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Previously, non-CAF cost centre symbol names contained a unique,
leading to non-deterministic object files which, among other issues,
can lead to an inconsistency causing linking failure when using cached
builds sourced from multiple machines, such as with nix. Now, each
cost centre symbol is annotated with the type of cost centre it
is (CAF, expression annotation, declaration annotation, or HPC) and,
when a single module has multiple cost centres with the same name and
type, a 0-based index.

Reviewers: bgamari, simonmar

Reviewed By: bgamari

Subscribers: niteria, simonmar, RyanGlScott, osa1, rwbarton, thomie, carter

GHC Trac Issues: #4012, #12935

Differential Revision: https://phabricator.haskell.org/D4388

This adds valid refinement substitution suggestions for typed holes and
documentation thereof.

Inspired by Agda's refinement facilities, this extends the typed holes
feature to be able to search for valid refinement substitutions, which
are substitutions that have one or more holes in them.

When the flag `-frefinement-level-substitutions=n` where `n > 0` is
passed, we also look for valid refinement substitutions, i.e.
substitutions that are valid, but adds more holes. Consider the
following:

  f :: [Integer] -> Integer
  f = _

Here the valid substitutions suggested will be (with the new
`-funclutter-valid-substitutions` flag for less verbosity set):

```
  Valid substitutions include
    f :: [Integer] -> Integer
    product :: forall (t :: * -> *).
              Foldable t => forall a. Num a => t a -> a
    sum :: forall (t :: * -> *).
          Foldable t => forall a. Num a => t a -> a
    maximum :: forall (t :: * -> *).
              Foldable t => forall a. Ord a => t a -> a
    minimum :: forall (t :: * -> *).
              Foldable t => forall a. Ord a => t a -> a
    head :: forall a. [a] -> a
    (Some substitutions suppressed; use -fmax-valid-substitutions=N or
-fno-max-valid-substitutions)
```

When the `-frefinement-level-substitutions=1` flag is given, we
additionally compute and report valid refinement substitutions:

```
  Valid refinement substitutions include
    foldl1 _ :: forall (t :: * -> *).
                Foldable t => forall a. (a -> a -> a) -> t a -> a
    foldr1 _ :: forall (t :: * -> *).
                Foldable t => forall a. (a -> a -> a) -> t a -> a
    head _ :: forall a. [a] -> a
    last _ :: forall a. [a] -> a
    error _ :: forall (a :: TYPE r).
                GHC.Stack.Types.HasCallStack => [Char] -> a
    errorWithoutStackTrace _ :: forall (a :: TYPE r). [Char] -> a
    (Some refinement substitutions suppressed; use
-fmax-refinement-substitutions=N or -fno-max-refinement-substitutions)
```

Which are substitutions with holes in them. This allows e.g. beginners
to discover the fold functions and similar.

We find these refinement suggestions by considering substitutions that
don't fit the type of the hole, but ones that would fit if given an
additional argument. We do this by creating a new type variable with
newOpenFlexiTyVarTy (e.g. `t_a1/m[tau:1]`), and then considering
substitutions of the type `t_a1/m[tau:1] -> v` where `v` is the type of
the hole. Since the simplifier is free to unify this new type variable
with any type (and it is cloned before each check to avoid
side-effects), we can now discover any identifiers that would fit if
given another identifier of a suitable type. This is then generalized
so that we can consider any number of additional arguments by setting
the `-frefinement-level-substitutions` flag to any number, and then
considering substitutions like e.g. `foldl _ _` with two additional
arguments.

This can e.g. help beginners discover the `fold` functions.
This could also help more advanced users figure out which morphisms
they can use when arrow chasing.
Then you could write `m = _ . m2 . m3` where `m2` and `m3` are some
morphisms, and not only get exact fits, but also help in finding
morphisms that might get you a little bit closer to where you want to
go in the diagram.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4357

When finishing up an implication constraint, it's a bit tricky to
decide which Derived constraints to retain (for error reporting) and
which to discard.  I got this wrong in commit
   f20cf982
   (Remove wc_insol from WantedConstraints)

The particular problem in Trac #14763 was that we were reporting as an
error a fundep-generated constraint
  (ex ~ T)
where 'ex' is an existentially-bound variable in a pattern match.
But this isn't really an error at all.

