Add a convenience API to build canonical pack types from slices

of canonical tuple type elements.

This has come up more than you might think.

Author: rjmccall

include/swift/AST/Types.h

@@ -6829,6 +6829,7 @@ class PackType final : public TypeBase, public llvm::FoldingSetNode,
 };
 BEGIN_CAN_TYPE_WRAPPER(PackType, Type)
   static CanPackType get(const ASTContext &ctx, ArrayRef<CanType> elements);
+  static CanPackType get(const ASTContext &ctx, CanTupleEltTypeArrayRef elts);
 
   CanType getElementType(unsigned elementNo) const {
     return CanType(getPointer()->getElementType(elementNo));

lib/AST/ASTContext.cpp

@@ -3303,6 +3303,12 @@ CanPackType CanPackType::get(const ASTContext &C, ArrayRef<CanType> elements) {
   return CanPackType(PackType::get(C, ncElements));
 }
 
+CanPackType CanPackType::get(const ASTContext &C,
+                             CanTupleEltTypeArrayRef elements) {
+  SmallVector<Type, 8> ncElements(elements.begin(), elements.end());
+  return CanPackType(PackType::get(C, ncElements));
+}
+
 PackType *PackType::get(const ASTContext &C, ArrayRef<Type> elements) {
   RecursiveTypeProperties properties;
   bool isCanonical = true;

lib/AST/ParameterPack.cpp

@@ -478,13 +478,6 @@ bool SILPackType::containsPackExpansionType() const {
 CanPackType
 CanTupleType::getInducedPackTypeImpl(CanTupleType tuple, unsigned start, unsigned count) {
   assert(start + count <= tuple->getNumElements() && "range out of range");
-
   auto &ctx = tuple->getASTContext();
-  if (count == 0) return CanPackType::get(ctx, {});
-
-  SmallVector<CanType, 4> eltTypes;
-  eltTypes.reserve(count);
-  for (unsigned i = start, e = start + count; i != e; ++i)
-    eltTypes.push_back(tuple.getElementType(i));
-  return CanPackType::get(ctx, eltTypes);
+  return CanPackType::get(ctx, tuple.getElementTypes().slice(start, count));
 }