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* Standalone kind signatures (#16794)wip/top-level-kind-signaturesVladislav Zavialov2019-09-252-5/+8
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Implements GHC Proposal #54: .../ghc-proposals/blob/master/proposals/0054-kind-signatures.rst With this patch, a type constructor can now be given an explicit standalone kind signature: {-# LANGUAGE StandaloneKindSignatures #-} type Functor :: (Type -> Type) -> Constraint class Functor f where fmap :: (a -> b) -> f a -> f b This is a replacement for CUSKs (complete user-specified kind signatures), which are now scheduled for deprecation. User-facing changes ------------------- * A new extension flag has been added, -XStandaloneKindSignatures, which implies -XNoCUSKs. * There is a new syntactic construct, a standalone kind signature: type <name> :: <kind> Declarations of data types, classes, data families, type families, and type synonyms may be accompanied by a standalone kind signature. * A standalone kind signature enables polymorphic recursion in types, just like a function type signature enables polymorphic recursion in terms. This obviates the need for CUSKs. * TemplateHaskell AST has been extended with 'KiSigD' to represent standalone kind signatures. * GHCi :info command now prints the kind signature of type constructors: ghci> :info Functor type Functor :: (Type -> Type) -> Constraint ... Limitations ----------- * 'forall'-bound type variables of a standalone kind signature do not scope over the declaration body, even if the -XScopedTypeVariables is enabled. See #16635 and #16734. * Wildcards are not allowed in standalone kind signatures, as partial signatures do not allow for polymorphic recursion. * Associated types may not be given an explicit standalone kind signature. Instead, they are assumed to have a CUSK if the parent class has a standalone kind signature and regardless of the -XCUSKs flag. * Standalone kind signatures do not support multiple names at the moment: type T1, T2 :: Type -> Type -- rejected type T1 = Maybe type T2 = Either String See #16754. * Creative use of equality constraints in standalone kind signatures may lead to GHC panics: type C :: forall (a :: Type) -> a ~ Int => Constraint class C a where f :: C a => a -> Int See #16758. Implementation notes -------------------- * The heart of this patch is the 'kcDeclHeader' function, which is used to kind-check a declaration header against its standalone kind signature. It does so in two rounds: 1. check user-written binders 2. instantiate invisible binders a la 'checkExpectedKind' * 'kcTyClGroup' now partitions declarations into declarations with a standalone kind signature or a CUSK (kinded_decls) and declarations without either (kindless_decls): * 'kinded_decls' are kind-checked with 'checkInitialKinds' * 'kindless_decls' are kind-checked with 'getInitialKinds' * DerivInfo has been extended with a new field: di_scoped_tvs :: ![(Name,TyVar)] These variables must be added to the context in case the deriving clause references tcTyConScopedTyVars. See #16731.
* base: Move Ix typeclass to GHC.IxBen Gamari2019-09-251-8/+8
| | | | | The `Ix` class seems rather orthogonal to its original home in `GHC.Arr`.
* PmCheck: Only ever check constantly many models against a single patternSebastian Graf2019-09-251-3/+0
| | | | | | | | | | | | | | | | | | | | | | | | | Introduces a new flag `-fmax-pmcheck-deltas` to achieve that. Deprecates the old `-fmax-pmcheck-iter` mechanism in favor of this new flag. From the user's guide: Pattern match checking can be exponential in some cases. This limit makes sure we scale polynomially in the number of patterns, by forgetting refined information gained from a partially successful match. For example, when matching `x` against `Just 4`, we split each incoming matching model into two sub-models: One where `x` is not `Nothing` and one where `x` is `Just y` but `y` is not `4`. When the number of incoming models exceeds the limit, we continue checking the next clause with the original, unrefined model. This also retires the incredibly hard to understand "maximum number of refinements" mechanism, because the current mechanism is more general and should catch the same exponential cases like PrelRules at the same time. ------------------------- Metric Decrease: T11822 -------------------------
* Module hierarchy: StgToCmm (#13009)Sylvain Henry2019-09-101-2/+2
| | | | | | Add StgToCmm module hierarchy. Platform modules that are used in several other places (NCG, LLVM codegen, Cmm transformations) are put into GHC.Platform.
* Doc: add Haddocks for quotRemWord2 primopSylvain Henry2019-08-221-2/+2
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* Introduce a type for "platform word size", use it instead of IntÖmer Sinan Ağacan2019-08-061-10/+11
| | | | | | | | We introduce a PlatformWordSize type and use it in platformWordSize field. This removes to panic/error calls called when platform word size is not 32 or 64. We now check for this when reading the platform config.