This patch fixes the problem. Indeed, since I had to understand
this rather tricky code, I took the opportunity to clean it up
and document better.  See
  isDroppableCt :: Ct -> Bool
and Note [Dropping derived constraints]

I also removed wl_deriv altogether from the WorkList data type.  It
was there in the hope of gaining efficiency by not even processing
lots of derived constraints, but it has turned out that most derived
constraints (notably equalities) must be processed anyway; see
Note [Prioritise equalities] in TcSMonad.

The two are coupled because to decide which constraints to put in
wl_deriv I was using another variant of isDroppableCt.  Now it's much
simpler -- and perhaps even more efficient too.

Ideally, I'd like to do

    type EvTerm = CoreExpr

and the type checker builds the evidence terms as it goes. This failed,
becuase the evidence for `Typeable` refers to local identifiers that are
added *after* the typechecker solves constraints. Therefore, `EvTerm`
stays a data type with two constructors: `EvExpr` for `CoreExpr`
evidence, and `EvTypeable` for the others.

Delted `Note [Memoising typeOf]`, its reference (and presumably
relevance) was removed in 8fa4bf9a.

Differential Revision: https://phabricator.haskell.org/D4341

This gives a 10% allocation improvement on MultiLayerModules.
The idea is to reuse existing tuples, instead of constantly
constructing new ones.

Test Plan: ./validate

Reviewers: simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: rwbarton, thomie, simonmar, carter

Differential Revision: https://phabricator.haskell.org/D4332

This implements SPJ's suggestion on the ticket (#14273). We find the
relevant constraints (ones that whose free unification variables are all
mentioned in the type of the hole), and then clone the free unification
variables of the hole and the relevant constraints. We then add a
subsumption constraints and run the simplifier, and then check whether
all the constraints were solved.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: RyanGlScott, rwbarton, thomie, carter

GHC Trac Issues: #14273

Differential Revision: https://phabricator.haskell.org/D4315

Trac #13032 pointed out that we sometimes generate unused
bindings for Givens, and (worse still) we can't always discard
them later (we don't drop a case binding unless we can prove
that the scrutinee is non-bottom.

It looks as if this may be a major reason for the performace
problems in #14338 (see comment:29).

This patch fixes the problem at source, by pruning away all the
dead Givens.  See Note [Delete dead Given evidence bindings]

Remarkably, compiler allocation falls by 23% in
perf/compiler/T12227!

I have not confirmed whether this change actualy helps with

In fixing Trac #14584 I found that it would be /much/ more
convenient if a "hole" in a coercion (much like a unification
variable in a type) acutally had a CoVar associated with it
rather than just a Unique.  Then I can ask what the free variables
of a coercion is, and get a set of CoVars including those
as-yet-un-filled in holes.

Once that is done, it makes no sense to stuff coercion holes
inside UnivCo.  They were there before so we could know the
kind and role of a "hole" coercion, but once there is a CoVar
we can get that info from the CoVar.  So I removed HoleProv
from UnivCoProvenance and added HoleCo to Coercion.

In summary:

* Add HoleCo to Coercion and remove HoleProv from UnivCoProvanance

* Similarly in IfaceCoercion

* Make CoercionHole have a CoVar in it, not a Unique

* Make tyCoVarsOfCo return the free coercion-hole variables
  as well as the ordinary free CoVars.  Similarly, remember
  to zonk the CoVar in a CoercionHole

We could go further, and remove CoercionHole as a distinct type
altogther, just collapsing it into HoleCo.  But I have not done
that yet.

This patch fixes Trac #14998, where we eventually decided that
the existential type variables of the signature of a pattern
synonym should not scope over the pattern synonym.

See Note [Pattern synonym existentials do not scope] in TcPatSyn.

