Multiple contexts are mis-handled

[nibro]

HSE complains about multiple class contexts in type signatures.

multipleCtx :: Eq a => Show a => a
multipleCtx = undefined

This should be accepted if explicit forall is enabled by any extension. HSE rejects it regardless of flags.

However, if the second context is parenthesised, i.e.

multipleCtxt :: Eq a => (Show a => a)

then HSE accepts it even without e.g. Rank2Types.

HSE's behavior should be the same with or without the parenthesis, and should exclude the type in both cases with no flags, and accept it in both cases when Rank2Types is not on.

[nibro]

As an update, I've now fixed it so that the second version (with parentheses) is accepted when e.g. Rank2Types is enabled, but not when it isn't. After digging in GHC a bit, I think the first version (without parentheses) is a bug, and should not parse correctly at all. I have posted a message to the ghc-users list asking for input on this, before I close this as fixed.

[nibro]

The news is that it should be allowed without the parentheses, so I've changed the production to allow chained ctypes. Case closed.