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Summary:
This contains two commits:

----

Make GHC's code-base compatible w/ `MonadFail`

There were a couple of use-sites which implicitly used pattern-matches
in `do`-notation even though the underlying `Monad` didn't explicitly
support `fail`

This refactoring turns those use-sites into explicit case
discrimations and adds an `MonadFail` instance for `UniqSM`
(`UniqSM` was the worst offender so this has been postponed for a
follow-up refactoring)

---

Turn on MonadFail desugaring by default

This finally implements the phase scheduled for GHC 8.6 according to

https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Transitionalstrategy

This also preserves some tests that assumed MonadFail desugaring to be
active; all ghc boot libs were already made compatible with this
`MonadFail` long ago, so no changes were needed there.

Test Plan: Locally performed ./validate --fast

Reviewers: bgamari, simonmar, jrtc27, RyanGlScott

Reviewed By: bgamari

Subscribers: bgamari, RyanGlScott, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5028

Somehow the level-2 headings were all missing a tilde, causing Sphinx to complain.

This is actually a decrease in the version number since a bump to 0.10
wasn't actually necessary.

Backpack is unable to type check signatures that expect a data which
is a type level literal. This was reported in issue #15138. These
commits are a fix for this. It also includes a minimal test case that
was mentioned in the issue.

Reviewers: bgamari, ezyang, goldfire

Reviewed By: bgamari, ezyang

Subscribers: simonpj, ezyang, rwbarton, thomie, carter

GHC Trac Issues: #15138

Differential Revision: https://phabricator.haskell.org/D4951

As can be seen on https://circleci.com/gh/ghc/ghc/7578, some
tests are failing on i686 due to too restrictive timeouts. This patch
tweaks those in the hope of solving the 4 failures from that URL due to
timeouts.

Test Plan: ./validate on i686

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5031

Enabling `-Weverything` does enable those warnings.

(cherry picked from commit b062bd10a88ea407ae91610f822f0c352909bcce)

The unhidden module GHC.OldList recommends using GHC.List instead.
In consequence we should also have haddocks for GHC.List.

(cherry picked from commit e3df129c8bf4c35693d01ea66238882f3e3b6fe1)

In the module signatures section, two modules were defined, `Str` and
`A`, but `A` was importing `Text`, not `Str`.

(cherry picked from commit 26ab3635ca342c88310321d7f310f1c12c23ec4c)

(cherry picked from commit 6fb2620dbc420c976dc9da90b0efc6eae533ebff)

Since we cast this to a gc_thread the compiler may assume that it's aligned.
Make sure that this is so. Fixes #15482.

Test Plan: make TEST=T12625

Reviewers: jstolarek, austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #12625

Differential Revision: https://phabricator.haskell.org/D5030

If we fail to initialize the liker properly, we still set the
`initLinkerDone`. In fact we even set that prior to actually
initializing the linker. However if the linker initialization fails, we
the `Done` state is still true. As such we run into the `Dynamic Linker
not initialised` error. Which while technically corret is confusing as
it pulls the attation away from the real issue.

This change puts the Done state into an MVar, and as such ensureing
that all parallel access needs to wait for the linker to be actually
initialized, or try to re-initilize if it fails.

Reviewers: bgamari, RyanGlScott, simonmar, hvr

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #9868, #10355, #13137, #13607, #13531

Differential Revision: https://phabricator.haskell.org/D5012

Summary:
In Python 3, subprocess.communicate() returns a pair of bytes, which
need to be decoded. In runtests.py, we were just calling str() instead,
which converts b'x' to "b'x'". As a result, the loop that was checking
pkginfo for lines starting with 'library-dirs' couldn't work.

Reviewers: bgamari, thomie, Phyx

Reviewed By: thomie

Subscribers: Phyx, rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5046

Test Plan: Harbormaster

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15469

Differential Revision: https://phabricator.haskell.org/D5039

Summary:
My very last commit to D4904 removed -fobject-code.  I should have tested this
more thoroughly, because it is required to do a fresh ghci load, as some code
uses unboxed tuples.

One of my motivations for doing this was that if you run the script without
passing -odir / -hidir, it would pollute the source tree with .hi and .o files.
This also appeared to break subsequent builds. I've made it much less likely
that this will happen by instead specifying -odir and -hidir within the ghci
script rather than on the commandline.

I plan to open a separate diff which adds a test that these scripts work.

