[PATCH] D15421: [Feature] Add a builtin for indexing into parameter packs

[Louis Dionne via cfe-commits]

ldionne added a comment.

We should probably consider changing the name from `__nth_element` to something else, perhaps something that contains the word "pack". This is especially true if we are to implement other intrinsics for manipulating parameter packs, like slicing (as proposed by Eric Fiselier). Unless someone with a clearer view of the project suggests something, I would probably go with `__pack_element` (and then we could add `__pack_slice`, etc...). I have no strong preference.

This patch also raises the question of whether we should have a more separated way of adding builtin templates. My job here was very easy because I just copied David's revision and tweaked a couple of things here and there, but I could imagine it not being obvious at all otherwise. Also, and most importantly, if we add other builtin templates, do we want these builtins to "clutter" the rest of the code instead of being somewhat isolated? I don't know the answer to these questions, but I'd just like to point them out.


================
Comment at: lib/AST/DeclTemplate.cpp:1245-1249
@@ +1244,7 @@
+createNthElement(const ASTContext &C, DeclContext *DC) {
+  // typename IndexType
+  auto *IndexType = TemplateTypeParmDecl::Create(
+      C, DC, SourceLocation(), SourceLocation(), /*Depth=*/0, /*Position=*/0,
+      /*Id=*/nullptr, /*Typename=*/true, /*ParameterPack=*/false);
+  IndexType->setImplicit(true);
+
----------------
It would be simpler and better (IMO) to have `template <std::size_t Index, typename ...T>` instead of `template <typename IndexType, IndexType Index, typename ...T>`, but I couldn't figure out how to create a non-type template parameter. Hence, I just adapted the code from David's revision for `__make_integer_seq`.

================
Comment at: lib/Sema/SemaTemplate.cpp:2101-2102
@@ +2100,4 @@
+    // We simply return the type at index `Index`.
+    // TODO:
+    // What are the implications of calling .getExtValue() on an APSInt?
+    assert(Index.getExtValue() == Index &&
----------------
More generally, what is the proper way to retrieve a non-type template argument as a number that I can then use as a normal `std::size_t` (or similar) in the code? I really just want to retrieve the index, which is given as a non-type template parameter to `__nth_element`, and convert it to a numeric index `n` in order to fetch the `n`-th element of the parameter pack.


http://reviews.llvm.org/D15421