It came back to life in 381c3ae3 by
mistake.

In #17343 we saw that we didn't handle the pattern guard `!_ <-
undefined` correctly: The `undefined` was never evaluated. Indeed,
elaboration failed to insert the invisible type aruments to `undefined`.
So `undefined` was trivially a normal-form and in turn never entered.

The problem is that we used to infer a sigma-type for the RHS of the
guard, the leading qualifiers of which will never be useful in a pattern
match situation. Hence we infer a rho-type now.

Fixes #17343.

This is a test for the current algorithm of GHCi command name resolution.

I add this test in preparation for updating GHCi command name resolution.

For the current algorithm, see https://downloads.haskell.org/ghc/latest/docs/html/users_guide/ghci.html#the-ghci-files

For backends maintaining the CFG during codegen
we can now find loops and their nesting level.

This is based on the Cmm CFG and dominator analysis.

As a result we can estimate edge frequencies a lot better
for methods, resulting in far better code layout.

Speedup on nofib: ~1.5%
Increase in compile times: ~1.9%

To make this feasible this commit adds:
* Dominator analysis based on the Lengauer-Tarjan Algorithm.
* An algorithm estimating global edge frequences from branch
probabilities - In CFG.hs

A few static branch prediction heuristics:

* Expect to take the backedge in loops.
* Expect to take the branch NOT exiting a loop.
* Expect integer vs constant comparisons to be false.

We also treat heap/stack checks special for branch prediction
to avoid them being treated as loops.

Fixes #17351.

Issue #17355 occurred because the control flow for
`TcValidity.check_valid_inst_head` was structured in such a way that
whenever it checked a special, built-in class (like `Generic` or
`HasField`), it would skip the most important check of all:
`checkValidTypePats`, which rejects nonsense like this:

```hs
instance Generic (forall a. a)
```

This fixes the issue by carving out `checkValidTypePats` from
`check_valid_inst_head` so that `checkValidTypePats` is always
invoked. `check_valid_inst_head` has also been renamed to
`check_special_inst_head` to reflect its new purpose of _only_
checking for instances headed by special classes.

Fixes #17355.

The latest installment in my quest to clean up the code in
`TcDeriv*`. This time, my sights are set on
`TcDerivInfer.inferConstraints`, which infers the context for derived
instances. This function is a wee bit awkward at the moment:

* It's not terribly obvious from a quick glance, but
  `inferConstraints` is only ever invoked when using the `stock` or
  `anyclass` deriving strategies, as the code for inferring the
  context for `newtype`- or `via`-derived instances is located
  separately in `mk_coerce_based_eqn`. But there's no good reason
  for things to be this way, so I moved this code from
  `mk_coerce_based_eqn` to `inferConstraints` so that everything
  related to inferring instance contexts is located in one place.
* In this process, I discovered that the Haddocks for the auxiliary
  function `inferConstraintsDataConArgs` are completely wrong. It
  claims that it handles both `stock` and `newtype` deriving, but
  this is completely wrong, as discussed above—it only handles
  `stock`. To rectify this, I renamed this function to
  `inferConstraintsStock` to reflect its actual purpose and created
  a new `inferConstraintsCoerceBased` function to specifically
  handle `newtype` (and `via`) deriving.

Doing this revealed some opportunities for further simplification:

* Removing the context-inference–related code from
  `mk_coerce_based_eqn` made me realize that the overall structure
  of the function is basically identical to `mk_originative_eqn`.
  In fact, I was easily able to combine the two functions into a
  single `mk_eqn_from_mechanism` function.

  As part of this merger, I now invoke
  `atf_coerce_based_error_checks` from `doDerivInstErrorChecks1`.
* I discovered that GHC defined this function:

  ```hs
  typeToTypeKind = liftedTypeKind `mkVisFunTy` liftedTypeKind
  ```

  No fewer than four times in different modules. I consolidated all
  of these definitions in a single location in `TysWiredIn`.

This is a very cheap job and can catch a number of "easy" failure modes
(e.g. missing imports in the compiler). Let's run it first.

The fixes for these issues both have user-facing consequences, so it
would be good to mention them in the release notes for GHC 8.10.1.

While I'm in town, also mention `UnboxedSums` in the release notes
entry related to `-fobject-code`.

