address Pavel feedback

Author: sophiapoirier

include/swift/Sema/ConstraintSystem.h

@@ -6269,7 +6269,7 @@ Type isPlaceholderVar(PatternBindingDecl *PB);
 void dumpAnchor(ASTNode anchor, SourceManager *SM, raw_ostream &out);
 
 /// Check whether the type is a tuple consisting of a single unlabeled element
-/// of \c PackExpansionType.
+/// of \c PackExpansionType or a type variable that represents a pack expansion type.
 bool isSingleUnlabeledPackExpansionTuple(Type type);
 
 /// \returns null if \c type is not a single unlabeled pack expansion tuple.

lib/Sema/CSSimplify.cpp

@@ -138,7 +138,7 @@ Type constraints::getPatternTypeOfSingleUnlabeledPackExpansionTuple(Type type) {
   if (isSingleUnlabeledPackExpansionTuple(type)) {
     auto tuple = type->getRValueType()->castTo<TupleType>();
     const auto &tupleElement = tuple->getElementType(0);
-    if (auto *expansion = tupleElement->castTo<PackExpansionType>()) {
+    if (auto *expansion = tupleElement->getAs<PackExpansionType>()) {
       return expansion->getPatternType();
     }
     if (auto *typeVar = tupleElement->getAs<TypeVariableType>()) {