>> >>>> template<typename T> T f() { return 13; }
>>> >>> >>>> template int f() { return 1; }
>>> >>> >>>>
>>> >>> >>>> It essentially parses the body of the explicit instantiation
>>> only to
>>> >>> >>>> ignore it.
>>> >>> >>>> Was this a conscious decision?
>>> >>> >>
>>> >>> >>
>>> >>> >> I just spoke to Richard Smith about this, and it appears that the
>>> >>> >> current
>>> >>> >> behavior is a bit different from the way I just described it
>>> above.
>>> >>> >> The declaration for the template instantiation is parsed, but the
>>> >>> >> 'template'
>>> >>> >> keyword is ignored, which leads to two different behaviors for the
>>> >>> >> calls f()
>>> >>> >> and f<int>().
>>> >>> >> While f() returns 1, f<int>() returns 13. f() picks up the
>>> declaration
>>> >>> >> "int
>>> >>> >> f() { return 1; }" while f<int>() picks up the template
>>> declaration
>>> >>> >> and
>>> >>> >> implicitly instantiates it.
>>> >>> >>
>>> >>> >>>
>>> >>> >>>
>>> >>> >>> No; please file a bug. You cannot define a function in an
>>> explicit
>>> >>> >>> instantiation.
>>> >>> >>
>>> >>> >>
>>> >>> >> I can quickly fix this or submit a patch. Should I still file a
>>> bug?
>>> >>> >>
>>> >>> >>
>>> >>> >> Fixing it is better. :)
>>> >>> >>
>>> >>> >>>> GCC 4.6.3 rejects the program with "expected ‘;’ before ‘{’
>>> token".
>>> >>> >>>
>>> >>> >>> Well, hopefully we can do better than that.
>>> >>> >>
>>> >>> >>
>>> >>> >> What do you have in mind?
>>> >>> >>
>>> >>> >>
>>> >>> >> "error: cannot implement a function in an explicit instantiation"
>>> >>> >>
>>> >>> >> or something along those lines.
>>> >>> >
>>> >>> > My suggestion was:
>>> >>> > * If the declarator-id is not a template-id, issue a "function
>>> cannot
>>> >>> > be defined in an explicit instantiation" diagnostic and recover by
>>> >>> > ignoring the 'template' keyword
>>> >>> > * If the declarator-id is a template-id, issue an "explicit
>>> >>> > specialization requires 'template<>'" diagnostic with a fixit to
>>> add
>>> >>> > the <>, and recover as if it were an explicit specialization
>>> >>>
>>> >>> Ah, yes, I wasn't thinking about the different recovery paths.
>>> >>> Good point.
>>> >>>
>>> >>> John.
>>> >>
>>> >>
>>> >>
>>> >> _______________________________________________
>>> >> cfe-dev mailing list
>>> >> cfe-dev at cs.uiuc.edu
>>> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>> >>
>>> >
>>> >
>>> >
>>> > --
>>> > Thanks,
>>> > --Serge
>>>
>>
>>
>
>
> --
> Thanks,
> --Serge
>
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