When the users guide fails to build (as in #17346), a
`docs/users_guide/.log` file will be generated with contents that
look something like this:

```
WARNING: unknown config value 'latex_paper_size' in override, ignoring
/home/rgscott/Software/ghc5/docs/users_guide/ghci.rst:3410: WARNING: u'ghc-flag' reference target not found: -pgmo ?option?
/home/rgscott/Software/ghc5/docs/users_guide/ghci.rst:3410: WARNING: u'ghc-flag' reference target not found: -pgmo ?port?

Encoding error:
'ascii' codec can't encode character u'\u27e8' in position 132: ordinal not in range(128)
The full traceback has been saved in /tmp/sphinx-err-rDF2LX.log, if you want to report the issue to the developers.
```

This definitely should not be checked in to version control, so let's
add this to `.gitignore`.

This commit explicitly adds description about double colon command
of GHCi.

[skip ci]

Statements can change the basic block in which instructions
are placed during instruction selection.

We have to keep track of this switch of the current basic block
as we need this information in order to properly update the CFG.

This commit implements this change and fixes #17334.

We do so by having stmtToInstr return the new block id
if a statement changed the basic block.

GHC has a wonderful-but-delicate mechanism for building recursive
dictionaries by adding a goal to the "solved dictionaries" before
solving the sub-goals.  See Note [Solved dictionaries] in TcSMonad

Ticket #17267 showed that if you use this mechanism for local
/quantified/ constraints you can get a loop -- or even unsafe
coerce.   This patch fixes the bug.

Specifically

* Make TcSMonad.addSolvedDict be conditional on using a
  /top level/ instance, not a quantified one.

* Moreover, we /also/ don't want to add a solved dict
  for equalities (a~b).

* Add lots more comments to Note [Solved dictionaries]
  to explain the above cryptic stuff.

* Extend InstanceWhat to identify those strange built-in
  equality instances.

A couple of other things along the way

* Delete the unused Type.isIPPred_maybe.

* Stop making addSolvedDict conditional on not being an
  impolicit parameter.  This comes from way back. But
  it's irrelevant now because IP dicts are never solved
  via an instance.

GHC is suposed to uphold the principle that an /inferred/ type
for a let-binding should obey the rules for that module.  E.g.
we should only accept an inferred higher rank type if we have
RankNTypes on.

But we were failing to check this: TcValidity.checkValidType
allowed arbitrary rank for inferred types.

This patch fixes the bug.  It might in principle cause some breakage,
but if so that's good: the user should add RankNTypes and/or a
manual signature.  (And almost every package has explicit user
signatures for all top-level things anyway.)  Let's see.

Fixes #17213.

Metric Decrease:
    T10370

Issue #17339 was caused by using a slightly different version of
`newDFunName` for derived instances that, confusingly enough, did not
take all arguments to the class into account when generating the
`DFun` name. I cannot think of any good reason for doing this, so
this patch uses `newDFunName` uniformly for both derived instances
and manually written instances alike.

Fixes #17339.

The CI job fails with:

	+++ rts/T13676.run/T13676.run.stderr.normalised	2019-10-09 12:27:56.000000000 -0700
	@@ -0,0 +1,4 @@
	+dyld: Library not loaded: @rpath/libHShaskeline-0.7.5.0-ghc8.9.0.20191009.dylib
	+  Referenced from: /Users/builder/builds/ewzE5N2p/0/ghc/ghc/inplace/lib/bin/ghc
	+  Reason: image not found
	+*** Exception: readCreateProcess: '/Users/builder/builds/ewzE5N2p/0/ghc/ghc/inplace/lib/bin/ghc' '-B/Users/builder/builds/ewzE5N2p/0/ghc/ghc/inplace/lib' '-e' ''/''$'/'' == '/''/x0024'/''' +RTS '-tT13676.t'  (exit -6): failed

Unable to reproduce locally.

 - No need to distinguish between gcc-llvm and clang. First of all,
   gcc-llvm is quite old and surely unmaintained by now. Second of all,
   none of the code actually care about that distinction!

   Now, it does make sense to consider C multiple frontends for LLVMs in
   the form of clang vs clang-cl (same clang, yes, but tweaked
   interface). But this is better handled in terms of "gccish vs
   mvscish" and "is LLVM", yielding 4 combinations. Therefore, I don't
   think it is useful saving the existing code for that.

 - Get the remaining CC_LLVM_BACKEND, and also TABLES_NEXT_TO_CODE in
   mk/config.h the normal way, rather than hacking it post-hoc. No point
   keeping these special cases around for now reason.

