Canonical way to prevent manual instances
For some classes (Typeable) the user must not provide hand-written instances, for other no hand-written instances in Safe mode (old Typeable) and for others no instances at all (Coercible). Currently the Typeable checks are in tcInstDecls1 in [source:compiler/typecheck/TcInstDcls.lhs]; but a more proper place would be in checkValidInstHead in [source:compiler/typecheck/TcValidity.lhs].
Why file a ticket and not just do it? The latter source location does not know whether the instance is a (stand-alone) deriving instance or a hand-written one, so this needs a bit of refactoring.
|Component||Compiler (Type checker)|