[cfe-dev] Explicit instantiation with body?

[Serge Pavlov]

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.
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?

Thanks,
--Serge



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.
>
>
>
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-- 
Thanks,
--Serge
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