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was fixed along with Trac #10224

Look through nested foralls when checking the validity of a partial type
signature. The combination of D836 and D613 prompts this change.

Test Plan: The test T10519 must pass

Reviewers: simonpj, alanz, austin

Reviewed By: simonpj, alanz, austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D994

GHC Trac Issues: #10519

Change the following tests from expect_fail to expect_broken: and list
the ticket number:

    * driver/sigof03m/sigof03 (#9252)
    * driver/static001 (#8127)
    * partial-sigs/should_compile/EqualityConstraint (#9478)
    * partial-sigs/should_compile/ExtraNumAMROn (#9478)
    * partial-sigs/should_compile/PatBind2 (#9478)
    * partial-sigs/should_fail/TidyClash2 (#9478)
    * simplCore/should_compile/T8832 (#8832)

The following tests are still marked as expect_fail, but it is not
clearly documented why so:

    * gadt/lazypatok
    * indexed-types/should_fail/SkolemOccursLoop

All other expect_fail tests are only expected to fail on either a
certain platform/os or for a certain way only.

Differential Revision: https://phabricator.haskell.org/D966

This reverts multiple commits from Simon:

  - 04a484ea Test Trac #10359
  - a9ccd37a Test Trac #10403
  - c0aae6f6 Test Trac #10248
  - eb6ca851 Make the "matchable-given" check happen first
  - ca173aa3 Add a case to checkValidTyCon
  - 51cbad15 Update haddock submodule
  - 6e1174da Separate transCloVarSet from fixVarSet
  - a8493e03 Fix imports in HscMain (stage2)
  - a154944b Two wibbles to fix the build
  - 5910a1bc Change in capitalisation of error msg
  - 130e93aa Refactor tuple constraints
  - 8da785d5 Delete commented-out line

These break the build by causing Haddock to fail mysteriously when
trying to examine GHC.Prim it seems.

The idea was promted by Trac #9939, but it was Christmas, so I did
some recreational programming that went much further.

The idea is to warn when a constraint in a user-supplied context is
redundant.  Everything is described in detail in
  Note [Tracking redundant constraints]
in TcSimplify.

Main changes:

 * The new ic_status field in an implication, of type ImplicStatus.
   It replaces ic_insol, and includes information about redundant
   constraints.

 * New function TcSimplify.setImplicationStatus sets the ic_status.

 * TcSigInfo has sig_report_redundant field to say whenther a
   redundant constraint should be reported; and similarly
   the FunSigCtxt constructor of UserTypeCtxt

 * EvBinds has a field eb_is_given, to record whether it is a given
   or wanted binding. Some consequential chagnes to creating an evidence
   binding (so that we record whether it is given or wanted).

 * AbsBinds field abs_ev_binds is now a *list* of TcEvBiinds;
   see Note [Typechecking plan for instance declarations] in
   TcInstDcls

 * Some significant changes to the type checking of instance
   declarations; Note [Typechecking plan for instance declarations]
   in TcInstDcls.

 * I found that TcErrors.relevantBindings was failing to zonk the
   origin of the constraint it was looking at, and hence failing to
   find some relevant bindings.  Easy to fix, and orthogonal to
   everything else, but hard to disentangle.

Some minor refactorig:

 * TcMType.newSimpleWanteds moves to Inst, renamed as newWanteds

 * TcClassDcl and TcInstDcls now have their own code for typechecking
   a method body, rather than sharing a single function. The shared
   function (ws TcClassDcl.tcInstanceMethodBody) didn't have much code
   and the differences were growing confusing.

 * Add new function TcRnMonad.pushLevelAndCaptureConstraints, and
   use it

 * Add new function Bag.catBagMaybes, and use it in TcSimplify

Summary:
Add support for Partial Type Signatures, i.e. holes in types, see:
https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures

This requires an update to the Haddock submodule.

Test Plan: validate

Reviewers: austin, goldfire, simonpj

Reviewed By: simonpj

Subscribers: thomie, Iceland_jack, dominique.devriese, simonmar, carter, goldfire

Differential Revision: https://phabricator.haskell.org/D168

GHC Trac Issues: #9478