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

[Roman Popov via cfe-dev]

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


... 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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