[cfe-dev] Explicit instantiation with body?

[Richard Smith]

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