Special-case implicit params in superclass expansion
This issue came up in Trac #11480, and is documented in Note [When superclasses help] in TcRnTypes. We were getting a spurious warning T11480.hs:1:1: warning: solveWanteds: too many iterations (limit = 4) The fix is easy. A bit of refactoring along the way. The original bug report in Trac #11480 appears to work fine in HEAD and the 8.0 branch but I added a regression test in this commit as well.
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