Fix 'SPECIALISE instance'
Trac #12944 showed that the DsBinds code that implemented a SPECIALISE pragma was inadequate if the constraints solving added let-bindings for dictionaries. The result was that we ended up with an unbound dictionary in a DFunUnfolding -- and Lint didn't even check for that! Fixing this was not entirely straightforward * In DsBinds.dsSpec we use a new function TcEvidence.collectHsWrapBinders to pick off the lambda binders from the HsWapper * dsWrapper now returns a (CoreExpr -> CoreExpr) function * CoreUnfold.specUnfolding now takes a (CoreExpr -> CoreExpr) function it can use to specialise the unfolding. On the whole the code is simpler than before.
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