[cfe-dev] Why ClassTemplatePartialSpecializationDecl derived from ClassTemplateSpecializationDecl and not from ClassTemplateDecl ?

[Roman Popov via cfe-dev]

Oh, now I see it.  Missed this point in first reply.

Thanks!

2017-10-05 17:16 GMT-07:00 Richard Smith <richard at metafoo.co.uk>:

> On 5 October 2017 at 14:55, Roman Popov via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Thanks for response.
>>
>> However, having the ClassTemplatePartialSpecializationDecl (or, in the
>>> template-around-a-ClassTemplateSpecializationDecl case, the
>>> ClassTemplateSpecializationDecl) be a TypeDecl is actually a sensible
>>> and essentially necessary choice -- it represents the type of the "injected
>>> class name" within the template, that is, the (dependent) type of "*this"
>>> within the definition of the template and its members.
>>
>>
>>
>> But in this case ClassTemplateDecl should also be a TypeDecl?  But it is
>> not according to diagram:
>> https://clang.llvm.org/doxygen/classclang_1_1ClassTemplateDe
>> cl.html#details
>>
>
> A ClassTemplateDecl is a wrapper around a CXXRecordDecl, which is a
> TypeDecl.
>
>
>> ... we need to identify that A<T*> refers to the (dependent, not yet
>>> instantiated) class template partial specialization we're currently
>>> defining in order to look up X and determine that it's a type, which allows
>>> "A<T*>::X *y;" to be parsed as a declaration rather than as a
>>> multiplication expression.
>>
>>
>> I understand. I don't yet looked into the parser code, so it was not
>> evident for me that it influences AST type hierarchy.
>>
>> 2017-10-05 14:02 GMT-07:00 Richard Smith <richard at metafoo.co.uk>:
>>
>>> On 5 October 2017 at 12:47, Roman Popov via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>>
>>>> Hi all,
>>>>
>>>> One thing that confused me in Clang AST is
>>>> that ClassTemplatePartialSpecializationDecl is derived
>>>> from ClassTemplateSpecializationDecl, and thus is a TypeDecl.
>>>>
>>>> What is the reasoning behind this decision?
>>>>
>>>
>>> I think the original design idea was that a
>>> ClassTemplatePartialSpecializationDecl is a templated form of
>>> ClassTemplateSpecializationDecl (in the same way that a
>>> ClassTemplateDecl is a templated form of CXXRecordDecl etc.) That makes
>>> sense if you think about it -- a partial specialization is a template whose
>>> templated declaration is a specialization of a template. Only it's not
>>> quite implemented like that, because ClassTemplatePartialSpecializationDecl
>>> isn't a template wrapper *around* a ClassTemplateSpecializationDecl --
>>> it's actually both a class and a template all at once.
>>>
>>> This non-uniformity of template representation is quite awkward, and I
>>> think we know enough about how it works out now to consider it a minor
>>> design error.
>>>
>>> However, having the ClassTemplatePartialSpecializationDecl (or, in the
>>> template-around-a-ClassTemplateSpecializationDecl case, the
>>> ClassTemplateSpecializationDecl) be a TypeDecl is actually a sensible
>>> and essentially necessary choice -- it represents the type of the "injected
>>> class name" within the template, that is, the (dependent) type of "*this"
>>> within the definition of the template and its members.
>>>
>>> I thought that partial specialization is still a template in C++, and
>>>> only when it is fully specialized it becomes a real class.
>>>>
>>>
>>> Yes, but correct semantic analysis of C++ templates requires modeling a
>>> family of dependent types too. Eg, within:
>>>
>>> template<typename T> struct A;
>>> template<typename T> struct A<T*> {
>>>   using X = int;
>>>   void f() {
>>>     A<T*>::X *y;
>>>   }
>>> };
>>>
>>> ... we need to identify that A<T*> refers to the (dependent, not yet
>>> instantiated) class template partial specialization we're currently
>>> defining in order to look up X and determine that it's a type, which allows
>>> "A<T*>::X *y;" to be parsed as a declaration rather than as a
>>> multiplication expression.
>>>
>>
>>
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>>
>
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