19079 DerivingVia: incorrectly accepts deriving via types with differing runtime representations

1 files changed, 5 insertions, 3 deletions

diff --git a/docs/users_guide/exts/deriving_via.rst b/docs/users_guide/exts/deriving_via.rst
index cc590987ea..26ff719780 100644
--- a/docs/users_guide/exts/deriving_via.rst
+++ b/docs/users_guide/exts/deriving_via.rst
@@ -13,9 +13,11 @@ Deriving via
     :since: 8.6.1
 
 This allows ``deriving`` a class instance for a type by specifying
-another type of equal runtime representation (such that there exists a
-``Coercible`` instance between the two: see :ref:`coercible`) that is
-already an instance of the that class.
+another type that is already an instance of that class.
+This only makes sense if the methods have identical runtime representations,
+in the sense that coerce (see The ``Coercible`` constraint) can convert
+the existing implementation into the desired implementation.
+The generated code will be rejected with a type error otherwise.
 
 :extension:`DerivingVia` is indicated by the use of the ``via``
 deriving strategy. ``via`` requires specifying another type (the ``via`` type)