[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

llvm-svn: 274316

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
index a4734a28124..2ade6df9456 100644
--- a/clang/lib/Lex/PPMacroExpansion.cpp
+++ b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -1695,6 +1695,7 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
           const LangOptions &LangOpts = getLangOpts();
           return llvm::StringSwitch<bool>(II->getName())
                       .Case("__make_integer_seq", LangOpts.CPlusPlus)
+                      .Case("__type_pack_element", LangOpts.CPlusPlus)
                       .Default(false);
         }
       });