Summary:
Suppose that you are typechecking A.hs, which transitively imports,
via B.hs, A.hs-boot.  When we poke on B.hs and discover that it
has a reference to a type from A, what TyThing should we wire
it up with?  Clearly, if we have already typechecked A, we
should use the most up-to-date TyThing: the one we freshly
generated when we typechecked A.  But what if we haven't typechecked
it yet?

For the longest time, GHC adopted the policy that this was
*an error condition*; that you MUST NEVER poke on B.hs's reference
to a thing defined in A.hs until A.hs has gotten around to checking
this.  However, actually ensuring this is the case has proven
to be a bug farm.  The problem was especially poignant with
type family consistency checks, which eagerly happen before
any typechecking takes place.

This patch takes a different strategy: if we ever try to access
an entity from A which doesn't exist, we just fall back on the
definition of A from the hs-boot file.  This means that you may
end up with a mix of A.hs and A.hs-boot TyThings during the
course of typechecking.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Test Plan: validate

Reviewers: simonpj, bgamari, austin, goldfire

Subscribers: thomie, rwbarton

GHC Trac Issues: #14396

Differential Revision: https://phabricator.haskell.org/D4154

As documented in #14490, the Data instances currently blow up
compilation time by too much to stomach. Alan will continue working on
this in a branch and we will perhaps merge to 8.2 before 8.2.1 to avoid
having to perform painful cherry-picks in 8.2 minor releases.

Reverts haddock submodule.

This reverts commit 47ad6578.
This reverts commit e3ec2e7a.
This reverts commit 438dd1cb.
This reverts commit 0ff152c9.

 * Document requirement to use the same binaries.
 * Fix some code comments.

Test Plan: ./validate

Reviewers: bgamari, mboes, hvr

Reviewed By: bgamari, mboes

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4172

See https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow

Trees that grow extension points are added for
- HsExpr

Updates haddock submodule

Test Plan: ./validate

Reviewers: bgamari, goldfire

Subscribers: rwbarton, thomie, shayan-najd, mpickering

Differential Revision: https://phabricator.haskell.org/D4177

This patch was provoked by Trac #14363.  Turned out that we were
kicking out too many constraints in TcSMonad.kickOutRewritable, and
that mean that the work-list never became empty: infinite loop!

That in turn made me look harder at the Main Theorem in
Note [Extending the inert equalities].

Main changes

* Replace TcType.isTyVarExposed by TcType.isTyVarHead.  The
  over-agressive isTyVarExposed is what caused Trac #14363.
  See Note [K3: completeness of solving] in TcSMonad.

* TcType.Make anyRewriteableTyVar role-aware.  In particular,
      a ~R ty
  cannot rewrite
      b ~R f a
  See Note [anyRewriteableTyVar must be role-aware].  That means
  it has to be given a role argument, which forces a little
  refactoring.

  I think this change is fixing a bug that hasn't yet been reported.
  The actual reported bug is handled by the previous bullet.  But
  this change is definitely the Right Thing

The main changes are in TcSMonad.kick_out_rewritable, and in TcType
(isTyVarExposed ---> isTyVarHead).

I did a little unforced refactoring:

 * Use the cc_eq_rel field of a CTyEqCan when it is available, rather
   than recomputing it.

 * Define eqCanRewrite :: EqRel -> EqRel -> EqRel, and use it, instead
   of duplicating its logic

This patch is a pure refactoring, which I've wanted to do for
some time.  The main payload is

* Remove the wc_insol field from WantedConstraints;
  instead put all the insolubles in wc_simple

* Remove inert_insols from InertCans
  Instead put all the insolubles in inert_irreds

* Add a cc_insol flag to CIrredCan, to record that
  the constraint is definitely insoluble

Reasons

* Quite a bit of code gets slightly simpler
* Fewer concepts to keep separate
* Insolubles don't happen at all in production code that is
  just being recompiled, so previously there was a lot of
  moving-about of empty sets

A couple of error messages acutally improved.

See Note [Given insolubles] in TcRnTypes

Fixes Trac #14325.