 - Get rid of hand-rolled `die` function and just use `AC_MSG_ERROR`.

 - Abstract check + flag override for unregisterised and tables next to
   code.

Oh, and as part of the above I also renamed/combined some variables
where it felt appropriate.

 - GccIsClang -> CcLlvmBackend. This is for `AC_SUBST`, like the other
 Camal case ones. It was never about gcc-llvm, or Apple's renamed clang,
 to be clear.

 - llvm_CC_FLAVOR -> CC_LLVM_BACKEND. This is for `AC_DEFINE`, like the
 other all-caps snake case ones. llvm_CC_FLAVOR was just silly
 indirection *and* an odd name to boot.

Fixes #17286.

Simon realised that the simple language composed of let bindings, bang
patterns and flat constructor patterns is enough to capture the
semantics of the source pattern language that are important for
pattern-match checking. Well, given that the Oracle is smart enough to
connect the dots in this less informationally dense form, which it is
now.

So we transform `translatePat` to return a list of `PmGrd`s relative to
an incoming match variable. `pmCheck` then trivially translates each of
the `PmGrd`s into constraints that the oracle understands.

Since we pass in the match variable, we incidentally fix #15884
(coverage checks for view patterns) through an interaction with !1746.

If you derive an instance like this:

```hs
deriving <...> instance Foo C
```

And the data constructors for `C` aren't in scope, then
`doDerivInstErrorChecks1` throws an error. Moreover, it will
_only_ throw an error if `<...>` is either `stock` or `newtype`.
This is because the code that the `anyclass` or `via` strategies
would generate would not require the use of the data constructors
for `C`.

However, `doDerivInstErrorChecks1` has another purpose. If you
write this:

```hs
import M (C(MkC1, ..., MkCn))

deriving <...> instance Foo C
```

Then `doDerivInstErrorChecks1` will call `addUsedDataCons` on
`MkC1` through `MkCn` to ensure that `-Wunused-imports` does not
complain about them. However, `doDerivInstErrorChecks1` was doing
this for _every_ deriving strategy, which mean that if `<...>` were
`anyclass` or `via`, then the warning about `MkC1` through `MkCn`
being unused would be suppressed!

The fix is simple enough: only call `addUsedDataCons` when the
strategy is `stock` or `newtype`, just like the other code paths
in `doDerivInstErrorChecks1`.

Fixes #17324.

This allows the stage1 compiler (which needs to run on the build
platform and produce code for the host) to depend upon properties of the
target. This is wrong. However, it's no more wrong than it was
previously and @Erichson2314 is working on fixing this so I'm going to
remove the guard so we can finally bootstrap HEAD with ghc-8.8 (see
issue #17146).

To avoid polluting the macro namespace

The vanilla COMPLETE set is attached to the representation TyCon of a
data family instance, whereas the user-defined COMPLETE sets are
attached to the parent data family TyCon itself.

Previously, we weren't trying particularly hard to get back to the
representation TyCon to the parent data family TyCon, resulting in bugs
like #17207. Now we should do much better.

Fixes the original issue in #17207, but I found another related bug that
isn't so easy to fix.

Previously, all constraints from all top-level groups (as
separated by top-level splices) were lumped together and solved
at the end. This could leak metavariables to TH, though, and
that's bad. This patch solves each group's constraints before
running the next group's splice.

Naturally, we now report fewer errors in some cases.

One nice benefit is that this also fixes #11680, but in a much
simpler way than the original fix for that ticket. Admittedly,
the error messages degrade just a bit from the fix from #11680
(previously, we informed users about variables that will be
brought into scope below a top-level splice, and now we just
report an out-of-scope error), but the amount of complexity
required throughout GHC to get that error was just not worth it.

This patch thus reverts much of
f93c9517.

Fixes #16980

Test cases: th/T16980{,a}

Currently, newtype constructors are not marked as used when they are
accessed under the hood by uses of `coerce`, as described in #10347.
This fixes #10347 by co-opting the `tcg_keep` field of `TcGblEnv`
to track uses of newtype constructors in the `Coercible` solver.
See `Note [Tracking unused binding and imports]` in `TcRnTypes`.

Since #10347 is fixed, I was able to simplify the code in `TcDeriv`
slightly, as the hack described in
`Note [Newtype deriving and unused constructors]`
is no longer necessary.

Rephrase a bunch of things in the unlifted ffi types documentation. Add a section on pinned byte arrays.