Sebastian Graf authored Mar 05, 2021 and Marge Bot committed Mar 14, 2021

This patch makes `guessConLikeUnivTyArgsFromResTy` consider required
Thetas of PatSynCons, by treating them as Wanted constraints to be
discharged with the constraints from the Nabla's TyState and saying
"does not match the match type" if the Wanted constraints are unsoluble.
It calls out into a new function `GHC.Tc.Solver.tcCheckWanteds` to do
so.

In pushing the failure logic around call sites of `initTcDsForSolver`
inside it by panicking, I realised that there was a bunch of dead code
surrounding `pmTopMoraliseType`: I was successfully able to delete the
`NoChange` data constructor of `TopNormaliseTypeResult`.

The details are in `Note [Matching against a ConLike result type]` and
`Note [Instantiating a ConLike].

The regression test is in `T19475`. It's pretty much a fork of `T14422`
at the moment.

Co-authored-by: Cale Gibbard <cgibbard@gmail.com>