How to perform template substitution in the clang library?

Question

Specifically, we have a C++ source file like this:

template <int n>
struct N {};

struct B {
    template <typename M>
    using A = typename std::conditional<std::is_same<M, N<4>>::value,
                                        int*, void*>::type;
};

template <typename T, T value>
struct F : B {};

template <>
struct F<decltype(&fopen), &fopen> : B {
    template <typename M>
    using A = double*;
};

template <>
struct F<decltype(&fclose), &fclose> : B {
    template <typename M>
    using A = typename std::conditional<std::is_same<M, N<16>>::value,
                                        void*, char**>::type;
};

// More specialization of 'F' follows.

It is easy to find the ClassTemplateDecls of N and F, and the QualType and FunctionDecl of the function pointers &fopen, &fclose, etc. But the problem is how to substitute these arguments into N, F and F::A without modifying the source code.

The question is:

• How do I evaluate F<decltype(&fprintf), &fprintf>::A<N<4>> and know that it is an int*?
• How do I evaluate F<decltype(&fopen), &fopen>::A<N<7>> and know that it is a double*?
• and so on...

Answer 1

I've got a partial solution, the only caveat is that, I can't get std::is_same<N<4>, N<4>>::value to return true. Well I can live with that, as I could just define a constexpr method which operates on the values directly. But I hope someone could provide a correct answer for this.

I have put the complete solution and the modified input to https://gist.github.com/4178490.

---

I've found that to substitute arguments into a class template and instantiate it, one would:

1. Use the arguments to turn the ClassTemplateDecl into ClassTemplateSpecializationDecl, and
2. Instantiate the specialization using Sema::InstantiateClass method.

The method Sema::RequireCompleteType does indirectly call InstantiateClass, and requires less input, so I call this method instead. Therefore, we would write:

/**
 * Instantiate a class template.
 */
ClassTemplateSpecializationDecl* instantiate(ASTContext& ast, Sema& sema, 
                                             DeclContext* parent,
                                             ClassTemplateDecl* decl, 
                                             ArrayRef<TemplateArgument> args) {
   void* ins_point;
   auto retval = decl->findSpecialization(args.data(), args.size(), ins_point);
   if (retval == nullptr) {
       retval = ClassTemplateSpecializationDecl::Create(ast, TTK_Class, parent, 
                                                        {}, {}, decl,
                                                        args.data(), args.size(),
                                                        nullptr);
       decl->AddSpecialization(retval, ins_point);
   }
   bool is_incomplete = sema.RequireCompleteType({}, ast.getTypeDeclType(retval),
                                                 diag::err_incomplete_type);
   return is_incomplete ? nullptr : retval;
}

This method only works for ClassTemplateDecl. In the question we also have a TypeAliasTemplateDecl. For this, I am going to invoke the TemplateDeclInstantiator directly, since this is the only object which knows a TypeAliasTemplateDecl. Perhaps this method also works with ClassTemplateDecl, but I can't be sure as it seems not enough work is done using TemplateDeclInstantiator alone.

/**
 * Instantiate a template alias (`template <...> using Foo = ...`).
 */
TypeAliasDecl* instantiate(ASTContext& ast, Sema& sema, DeclContext* parent,
                           TypeAliasTemplateDecl* decl,
                           ArrayRef<TemplateArgument> args) {
    auto args_count = static_cast<unsigned>(args.size());
    TemplateArgumentList arg_list {TemplateArgumentList::OnStack,
                                   args.data(), args_count};
    MultiLevelTemplateArgumentList multi_arg_list {arg_list};
    TemplateDeclInstantiator instantiator {sema, parent, multi_arg_list};
    auto instantiated = instantiator.Visit(decl);
    if (auto inst_decl = dyn_cast<TypeAliasTemplateDecl>(instantiated)) {
        return inst_decl->getTemplatedDecl();
    }
    return nullptr;
}

(I skipped FunctionTemplateDecl, it is out of the scope of my question.)