[AST] Allow tuple types with consecutive pack expansions.

Author: hborla

lib/AST/ASTContext.cpp

@@ -3136,15 +3136,6 @@ TupleType *TupleType::get(ArrayRef<TupleTypeElt> Fields, const ASTContext &C) {
     properties |= eltTy->getRecursiveProperties();
   }
 
-  // Enforce an invariant.
-  for (unsigned i = 0, e = Fields.size(); i < e; ++i) {
-    if (Fields[i].getType()->is<PackExpansionType>()) {
-      assert(i == e - 1 || Fields[i + 1].hasName() &&
-             "Tuple element with pack expansion type cannot be followed "
-             "by an unlabeled element");
-    }
-  }
-
   auto arena = getArena(properties);
 
   void *InsertPos = nullptr;

test/Constraints/pack-expansion-expressions.swift

@@ -8,8 +8,8 @@ func prepend<First, Rest...>(value: First, to rest: Rest...) -> (First, Rest...)
   return (value, rest...)
 }
 
-func concatenate<T..., U...>(_ first: T..., with second: U...) -> ((T, U)...) {
-  return ((first, second)...)
+func concatenate<T..., U...>(_ first: T..., with second: U...) -> (T..., U...) {
+  return (first..., second...)
 }
 
 func zip<T..., U...>(_ first: T..., with second: U...) -> ((T, U)...) {