* Add Note [RuntimeRep and PrimRep] in RepTypeRichard Eisenberg2019-07-291-0/+6
| | | | | | | | | | Also adds Note [Getting from RuntimeRep to PrimRep], which deocuments a related thorny process. This Note addresses #16964, which correctly observes that documentation for this thorny design is lacking. Documentation only.
* Create {Int,Word}32RepJohn Ericson2019-07-174-42/+58
| | | | | | | This prepares the way for making Int32# and Word32# the actual size they claim to be. Updates binary submodule for (de)serializing the new runtime reps.
* primops: haddock: Fix typo in referenced function.Niklas Hambüchen2019-07-141-1/+1
| | | Found by @lehins.
* Remove purely external primopsJohn Ericson2019-07-141-36/+38
| | | | | | | | | | | | The compiler doesn't create uses nor compiles the uses that exist specially. These are just plain C-- FFI. These `await*` ones are especially important to so convert because "true" primops are hard to make platform-specific currently. The other exports are part of this commit so this module always exports something, which avoids silly CPP elsewhere. More will be added later once `foreign import prim` is extended.
* Don't eta-expand unsaturated primopsBen Gamari2019-06-253-3/+52
| | | | | | | | | | | Previously, as described in Note [Primop wrappers], `hasNoBinding` would return False in the case of `PrimOpId`s. This would result in eta expansion of unsaturated primop applications during CorePrep. Not only did this expansion result in unnecessary allocations, but it also meant lead to rather nasty inconsistencies between the CAFfy-ness determinations made by TidyPgm and CorePrep. This fixes #16846.
* ghci: Don't rely on resolution of System.IO to base moduleBen Gamari2019-06-221-1/+2
| | | | | | | | | Previously we would hackily evaluate a textual code snippet to compute actions to disable I/O buffering and flush the stdout/stderr handles. This broke in a number of ways (#15336, #16563). Instead we now ship a module (`GHC.GHCi.Helpers`) with `base` containing the needed actions. We can then easily refer to these via `Orig` names.
* Move 'Platform' to ghc-bootJohn Ericson2019-06-191-1/+1
| | | | | | | ghc-pkg needs to be aware of platforms so it can figure out which subdire within the user package db to use. This is admittedly roundabout, but maybe Cabal could use the same notion of a platform as GHC to good affect too.
* PrelRules: Don't break let/app invariant in shiftRuleBen Gamari2019-06-142-3/+43
| | | | | | | | | | | | | | | | | | | Previously shiftRule would rewrite as invalid shift like ``` let x = I# (uncheckedIShiftL# n 80) in ... ``` to ``` let x = I# (error "invalid shift") in ... ``` However, this breaks the let/app invariant as `error` is not okay-for-speculation. There isn't an easy way to avoid this so let's not try. Instead we just take advantage of the undefined nature of invalid shifts and return zero. Fixes #16742.
* Implement the -XUnliftedNewtypes extension.Andrew Martin2019-06-142-6/+27
| | | | | | | | | | | | | | | | | | | | | | | | | | | GHC Proposal: 0013-unlifted-newtypes.rst Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/98 Issues: #15219, #1311, #13595, #15883 Implementation Details: Note [Implementation of UnliftedNewtypes] Note [Unifying data family kinds] Note [Compulsory newtype unfolding] This patch introduces the -XUnliftedNewtypes extension. When this extension is enabled, GHC drops the restriction that the field in a newtype must be of kind (TYPE 'LiftedRep). This allows types like Int# and ByteArray# to be used in a newtype. Additionally, coerce is made levity-polymorphic so that it can be used with newtypes over unlifted types. The bulk of the changes are in TcTyClsDecls.hs. With -XUnliftedNewtypes, getInitialKind is more liberal, introducing a unification variable to return the kind (TYPE r0) rather than just returning (TYPE 'LiftedRep). When kind-checking a data constructor with kcConDecl, we attempt to unify the kind of a newtype with the kind of its field's type. When typechecking a data declaration with tcTyClDecl, we again perform a unification. See the implementation note for more on this. Co-authored-by: Richard Eisenberg <rae@richarde.dev>
* PrelRules: Ensure that string unpack/append rule fires with source notesBen Gamari2019-06-131-11/+18
| | | | | | | | | | | | | | | | | | | | Previously the presence of source notes could hide nested applications of `unpackFoldrCString#` from our constant folding logic. For instance, consider the expression: ```haskell unpackFoldrCString# "foo" c (unpackFoldrCString# "baz" c n) ``` Specifically, ticks appearing in two places can defeat the rule: a. Surrounding the inner application of `unpackFoldrCString#` b. Surrounding the fold function, `c` The latter caused the `str_rules` testcase to fail when `base` was built with `-g3`. Fixes #16740.
