diff --git a/llvm/include/llvm/ADT/PointerUnion.h b/llvm/include/llvm/ADT/PointerUnion.h
index 24caa960c718d..ca0e1ed3d49d1 100644
--- a/llvm/include/llvm/ADT/PointerUnion.h
+++ b/llvm/include/llvm/ADT/PointerUnion.h
@@ -88,9 +88,6 @@ namespace pointer_union_detail {
   };
 }
 
-// This is a forward declaration of CastInfoPointerUnionImpl
-// Refer to its definition below for further details
-template <typename... PTs> struct CastInfoPointerUnionImpl;
 /// A discriminated union of two or more pointer types, with the discriminator
 /// in the low bit of the pointer.
 ///