I would say you do indeed need impredicative types to compile the instance you request -- you need to instantiate a type variable with a forall-type. But, whether or not we should require users to actually specify ImpredicativeTypes to compile code that we're generating is another question. And, the error message is terrible, regardless. I'll post. Thanks for bringing this up! Richard On Dec 26, 2013, at 10:30 PM, Nathan Howell wrote:

I can't file a trac ticket at the meny but wanted to see if this is a bug... This is an excerpt from llvm-general that compiles on 7.6.3 but doesn't without ImpredicativeTypes enabled on HEAD: https://gist.github.com/NathanHowell/8138977

thanks, -n

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