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Fixes #16445.

Issues #16289 and #15713 are proof that the pattern match checker did
an unsound job of estimating the value set abstraction corresponding to
the uncovered set.

The reason is that the fix from #11303 introducing `NLit` was
incomplete: The `LitCon` case desugared to `Var` rather than `LitVar`,
which would have done the necessary case splitting analogous to the
`ConVar` case.

This patch rectifies that by introducing the fresh unification variable
in `LitCon` in value abstraction position rather than pattern postition,
recording a constraint equating it to the constructor expression rather
than the literal. Fixes #16289 and #15713.

This fixes #16440, where the build system incorrectly concluded that the
`.subsections_via_symbols` assembler directive was supported on a Linux
system. This was caused by the fact that gcc was invoked with `-flto`;
when so-configured gcc does not call the assembler but rather simply
serialises its AST for compilation during the final link.

This is described in Note [autoconf assembler checks and -flto].

The law as it is currently written is meaningless, because nowhere have
we defined the implementation of 'ap'. The reader of the Control.Monad
documentation is provided with only a type signature,

> ap :: Monad m => m (a -> b) -> m a -> m b

an informal description,

> In many situations, the liftM operations can be replaced by uses of
> ap, which promotes function application.

and a relationship between 'ap' and the 'liftM' functions

> return f `ap` x1 `ap` ... `ap` xn
> is equivalent to
> liftMn f x1 x2 ... xn

Without knowing how 'ap' is defined, a law involving 'ap' cannot
provide any guidance for how to write a lawful Monad instance, nor can
we conclude anything from the law.

I suspect that a reader equipped with the understanding that 'ap' was
defined prior to the invention of the Applicative class could deduce
that 'ap' must be defined in terms of (>>=), but nowhere as far as I can
tell have we written this down explicitly for readers without the
benefit of historical context.

If the law is meant to express a relationship among (<*>), (>>=), and
'return', it seems that it is better off making this statement directly,
sidestepping 'ap' altogether.

`substBndr` should rename bindings which shadow existing ids.
However while it was renaming the bindings it was not adding proper substitutions
for renamed bindings.
Instead of adding a substitution of the form `old -> new` for renamed
bindings it mistakenly added `old -> old` if no replacement had taken
place while adding none if `old` had been renamed.

As a byproduct this should improve performance, as we no longer add
useless substitutions for unshadowed bindings.

Previously, when needing ghc as a builder, the ghcDeps (Files
the GHC binary depends on) for the current stage were needed.
This is incorrect as the previous stage's ghc is used for building.
This commit fixes the issue, needing the previous stage's ghcDeps.

This:
- Hoists part of the condition outside of the initialization loop in
  `stg_newSmallArrayzh`.
- Annotates one of the unlikely branches as unlikely, also in
  `stg_newSmallArrayzh`.
- Adds a couple of annotations to `allocateMightFail` indicating which
  branches are likely to be taken.

Together this gives about 5% improvement.
Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

This bumps `array` to version 0.5.4.0 so that we can distinguish
it with `MIN_VERSION_array` (as it introduces some changes to the
`Show` instance for `UArray`).

Whoops.

This moves URL references to old Trac to their corresponding
GitLab counterparts.

This patch does not update the submodule library, such as
libraries/Cabal.

See also !539, !606, !618

[ci skip]

Using this feature requires fsatrace (e.g. https://github.com/jacereda/fsatrace).
Simply use the `--lint-fsatrace` option when running hadrian.
Shake version >= 0.17.7 is required to support linting out of tree build dirs.

Also removes a couple unnecessary MagicHash pragmas

    This commit includes the necessary changes in code and
    documentation to support a primop that reverses a word's
    bits. It also includes a test.

See #16199.

This ensures that the release dates in the library changelogs are
properly set.

Fixes #16228. The included test case is adapted from the reproduction in
the issue, and fails without this patch.

------

We compute an initial visilibity mapping for units based on what is
present in the package databases. To seed this, we compute a set of all
the package configs to add visibilities for.

However, this set was keyed off the unit's *package name*. This is
correct, since we compare packages across databases by version. However,
we would only ever consider a single, most-preferable unit from the
database in which it was found.

The effect of this was that only one of the libraries in a Cabal package
would be added to this initial set. This would cause attempts to use
modules from the omitted libraries to fail, claiming that the package
was hidden (even though `ghc-pkg` would correctly show it as visible).

A solution is to do the selection of the most preferable packages
separately, and then be sure to consider exposing all units in the
same package in the same package db. We can do this by picking a
most-preferable unit for each package name, and then considering
exposing all units that are equi-preferable with that unit.

------

Why wasn't this bug apparent to all people trying to use sub-libraries
in Cabal? The answer is that Cabal explicitly passes `-package` and
`-package-id` flags for all the packages it wants to use, rather than
relying on the state of the package database. So this bug only really
affects people who are trying to use package databases produced by Cabal
outside of Cabal itself.

One particular example of this is the way that the
Nixpkgs Haskell infrastructure provides wrapped GHCs: typically these
are equipped with a package database containing all the needed
package dependencies, and the user is not expected to pass
`-package` flags explicitly.

This patch only attempts to fix links that don't automatically re-direct to the correct URL.

- `emitCopySmallArray` now checks size before generating code and
  doesn't generate any code when size is 0. `emitCopyArray` already does
  this so this makes small/large array cases the same in argument
  checking.

- In both `emitCopySmallArray` and `emitCopyArray` read the `dflags`
  after checking the argument.

Any user can now trivially build any number of Haskell packages with
`-split-sections` by using `splitSections`/`splitSectionsIf` on any
existing or new flavour:

    -- build all packages but the ghc library with -split-sections
    splitSections :: Flavour -> Flavour

    -- build all packages that satisfy the given predicate
    -- with --split-sections
    splitSectionsIf :: (Package -> Bool) -> Flavour -> Flavour

See the new section in `doc/user-settings.md`.