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

[Richard Smith via cfe-dev]

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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