Until this patch is merged, the workaround is to do `./utils/ghc-in-ghci/run.sh -fobject-code`

Reviewers: bgamari, alpmestan, monoidal

Reviewed By: alpmestan, monoidal

Subscribers: alpmestan, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5015

Somehow, this escaped my notice before.

Summary:
I have some pending commits which will debut in GHC 8.8.1,
but we don't yet have release notes for this. This adds them, and
deletes the stale 8.4.1 and 8.4.2 release notes.

Test Plan: Read it

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5025

This patch implements GHC proposal 29:
(sorry, URL is too long for the commit message linter)
and fixess #15050.

The change is simple: Just use a different meta variable form.

Test suite and documentation updated.

Differential Revision: https://phabricator.haskell.org/D4980

Summary: This was a cut-and-paste error.

Reviewers: alpmestan

Reviewed By: alpmestan

Subscribers: alpmestan, rwbarton, thomie, carter

GHC Trac Issues: #15466

Differential Revision: https://phabricator.haskell.org/D5037

Summary:
Split into getMinimalImports and printMinimalImports.
Export both functions from RnNames module.

Reviewers: bgamari, mpickering

Reviewed By: mpickering

Subscribers: mpickering, rwbarton, carter

GHC Trac Issues: #15439

Differential Revision: https://phabricator.haskell.org/D5045

TypeInType came with a new function: decideKindGeneralisationPlan.
This type-level counterpart to the term-level decideGeneralisationPlan
chose whether or not a kind should be generalized. The thinking was
that if `let` should not be generalized, then kinds shouldn't either
(under the same circumstances around -XMonoLocalBinds).

However, this is too conservative -- the situation described in the
motivation for "let should be be generalized" does not occur in types.

This commit thus removes decideKindGeneralisationPlan, always
generalizing.

One consequence is that tc_hs_sig_type_and_gen no longer calls
solveEqualities, which reports all unsolved constraints, instead
relying on the solveLocalEqualities in tcImplicitTKBndrs. An effect
of this is that reporing kind errors gets delayed more frequently.
This seems to be a net benefit in error reporting; often, alongside
a kind error, the type error is now reported (and users might find
type errors easier to understand).

Some of these errors ended up at the top level, where it was
discovered that the GlobalRdrEnv containing the definitions in the
local module was not in the TcGblEnv, and thus errors were reported
with qualified names unnecessarily. This commit rejiggers some of
the logic around captureTopConstraints accordingly.

One error message (typecheck/should_fail/T1633)
is a regression, mentioning the name of a default method. However,
that problem is already reported as #10087, its solution is far from
clear, and so I'm not addressing it here.

This commit fixes #15141. As it's an internal refactor, there is
no concrete test case for it.

Along the way, we no longer need the hsib_closed field of
HsImplicitBndrs (it was used only in decideKindGeneralisationPlan)
and so it's been removed, simplifying the datatype structure.

Along the way, I removed code in the validity checker that looks
at coercions. This isn't related to this patch, really (though
it was, at one point), but it's an improvement, so I kept it.

This updates the haddock submodule.

This was missed by 0960a378

[skip ci]

Test Plan: Validate

Reviewers: goldfire, simonpj, bgamari

Reviewed By: simonpj

Subscribers: RyanGlScott, rwbarton, thomie, carter

GHC Trac Issues: #15415

Differential Revision: https://phabricator.haskell.org/D5022

`checkEmptyCase'` (the code path for coverage-checking
`EmptyCase` expressions) had a fair bit of code duplication from the
code path for coverage-checking non-`EmptyCase` expressions, and to
make things worse, it behaved subtly different in some respects (for
instance, emitting different warnings under unsatisfiable
constraints, as shown in #15450). This patch attempts to clean up
both this discrepancy and the code duplication by doing the
following:

* Factor out a `pmInitialTmTyCs` function, which returns the initial
  set of term and type constraints to use when beginning coverage
  checking. If either set of constraints is unsatisfiable, we use an
  empty set in its place so that we can continue to emit as many
  warnings as possible. (The code path for non-`EmptyCase`
  expressions was doing this already, but not the code path for
  `EmptyCase` expressions, which is the root cause of #15450.)

  Along the way, I added a `Note` to explain why we do this.
* Factor out a `pmIsSatisfiable` constraint which checks if a set of
  term and type constraints are satisfiable. This does not change any
  existing behavior; this is just for the sake of deduplicating code.

