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This patch began as a modest refactoring of HsType and friends, to
clarify and tidy up exactly where quantification takes place in types.
Although initially driven by making the implementation of wildcards more
tidy (and fixing a number of bugs), I gradually got drawn into a pretty
big process, which I've been doing on and off for quite a long time.

There is one compiler performance regression as a result of all
this, in perf/compiler/T3064.  I still need to look into that.

* The principal driving change is described in Note [HsType binders]
  in HsType.  Well worth reading!

* Those data type changes drive almost everything else.  In particular
  we now statically know where

       (a) implicit quantification only (LHsSigType),
           e.g. in instance declaratios and SPECIALISE signatures

       (b) implicit quantification and wildcards (LHsSigWcType)
           can appear, e.g. in function type signatures

* As part of this change, HsForAllTy is (a) simplified (no wildcards)
  and (b) split into HsForAllTy and HsQualTy.  The two contructors
  appear when and only when the correponding user-level construct
  appears.  Again see Note [HsType binders].

  HsExplicitFlag disappears altogether.

* Other simplifications

     - ExprWithTySig no longer needs an ExprWithTySigOut variant

     - TypeSig no longer needs a PostRn name [name] field
       for wildcards

     - PatSynSig records a LHsSigType rather than the decomposed
       pieces

     - The mysterious 'GenericSig' is now 'ClassOpSig'

* Renamed LHsTyVarBndrs to LHsQTyVars

* There are some uninteresting knock-on changes in Haddock,
  because of the HsSyn changes

I also did a bunch of loosely-related changes:

* We already had type synonyms CoercionN/CoercionR for nominal and
  representational coercions.  I've added similar treatment for

      TcCoercionN/TcCoercionR

      mkWpCastN/mkWpCastN

  All just type synonyms but jolly useful.

* I record-ised ForeignImport and ForeignExport

* I improved the (poor) fix to Trac #10896, by making
  TcTyClsDecls.checkValidTyCl recover from errors, but adding a
  harmless, abstract TyCon to the envt if so.

* I did some significant refactoring in RnEnv.lookupSubBndrOcc,
  for reasons that I have (embarrassingly) now totally forgotten.
  It had to do with something to do with import and export

Updates haddock submodule.

The tests weren't explicit enough about comparing under different
unique allocation strategies. This led to some confusion on my part when
I started overriding flags in `testsuite/mk/test.mk`.
Includes a `.gitignore` rule.

Test Plan: harbormaster

Reviewers: austin, simonmar, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1555

Summary: It's just a docufix.

Test Plan: just a docufix

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1550

* Add stage specific versions of SRC_HC_OPTS. These are currently only
  used for -Werror. The previous combination of GhcStage2HcOpts and
  GhcLibHcOpts didn't apply to utils/*.

* Add stage specific versions of SRC_HC_WARNING_OPTS. These will later be
  used for new warning supression flags that should not be passed to the
  bootstrap compiler.

* Move -Wall (and -Werror) related code back to mk/warnings.mk, where it
  was before 987d5427. Now all warning related code is nicely together.
  Include mk/warnings.mk after mk/custom-settings.mk to make this work.

Reviewed By: bgamari, hvr

Differential Revision: https://phabricator.haskell.org/D1536

This patch is similar to the AMP patch (#8004), which offered two
functions:

  1. Warn when an instance of a class has been given, but the type does
     not have a certain superclass instance
  2. Warn when top-level definitions conflict with future Prelude names

These warnings are issued as part of the new `-Wcompat` warning group.

Reviewers: hvr, ekmett, austin, bgamari

Reviewed By: hvr, ekmett, bgamari

Subscribers: ekmett, thomie

Differential Revision: https://phabricator.haskell.org/D1539

GHC Trac Issues: #11139

This patch does two improvements:

- We now show ToDos in `CoreDoPasses`. This is pretty important,
  otherwise `CoreDoPasses` makes debugging impossible in some cases.

