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

[Richard Smith via cfe-dev]

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_1ClassTemplateDecl.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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