Re: [C++ Patch/RFC] PR 71569 ("[6/7/8 regression] Crash: External definition of template member from template struct")

[Jason Merrill <jason at redhat dot com>]

On Wed, Feb 28, 2018 at 11:18 AM, Paolo Carlini
<paolo.carlini@oracle.com> wrote:
> On 28/02/2018 16:53, Jason Merrill wrote:
>> On Wed, Feb 28, 2018 at 9:36 AM, Paolo Carlini <paolo.carlini@oracle.com>
>> wrote:
>>>
>>> in this P2 ICE on invalid we fail to diagnose an ill-formed variable
>>> template partial specialization and the assert at the beginning of
>>> determine_specialization triggers. Indeed, we didn't diagnose the problem
>>> via check_specialization_scope (there are no template <>, thus
>>> begin_specialization isn't involved). Simply adding a specific check to
>>> check_explicit_specialization exactly when we recognize a variable template
>>> partial specialization seems a safe thing to do and passes testing. Not sure
>>> if the test itself is as simple as possible or is better done somewhere
>>> else...
>>
>> Hmm, this doesn't look like the user intended to write a partial
>> specialization, but rather that they were trying to define the member
>> template and wrote the template arguments out by mistake.
>
> I see your point. If I understand correctly, in such cases clang somehow
> talks about both possibilities:
>
> error: variable template partial specialization does not specialize any
> template argument; to define the primary template, remove the template
> argument list
>
> What do you think, should we do something similar?

Sounds good.

> (by the way, yesterday
> got a bit confused because in other vaguely related cases clang also has
> this kind of two-part error message and we don't. Eg, I cooked up:
>
> template <class T>
> class Foo
> {
>   template <class U>
>   class Bar;
> };
>
> template<class T>
> template<class U>
> class Foo<T>::Bar<U>;

And the same would be good for this example.

Jason