Help needed: parsing pattern synonym contexts

[Dr. ERDI Gergo]

On Tue, 24 Jun 2014, Simon Peyton Jones wrote:

> What do you mean by "doesn't work"?  Crashes? Fails to build with some 
> error?  Builds but doesn't parse what you expect?

Yeah, sorry, when I wrote

>> fails with a parse error on "::".

what I meant was that the parser builds & runs, but fails with a parse 
error on the double-colon.

I'll get back to you on the other points when I've had time to try them.