This patch does two things:

* When reporting a hole, we now include its kind if the
  kind is not just '*'.  This addresses Trac #14265

* When reporting things like "'a' is a rigid type varaible
  bound by ...", this patch arranges to group the type variables
  together, so we don't repeat the "bound by..." stuff endlessly

This allows template-haskell code to add plugins to the compilation
pipeline. Otherwise, the user would have to pass -fplugin=... to ghc.

For now, plugin modules in the current package can't be used. This is
because when TH runs, it is too late to let GHC know that the plugin
modules needed to be compiled first.

Test Plan: ./validate

Reviewers: simonpj, bgamari, austin, goldfire

Reviewed By: bgamari

Subscribers: angerman, rwbarton, mboes, thomie

GHC Trac Issues: #13608

Differential Revision: https://phabricator.haskell.org/D3821

This builds on the previous "Valid substitutions include..." functionality,
but add subsumption checking as well, so that the suggested valid substitutions
show not only exact matches, but also identifiers that fit the hole by virtue of
subsuming the type of the hole (i.e. being more general than the type of the
hole).

Building on the previous example, in the given program

```
ps :: String -> IO ()
ps = putStrLn

ps2 :: a -> IO ()
ps2 _ = putStrLn "hello, world"

main :: IO ()
main = _ "hello, world"
```

The results would be something like

```
    • Found hole: _ :: [Char] -> IO ()
    • In the expression: _
      In the expression: _ "hello, world"
      In an equation for ‘main’: main = _ "hello, world"
    • Relevant bindings include main :: IO () (bound at t1.hs:8:1)
      Valid substitutions include
        ps :: String -> IO () (defined at t1.hs:2:1)
        ps2 :: forall a. a -> IO () (defined at t1.hs:5:1)
        putStrLn :: String -> IO ()
          (imported from ‘Prelude’ at t1.hs:1:1
           (and originally defined in ‘System.IO’))
        fail :: forall (m :: * -> *). Monad m => forall a. String -> m a
          (imported from ‘Prelude’ at t1.hs:1:1
           (and originally defined in ‘GHC.Base’))
        mempty :: forall a. Monoid a => a
          (imported from ‘Prelude’ at t1.hs:1:1
           (and originally defined in ‘GHC.Base’))
        print :: forall a. Show a => a -> IO ()
          (imported from ‘Prelude’ at t1.hs:1:1
           (and originally defined in ‘System.IO’))
        (Some substitutions suppressed;
         use -fmax-valid-substitutions=N or -fno-max-valid-substitutions)
```
Signed-off-by: Matthías Páll Gissurarson <mpg@mpg.is>

Modified according to suggestions from Simon PJ

Accept tests that match the expectations, still a few to look better at

Swithced to using tcLookup, after sit down with SPJ at ICFP. Implications are WIP.

Now works with polymorphism and constraints!

We still need to merge the latest master, before we can make a patch.

Wrap the type of the hole, instead of implication shenanigans,

As per SPJs suggestion, this is simpler and feels closer to
what we actually want to do.

Updated tests with the new implementation

Remove debugging trace and update documentation

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: RyanGlScott, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3930

This switches the compiler/ component to get compiled with
-XNoImplicitPrelude and a `import GhcPrelude` is inserted in all
modules.

This is motivated by the upcoming "Prelude" re-export of
`Semigroup((<>))` which would cause lots of name clashes in every
modulewhich imports also `Outputable`

Reviewers: austin, goldfire, bgamari, alanz, simonmar

Reviewed By: bgamari

Subscribers: goldfire, rwbarton, thomie, mpickering, bgamari

Differential Revision: https://phabricator.haskell.org/D3989

I found (Trac #14175) that the tidying we do when reporting
type error messages could cause a well-kinded type to become
ill-kinded. Reason: we initialised the tidy-env with a
completely un-zonked TidyEnv accumulated in the TcLclEnv
as we come across lexical type-varialbe bindings.

Solution: zonk them.

But I ended up refactoring a bit:

* Get rid of tcl_tidy :: TidyEnv altogether

* Instead use tcl_bndrs :: TcBinderStack
  This used to contain only Ids, but now I've added those
  lexically scoped TyVars too.

