[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

2 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 8650ce75769..27ef02f5b06 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -6428,6 +6428,9 @@ static Decl *getPredefinedDecl(ASTContext &Context, PredefinedDeclIDs ID) {
 
   case PREDEF_DECL_CF_CONSTANT_STRING_TAG_ID:
     return Context.getCFConstantStringTagDecl();
+
+  case PREDEF_DECL_TYPE_PACK_ELEMENT_ID:
+    return Context.getTypePackElementDecl();
   }
   llvm_unreachable("PredefinedDeclIDs unknown enum value");
 }
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 4fa016cc896..ef51225b60c 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -4191,6 +4191,8 @@ uint64_t ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
                      PREDEF_DECL_CF_CONSTANT_STRING_ID);
   RegisterPredefDecl(Context.CFConstantStringTagDecl,
                      PREDEF_DECL_CF_CONSTANT_STRING_TAG_ID);
+  RegisterPredefDecl(Context.TypePackElementDecl,
+                     PREDEF_DECL_TYPE_PACK_ELEMENT_ID);
 
   // Build a record containing all of the tentative definitions in this file, in
   // TentativeDefinitions order.  Generally, this record will be empty for