Test Plan: make test TEST=T15450

Reviewers: simonpj, bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15450

Differential Revision: https://phabricator.haskell.org/D5017

When compiling and linking files in `ghci`, we keep adding rpath
arguments to the linker command invoation.  If those are identical we
should `nub` them out.  Otherwise we not only risk overflowing the
argument limit, but also embed huge amounts of identical rpath values
into the dynamic library, eventually leading to the overflow of the load
command size limit, due to the number of rpath entries alone.

A further improvement could be to pass `-Xlinker -dead_strip_dylibs`;
that however might be stipping too aggressively, and potentially lead to
missing symbols?

For the time being I suggest to only do the nubbing and if need be to
provide -Wl,-dead_strip_dylibs when invoking ghci.

Test Plan: ./validate

Reviewers: bgamari, hvr

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15446

Differential Revision: https://phabricator.haskell.org/D5021

We need to store the used plugins so that we recompile
a module when a plugin that it uses is recompiled.

However, storing the `ModuleName`s of the plugins used by a
module in the `dep_mods` field made the rest of GHC think
that they belong in the HPT, causing at least the issues
reported in #15234

We therefor store the `ModuleName`s of the plugins in a
new field, `dep_plgins`, which is only used the the
recompilation logic.

Reviewers: mpickering, bgamari

Reviewed By: mpickering, bgamari

Subscribers: alpmestan, rwbarton, thomie, carter

GHC Trac Issues: #15234

Differential Revision: https://phabricator.haskell.org/D4937

Bug #15380 hangs because a knot-tied TyCon ended up in a kind.
Looking at the code in tcInferApps, I'm amazed this hasn't happened
before! I couldn't think of a good way to fix it (with dependent
types, we can't really keep types out of kinds, after all), so
I just went ahead and removed the knot.

This was remarkably easy to do. In tcTyVar, when we find a TcTyCon,
just use it. (Previously, we looked up the knot-tied TyCon and used
that.) Then, during the final zonk, replace TcTyCons with the real,
full-blooded TyCons in the global environment. It's all very easy.

The new bit is explained in the existing
Note [Type checking recursive type and class declarations]
in TcTyClsDecls.

Naturally, I removed various references to the knot and the
zonkTcTypeInKnot (and related) functions. Now, we can print types
during type checking with abandon!

NB: There is a teensy error message regression with this patch,
around the ordering of quantified type variables. This ordering
problem is fixed (I believe) with the patch for #14880. The ordering
affects only internal variables that cannot be instantiated with
any kind of visible type application.

There is also a teensy regression around the printing of types
in TH splices. I think this is really a TH bug and will file
separately.

Test case: dependent/should_fail/T15380

This commit causes significant performance regressions:
```
bytes allocated value is too high:
    Expected    T9872d(normal) bytes allocated: 578498120 +/-5%
    Lower bound T9872d(normal) bytes allocated: 549573214
    Upper bound T9872d(normal) bytes allocated: 607423026
    Actual      T9872d(normal) bytes allocated: 677179968
    Deviation   T9872d(normal) bytes allocated:      17.1 %
bytes allocated value is too high:
    Expected    T9872c(normal) bytes allocated: 3096670112 +/-5%
    Lower bound T9872c(normal) bytes allocated: 2941836606
    Upper bound T9872c(normal) bytes allocated: 3251503618
    Actual      T9872c(normal) bytes allocated: 3601872536
    Deviation   T9872c(normal) bytes allocated:       16.3 %
bytes allocated value is too high:
    Expected    T9872b(normal) bytes allocated: 3730686224 +/-5%
    Lower bound T9872b(normal) bytes allocated: 3544151912
    Upper bound T9872b(normal) bytes allocated: 3917220536
    Actual      T9872b(normal) bytes allocated: 4374298272
    Deviation   T9872b(normal) bytes allocated:       17.3 %
bytes allocated value is too high:
    Expected    T9872a(normal) bytes allocated: 2729927408 +/-5%
    Lower bound T9872a(normal) bytes allocated: 2593431037
    Upper bound T9872a(normal) bytes allocated: 2866423779
    Actual      T9872a(normal) bytes allocated: 3225788896
    Deviation   T9872a(normal) bytes allocated:       18.2 %
```
It's not clear that this was intentional so I'm going to revert for now.

This reverts commit 2110738b.

This documentation was a bit unprofessional and the markup wasn't
correct.

Reviewers: hvr, alpmestan

Reviewed By: alpmestan

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5026

As mentioned in #15402.

[no ci]

Test Plan: Read it.

Reviewers: alpmestan

Reviewed By: alpmestan

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15402

Differential Revision: https://phabricator.haskell.org/D5027