* Change names:
     TcIdBinderStack -> TcBinderStack
     TcIdBinder      -> TcBinder
     extendTcIdBndrs -> extendTcBinderStack

* Now tcInitTidyEnv can grab those TyVars from the
  tcl_bndrs, zonk, and tidy them.

The only annoyance is that I had to add TcEnv.hs-boot, to
break the recursion between the zonking code and the
TrRnMonad functions like addErrTc that call tcInitTidyEnv.
Tiresome, but in fact that file existed already.

IOW, code compiles -Wnoncanonical-monoidfail-instances clean now

This is easy now since we require GHC 8.0/base-4.9 or later
for bootstrapping.

Note that we can easily enable `MonadFail` via

  default-extensions: MonadFailDesugaring

in compiler/ghc.cabal.in

which currently would point out that NatM doesn't have
a proper `fail` method, even though failable patterns
are made use of:

  compiler/nativeGen/SPARC/CodeGen.hs:425:25: error:
    * No instance for (Control.Monad.Fail.MonadFail NatM)
        arising from a do statement
        with the failable pattern ‘(dyn_c, [dyn_r])’

This is another take on https://phabricator.haskell.org/D3844.

This patch removes then need for haddock to reimplement the calculation
of exported names from modules. Instead when renaming export lists ghc
annotates each IE with its exported names.

Haddocks current  export logic has caused lots of trouble in the past
(on the Github issue tracker):
  - https://github.com/haskell/haddock/issues/121
  - https://github.com/haskell/haddock/issues/174
  - https://github.com/haskell/haddock/issues/225
  - https://github.com/haskell/haddock/issues/344
  - https://github.com/haskell/haddock/issues/584
  - https://github.com/haskell/haddock/issues/591
  - https://github.com/haskell/haddock/issues/597

Updates haddock submodule.

Reviewers: austin, bgamari, ezyang

Reviewed By: bgamari, ezyang

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3864

We pretty-print a type by converting it to an IfaceType and
pretty-printing that.  But
 (a) that's a bit indirect, and
 (b) delibrately loses information about (e.g.) the kind
      on the /occurrences/ of a type variable

So this patch implements debugPprType, which pretty prints
the type directly, with no fancy formatting.  It's just used
for debugging.

I took the opportunity to refactor the debug-pretty-printing
machinery a little.  In particular, define these functions
and use them:

  ifPprDeubug :: SDoc -> SDOc -> SDoc
    -- Says what to do with and without -dppr-debug
  whenPprDebug :: SDoc -> SDoc
    -- Says what to do with  -dppr-debug; without is empty
  getPprDebug :: (Bool -> SDoc) -> SDoc

getPprDebug used to be called sdocPprDebugWith
whenPprDebug used to be called ifPprDebug

So a lot of files get touched in a very mechanical way

Debug-only; no change in mainstream behaviour

GHC 8.2.1 is out, so now GHC's support window only extends back to GHC
8.0. This means we can delete gobs of code that was only used for GHC
7.10 support. Hooray!

Test Plan: ./validate

Reviewers: hvr, bgamari, austin, goldfire, simonmar

Reviewed By: bgamari

Subscribers: Phyx, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3781

Trac #14000 showed up two errors

* In TcRnTypes.dropInsolubles we dropped all implications, which
  might contain the very insolubles we wanted to keep.  This was
  an outright error, and is why the out-of-scope error was actually
  lost altogether in Trac #14000

* In TcSimplify.simplifyInfer, if there are definite (insoluble)
  errors, it's better to suppress the following ambiguity test,
  because the type may be bogus anyway.  See TcSimplify
  Note [Quantification with errors].  This fix seems a bit clunky,
  but it'll do for now.

A type equality error can arise from a mismatch between
*invisible* arguments just as easily as from visible arguments.
But we should really prefer printing out errors from visible
arguments over invisible ones. Suppose we have a mismatch between
`Proxy Int` and `Proxy Maybe`. Would you rather get an error
between `Int` and `Maybe`? Or between `*` and `* -> *`? I thought
so, too.