* Use DeriveFunctor throughout the codebase (#15654)Krzysztof Gogolewski2019-06-121-4/+3
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* Remove CPP ensuring word size is 32 or 64 bits around Addr# <-> int# primopsJohn Ericson2019-06-091-2/+0
| | | | | | | | | | | It shouldn't be needed these days, and those primops are "highly deprecated" anyways. This fits with my plans because it removes one bit of target-dependence of the builtin primops, and this is the hardest part of GHC to make multi-target. CC @carter
* Introduce log1p and expm1 primopschessai2019-06-091-0/+22
| | | | | Previously log and exp were primitives yet log1p and expm1 were FFI calls. Fix this non-uniformity.
* Fix #16700: Tiny errors in output of GHCi commands :forward and :infoRoland Senn2019-06-071-1/+2
| | | | | | `:info Coercible` now outputs the correct section number of the GHCi User's guide together with the secion title. `:forward x` gives the correct syntax hint.
* [skip ci] Improve the documentation of the CNF primops. In this context, the ↵Andrew Martin2019-06-071-23/+40
| | | | term "size" is ambiguous and is now avoided. Additionally, the distinction between a CNF and the blocks that comprise it has been emphasize. The vocabulary has been made more consistent with the vocabulary in the C source for CNF.
* Fix rewriting invalid shifts to errorsÖmer Sinan Ağacan2019-06-011-1/+4
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Fixes #16449. 5341edf3 removed a code in rewrite rules for bit shifts, which broke the "silly shift guard", causing generating invalid bit shifts or heap overflow in compile time while trying to evaluate those invalid bit shifts. The "guard" is explained in Note [Guarding against silly shifts] in PrelRules.hs. More specifically, this was the breaking change: --- a/compiler/prelude/PrelRules.hs +++ b/compiler/prelude/PrelRules.hs @@ -474,12 +474,11 @@ shiftRule shift_op ; case e1 of _ | shift_len == 0 -> return e1 - | shift_len < 0 || wordSizeInBits dflags < shift_len - -> return (mkRuntimeErrorApp rUNTIME_ERROR_ID wordPrimTy - ("Bad shift length" ++ show shift_len)) This patch reverts this change. Two new tests added: - T16449_1: The original reproducer in #16449. This was previously casing a heap overflow in compile time when CmmOpt tries to evaluate the large (invalid) bit shift in compile time, using `Integer` as the result type. Now it builds as expected. We now generate an error for the shift as expected. - T16449_2: Tests code generator for large (invalid) bit shifts.
* Fix typo of primop formatmizunashi_mana2019-05-271-1/+1
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* [skip ci] correct the docs for casArray a little more. clarify that the ↵Andrew Martin2019-04-211-3/+5
| | | | returned element may be two different things
* [skip ci] correct formatting of casArray# in docs for casSmallArray#Andrew Martin2019-04-211-1/+1
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* [skip ci] say "machine words" instead of "Int units" in the primops docsAndrew Martin2019-04-211-16/+19
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* improve docs for casArray and casSmallArrayAndrew Martin2019-04-211-1/+8
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* TH: make `Lift` and `TExp` levity-polymorphicAlec Theriault2019-04-181-4/+7
| | | | | | | | | | | | | | | | | | | | | | | | | | | | Besides the obvious benefits of being able to manipulate `TExp`'s of unboxed types, this also simplified `-XDeriveLift` all while making it more capable. * `ghc-prim` is explicitly depended upon by `template-haskell` * The following TH things are parametrized over `RuntimeRep`: - `TExp(..)` - `unTypeQ` - `unsafeTExpCoerce` - `Lift(..)` * The following instances have been added to `Lift`: - `Int#`, `Word#`, `Float#`, `Double#`, `Char#`, `Addr#` - unboxed tuples of lifted types up to arity 7 - unboxed sums of lifted types up to arity 7 Ideally we would have levity-polymorphic _instances_ of unboxed tuples and sums. * The code generated by `-XDeriveLift` uses expression quotes instead of generating large amounts of TH code and having special hard-coded cases for some unboxed types.
