Also, the “the instance is sometimes used” problem never materializes if you are careful to always define a new class and a sole fully universal instance for it.

I’ve used this fairly extensively for years, and I know it predates me in the Haskell community by a few years.

It doesn’t solve everything the class alias proposals attempt to solve though: You can’t retroactively define a superclass to an existing class for one; if you went to take Monad and rip it into a Bindable and Returnable class and glue that together with this style of alias, all the implementations of Monad would have to be refactored as well.

However I’m wondering what happens if I import a module which use it. Do I fall into the undecidable world? In particular if a module use UndecidableInstances but only export decidable instances, is this module safe to import?

Are you saying that ContextSynonyms should imply UndecidableInstances? I wonder how difficult it is to implement ContextSynonyms so that UndecidableInstances? is on only locally to the instance definition.

I used it here: http://hackage.haskell.org/packages/archive/ListTree/0.1/doc/html/Data-List-Tree.html

“class (List t, List (ItemM t)) => Tree t”