- Before running ToDos we run a cleanup pass on ToDos to remove
  `CoreDoNothing`s and flatten `CoreDoPasses`. This removes a lot of
  noise from `[CoreToDo]` argument passed to plugins.

Reviewers: simonpj, bgamari, austin

Reviewed By: bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1548

In so doing ensure that the help text can't fall out of sync with the
implementation.

Test Plan: Validate and play in ghci

Reviewers: austin, thomie

Reviewed By: austin, thomie

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1538

GHC Trac Issues: #11111

`#if __GLASGOW_HASKELL__ > xxx` macros make little sense inside `base`
currently.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1541

This pulls in new NFData instances for NonEmpty and Semigroup newtype wrappers

Differential Revision: https://phabricator.haskell.org/D1546

Differential Revision: https://phabricator.haskell.org/D1545

In the function `tcUserStmt` in compiler/typecheck/TcRnDriver.hs, before
going over the different ways ('plans') to lift an expression typed at
the prompt into the GHCi monad, `Opt_DeferTypeErrors` is disabled. Here is
the accompanying comment:

```
-- Ensure that type errors don't get deferred when type checking the
-- naked expression. Deferring type errors here is unhelpful because the
-- expression gets evaluated right away anyway. It also would potentially
-- emit redundant type-error warnings, one from each plan.
; plan <- unsetGOptM Opt_DeferTypeErrors $
```

Since `Opt_DeferTypeErrors` implies `Opt_DeferTypedHoles`,
`Opt_DeferTypedHoles` should be disabled here as well. This improves
the error message for T10248 (it doesn't mention ghciStepIO anymore).
This is a partial fix for #10249, and a complete fix for #11130.

Depends on D1526

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: simonpj

Differential Revision: https://phabricator.haskell.org/D1527

GHC Trac Issues: #10249, #11130

In spirit, this reverts 9ba922ee

The syntax has been deprecated with a warning since 2010.

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1530

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1534

This is an updated version of @jlengyel's original patch adding support
for prompt functions.

Since f16ddcee / D876, `ghc-stage1`
supports a subset of `-XTemplateHaskell`, but since we need Cabal to be
able detect (so `.cabal` files can be specified accordingly, see also
GHC #11102 which omits `TemplateHaskell` from `--supported-extensions`)
whether GHC provides full or only partial `-XTemplateHaskell` support,
the proper way to accomplish this is to split off the
quotation/non-splicing `TemplateHaskell` feature-subset into a new
language pragma `TemplateHaskellQuotes`.

Moreover, `-XTemplateHaskellQuotes` is considered safe under SafeHaskell

This addresses #11121

Reviewers: goldfire, ezyang, dterei, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1511

GHC Trac Issues: #11121

Differential Revision: https://phabricator.haskell.org/D1542

Fixes Trac #11136.  We should check arity before
doing tcTyClTyVars, because the latter crahes if
the arity isn't right.

Most notably this pulls in `MonadFail` instances
(see also #10751)

- add MonadFail instance for ContT
- re-order methods for consistency
- Add `MonadFail` instances
- Canonicalise Monad instances
- instance Bifunctor Constant

This one slipped through testing.

This is a first draft of the user manual documentation for
DuplicateRecordFields. Feedback welcome. I've intentionally made this a
minimal change, but I wonder if it might make sense to pull out all the
subsections on record system extensions ("Traditional record syntax",
and "Record field disambiguation" through to "Record wildcards") into a
new section? That is, is it worth breaking down the rather monolithic
"Syntactic extensions" section?

Test Plan: n/a

Reviewers: simonpj, bgamari, austin

Reviewed By: bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1522

Test Plan: IIAM

Reviewers: austin, bgamari, quchen

Reviewed By: bgamari, quchen

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1529

GHC Trac Issues: #10751

Test Plan: Read it

Reviewers: austin, fryguybob

Reviewed By: austin

Subscribers: thomie, fryguybob

Differential Revision: https://phabricator.haskell.org/D1523

GHC Trac Issues: #10639, #367

This patch does some signficant refactoring to the treatment
of default methods in class declarations, and more generally
to the type checking of type/class decls.

