+Note [Flags and equational constraints]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If there are equational constraints, we take account of them
+regardless of flag settings; -XGADTs etc applies only to the
+*definition* of a data type.
+
+An alternative would be also to reject a program that *used*
+constructors with equational constraints. But want we should avoid at
+all costs is simply to *ignore* the constraints, since that gives
+incomprehensible errors (Trac #2004).
+