Add support for class templates

[jbcoe]

Currently fails to correctly get class template argument count

[jbcoe]

cursor.get_num_template_arguments() is currently returning -1 which looks like a bug, maybe in libclang or cindex.py.

Maybe we can try building libclang locally to see what's going wrong?

https://clang.llvm.org/get_started.html has instructions.

[jbcoe]

clang has

int clang_Cursor_getNumTemplateArguments(CXCursor C) {
  CXCursorKind kind = clang_getCursorKind(C);
  if (kind != CXCursor_FunctionDecl && kind != CXCursor_StructDecl &&
      kind != CXCursor_ClassDecl &&
      kind != CXCursor_ClassTemplatePartialSpecialization) {
    return -1;
  }

  if (const auto *FD =
          llvm::dyn_cast_if_present<clang::FunctionDecl>(getCursorDecl(C))) {
    const FunctionTemplateSpecializationInfo *SpecInfo =
        FD->getTemplateSpecializationInfo();
    if (!SpecInfo) {
      return -1;
    }
    return SpecInfo->TemplateArguments->size();
  }
    
  if (const auto *SD =
          llvm::dyn_cast_if_present<clang::ClassTemplateSpecializationDecl>(
              getCursorDecl(C))) {
    return SD->getTemplateArgs().size();
  }
    
  return -1;
} 

which won't work for CXCursor_ClassTemplate which is what our Python code picks up.

I'm not sure what we should do differently.

[jbcoe]

This clang-commit looks relevant: https://reviews.llvm.org/D5621

[jbcoe]

The issue here is the distinction between template arguments and template parameters.

libclang's docs in clang/include/clang-c/Index.h say

/**
 * Returns the number of template args of a function, struct, or class decl
 * representing a template specialization.
 *
 * If the argument cursor cannot be converted into a template function
 * declaration, -1 is returned.
 *
 * For example, for the following declaration and specialization:
 *   template <typename T, int kInt, bool kBool>
 *   void foo() { ... }
 *
 *   template <>
 *   void foo<float, -7, true>();
 *
 * The value 3 would be returned from this call.
 */
CINDEX_LINKAGE int clang_Cursor_getNumTemplateArguments(CXCursor C);

I think we need to add and expose clang_Cursor_getNumTemplateParameters to access parameters from py_cppmodel.