Generalised wildcards (PartialTypeSignatures) in the binder type annotation of a RULE cause panics.

Minimal example:

module WildcardInRuleBndrSig where
{-# RULES
"map/empty" forall (f :: a -> _). map f [] = []
  #-}

Output:

WildcardInRuleBndrSig.hs:3:31:ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 7.11.20150209 for x86_64-unknown-linux):
	No skolem info: w__alY[sk]

When a wildcard is generalised over, the error message reporting the inferred type gives some extra info about the type variables occurring in the inferred type. This extra information is retrieved by looking up the skolem information (getSkolemInfo) for the type variables in the enclosing implications (cec_encl). The problem is that there are no enclosing implications in this case, hence the panic.

Note that with the flags -XPartialTypeSignatures and -fno-warn-partial-type-signatures enabled, there is no panic, as no error/warning message is constructed.