Highlights:

* When the class has a generic-default method, such as
     class C a where
       op :: a -> a -> Bool
       default op :: Ord a => a -> a -> a
  the ClassOpItem records the type of the generic-default,
  in this case the type (Ord a => a -> a -> a)

* I killed off Class.DefMeth in favour of the very-similar
  BasicTypes.DefMethSpec.  However it turned out to be better
  to use a Maybe, thus
      Maybe (DefMethSpec Type)
  with Nothing meaning "no default method".

* In TcTyClsDecls.tcTyClGroup, we used to accumulate a [TyThing],
  but I found a way to make it much simpler, accumulating only
  a [TyCon].  Much less wrapping and unwrapping.

* On the way I also fixed Trac #10896 in a better way. Instead
  of killing off all ambiguity checks whenever there are any type
  errors (the fix in commit 8e8b9ed9), I instead recover in
  TcTyClsDecls.checkValidTyCl.

There was a lot of associated simplification all round

[skip ci]

Simplify and tidy up the MonadFail code.
See TcMatches.tcMonadFailOp

Less, code; and more robust.

This incidentally fixes a bug; see the change
to MonadFailErrors.stderr

This should catch duplicate uniques in the future before Bad Things
happen.

Test Plan: Introduce a duplicate unique and validate

Reviewers: austin, hvr, thomie

Reviewed By: hvr, thomie

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1521

Reviewers: hvr, austin, thomie, bgamari

Reviewed By: hvr, austin, thomie, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1495

GHC Trac Issues: #11000

This puts the "Relevant bindings" section at the end.

It uses a TcErrors.Report Monoid to divide messages by importance and
then mappends them together.  This is not the most efficient way since
there are various intermediate Reports and list appends, but it probably
doesn't matter since error messages shouldn't get that large, and are
usually prepended.  In practice, everything is `important` except
`relevantBindings`, which is `supplementary`.

ErrMsg's errMsgShortDoc and errMsgExtraInfo were extracted into ErrDoc,
which has important, context, and suppelementary fields.  Each of those
three sections is marked with a bullet character, '•' on unicode
terminals and '*' on ascii terminals.  Since this breaks tons of tests,
I also modified testlib.normalise_errmsg to strip out '•'s.

--- Additional notes:

To avoid prepending * to an empty doc, I needed to filter empty docs.
This seemed less error-prone than trying to modify everyone who produces
SDoc to instead produce Maybe SDoc.  So I added `Outputable.isEmpty`.
Unfortunately it needs a DynFlags, which is kind of bogus, but otherwise
I think I'd need another Empty case for SDoc, and then it couldn't be a
newtype any more.

ErrMsg's errMsgShortString is only used by the Show instance, which is
in turn only used by Show HscTypes.SourceError, which is in turn only
needed for the Exception instance.  So it's probably possible to get rid
of errMsgShortString, but that would a be an unrelated cleanup.

Fixes #11014.

Test Plan: see above

Reviewers: austin, simonpj, thomie, bgamari

Reviewed By: thomie, bgamari

Subscribers: simonpj, nomeata, thomie

Differential Revision: https://phabricator.haskell.org/D1427

GHC Trac Issues: #11014

Warn about incoherent/non-canonical 'Applicative'/'Monad' instance
declarations. Specifically the following invariants are checked:

In 'Monad' instances declarations warn if the any of the following
conditions does not hold:

 * If `return` is overridden it must be canonical (i.e. `return = pure`).
 * If `(>>)` is overridden it must be canonical (i.e. `(>>) = (*>)`).

In 'Applicative' instance declarations:

 * Warn if 'pure' is defined backwards (i.e. `pure = return`).
 * Warn if '(*>)' is defined backwards (i.e. `(*>) = (>>)`).

NB, this warning flag is not enabled via `-Wall` nor `-Wcompat`.

This addresses #11128

Reviewers: quchen, austin, bgamari

Reviewed By: bgamari

Differential Revision: https://phabricator.haskell.org/D1516