There is a fair amount of plumbing with this one, but I think
it's worth it.

This commit introduces a performance regression in test
perf/compiler/T5631. The cause of the regression is not the
new visibility stuff, directly: it's due to a change from
zipWithM to zipWith3M in TcUnify. To my surprise, zipWithM
is nicely optimized (it fuses away), but zipWith3M is not.
There are other examples of functions that could be made faster,
so I've posted a separate ticket, #14037, to track these
improvements. For now, I've accepted the small (6.6%) regression.

The uo_thing field of TypeEqOrigin is used to track the
"thing" (either term or type) that has the type (kind) stored
in the TypeEqOrigin fields. Previously, this was sometimes a
proper Core Type, which needed zonking and tidying. Now, it
is only HsSyn: much simpler, and the error messages now use
the user-written syntax.

But this aspect of uo_thing didn't cause #13819; it was the
sibling field uo_arity that did. uo_arity stored the number
of arguments of uo_thing, useful when reporting something
like "should have written 2 fewer arguments". We wouldn't want
to say that if the thing didn't have two arguments. However,
in practice, GHC was getting this wrong, and this message
didn't seem all that helpful. Furthermore, the calculation
of the number of arguments is what caused #13819 to fall over.
This patch just removes uo_arity. In my opinion, the change
to error messages is a nudge in the right direction.

Test case: typecheck/should_fail/T13819

Previously, when canonicalizing (or unifying, in uType) a
heterogeneous equality, we emitted a kind equality and used the
resulting coercion to cast one side of the heterogeneous equality.

While sound, this led to terrible error messages. (See the bugs
listed below.) The problem is that using the coercion built from
the emitted kind equality is a bit like a wanted rewriting a wanted.
The solution is to keep heterogeneous equalities as irreducible.

See Note [Equalities with incompatible kinds] in TcCanonical.

This commit also removes a highly suspicious switch to FM_SubstOnly
when flattening in the kinds of a type variable. I have no idea
why this was there, other than as a holdover from pre-TypeInType.
I've not left a Note because there is simply no reason I can conceive
of that the FM_SubstOnly should be there.

One challenge with this patch is that the emitted derived equalities
might get emitted several times: when a heterogeneous equality is
in an implication and then gets floated out from the implication,
the Derived is present both in and out of the implication. This
causes a duplicate error message. (Test case:
typecheck/should_fail/T7368) Solution: track the provenance of
Derived constraints and refuse to float out a constraint that has
an insoluble Derived.

Lastly, this labels one test (dependent/should_fail/RAE_T32a)
as expect_broken, because the problem is really #12919. The
different handling of constraints in this patch exposes the error.

This fixes bugs #11198, #12373, #13530, and #13610.

test cases:
typecheck/should_fail/{T8262,T8603,tcail122,T12373,T13530,T13610}

This fixes #13915, where the promoted tycons belonging to data family
instances wouldn't get Typeable bindings, resulting in missing
declarations.

Test Plan: Validate with included testcases

Reviewers: austin, simonpj

Reviewed By: simonpj

Subscribers: simonpj, RyanGlScott, rwbarton, thomie

GHC Trac Issues: #13915

Differential Revision: https://phabricator.haskell.org/D3759

The IdBindingInfo field of ATcId serves two purposes

- to control generalisation when we have -XMonoLocalBinds
- to check for floatability when dealing with (static e)

These are related, but not the same, and they'd becomme confused.
Trac #13804 showed this up via an example like this:

  f periph = let sr :: forall a. [a] -> [a]
                 sr = if periph then reverse else id

                 sr2 = sr
                 -- The question: is sr2 generalised?
                 -- It should be, because sr has a type sig
                 -- even though it has periph free
             in
             (sr2 [True], sr2 "c")

Here sr2 should be generalised, despite the free var 'periph'
in 'sr' because 'sr' has a closed type signature.

I documented all this very carefully this time, in TcRnTypes:
  Note [Meaning of IdBindingInfo]
  Note [Bindings with closed types: ClosedTypeId]