Add optimization to sizeof...(X) handling: if none of parameter pack X's

corresponding arguments are unexpanded pack expansions, we can compute the
result without substituting them. This significantly improves the memory usage
and performance of make_integer_sequence implementations that do this kind of
thing:

  using result = integer_sequence<T, Ns ..., sizeof...(Ns) + Ns ...>;

... but note that such an implementation will still perform O(sizeof...(Ns)^2)
work while building the second pack expansion (we just have a somewhat lower
constant now).

In principle we could get this down to linear time by caching whether the
number of expansions of a pack is constant, or checking whether we're within an
alias template before scanning the pack for pack expansions (since that's the
only case in which we do substitutions within a dependent context at the
moment), but this patch doesn't attempt that.

llvm-svn: 284653

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/include/clang/Sema/Sema.h b/clang/include/clang/Sema/Sema.h
index 78080f5e4a4..78be700ec3a 100644
--- a/clang/include/clang/Sema/Sema.h
+++ b/clang/include/clang/Sema/Sema.h
@@ -6499,6 +6499,14 @@ public:
       SourceLocation &Ellipsis,
       Optional<unsigned> &NumExpansions) const;
 
+  /// Given a template argument that contains an unexpanded parameter pack, but
+  /// which has already been substituted, attempt to determine the number of
+  /// elements that will be produced once this argument is fully-expanded.
+  ///
+  /// This is intended for use when transforming 'sizeof...(Arg)' in order to
+  /// avoid actually expanding the pack where possible.
+  Optional<unsigned> getFullyPackExpandedSize(TemplateArgument Arg);
+
   //===--------------------------------------------------------------------===//
   // C++ Template Argument Deduction (C++ [temp.deduct])
   //===--------------------------------------------------------------------===//