[Feature] Add a builtin for indexing into parameter packs. Patch by Louis Dionne.

This patch adds a __nth_element builtin that allows fetching the n-th type of a
parameter pack with very little compile-time overhead. The patch was inspired by
r252036 and r252115 by David Majnemer, which add a similar __make_integer_seq
builtin for efficiently creating a std::integer_sequence.

Reviewed as D15421. http://reviews.llvm.org/D15421



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@274316 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 16 insertions, 0 deletions

diff --git a/test/PCH/type_pack_element.cpp b/test/PCH/type_pack_element.cpp
new file mode 100644
index 0000000000..c4ed6c81e2
--- /dev/null
+++ b/test/PCH/type_pack_element.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++14 -x c++-header %s -emit-pch -o %t.pch
+// RUN: %clang_cc1 -std=c++14 -x c++ /dev/null -include-pch %t.pch
+
+template <int i>
+struct X { };
+
+using SizeT = decltype(sizeof(int));
+
+template <SizeT i, typename ...T>
+using TypePackElement = __type_pack_element<i, T...>;
+
+void fn1() {
+  X<0> x0 = TypePackElement<0, X<0>, X<1>, X<2>>{};
+  X<1> x1 = TypePackElement<1, X<0>, X<1>, X<2>>{};
+  X<2> x2 = TypePackElement<2, X<0>, X<1>, X<2>>{};
+}