* Add support for bitreverse primopAlexandre2019-04-011-0/+11
| | | | | | 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.
* Update Wiki URLs to point to GitLabTakenobu Tani2019-03-251-2/+2
| | | | | | | | | | | | | | | | | | | | | | | This moves all URL references to Trac Wiki to their corresponding GitLab counterparts. This substitution is classified as follows: 1. Automated substitution using sed with Ben's mapping rule [1] Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy... New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy... 2. Manual substitution for URLs containing `#` index Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...#Zzz New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...#zzz 3. Manual substitution for strings starting with `Commentary` Old: Commentary/XxxYyy... New: commentary/xxx-yyy... See also !539 [1]: https://gitlab.haskell.org/bgamari/gitlab-migration/blob/master/wiki-mapping.json
* base: Remove `Monad(fail)` method and reexport `MonadFail(fail)` insteadHerbert Valerio Riedel2019-03-221-9/+5
| | | | | | As per https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail Coauthored-by: Ben Gamari <ben@well-typed.com>
* Replace nOfThem by replicateKrzysztof Gogolewski2019-03-191-3/+3
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* ghc-heap: Introduce closureSizeBen Gamari2019-03-171-0/+7
| | | | | | This function allows the user to compute the (non-transitive) size of a heap object in words. The "closure" in the name is admittedly confusing but we are stuck with this nomenclature at this point.
* Update Trac ticket URLs to point to GitLabRyan Scott2019-03-154-16/+16
| | | | | This moves all URL references to Trac tickets to their corresponding GitLab counterparts.
* Visible dependent quantificationRyan Scott2019-03-012-54/+55
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This implements GHC proposal 35 (https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0035-forall-arrow.rst) by adding the ability to write kinds with visible dependent quantification (VDQ). Most of the work for supporting VDQ was actually done _before_ this patch. That is, GHC has been able to reason about kinds with VDQ for some time, but it lacked the ability to let programmers directly write these kinds in the source syntax. This patch is primarly about exposing this ability, by: * Changing `HsForAllTy` to add an additional field of type `ForallVisFlag` to distinguish between invisible `forall`s (i.e, with dots) and visible `forall`s (i.e., with arrows) * Changing `Parser.y` accordingly The rest of the patch mostly concerns adding validity checking to ensure that VDQ is never used in the type of a term (as permitting this would require full-spectrum dependent types). This is accomplished by: * Adding a `vdqAllowed` predicate to `TcValidity`. * Introducing `splitLHsSigmaTyInvis`, a variant of `splitLHsSigmaTy` that only splits invisible `forall`s. This function is used in certain places (e.g., in instance declarations) to ensure that GHC doesn't try to split visible `forall`s (e.g., if it tried splitting `instance forall a -> Show (Blah a)`, then GHC would mistakenly allow that declaration!) This also updates Template Haskell by introducing a new `ForallVisT` constructor to `Type`. Fixes #16326. Also fixes #15658 by documenting this feature in the users' guide.
* Add AnonArgFlag to FunTySimon Peyton Jones2019-02-234-27/+38
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The big payload of this patch is: Add an AnonArgFlag to the FunTy constructor of Type, so that (FunTy VisArg t1 t2) means (t1 -> t2) (FunTy InvisArg t1 t2) means (t1 => t2) The big payoff is that we have a simple, local test to make when decomposing a type, leading to many fewer calls to isPredTy. To me the code seems a lot tidier, and probably more efficient (isPredTy has to take the kind of the type). See Note [Function types] in TyCoRep. There are lots of consequences * I made FunTy into a record, so that it'll be easier when we add a linearity field, something that is coming down the road. * Lots of code gets touched in a routine way, simply because it pattern matches on FunTy. * I wanted to make a pattern synonym for (FunTy2 arg res), which picks out just the argument and result type from the record. But alas the pattern-match overlap checker has a heart attack, and either reports false positives, or takes too long. In the end I gave up on pattern synonyms. There's some commented-out code in TyCoRep that shows what I wanted to do. * Much more clarity about predicate types, constraint types and (in particular) equality constraints in kinds. See TyCoRep Note [Types for coercions, predicates, and evidence] and Note [Constraints in kinds]. This made me realise that we need an AnonArgFlag on AnonTCB in a TyConBinder, something that was really plain wrong before. See TyCon Note [AnonTCB InivsArg] * When building function types we must know whether we need VisArg (mkVisFunTy) or InvisArg (mkInvisFunTy). This turned out to be pretty easy in practice. * Pretty-printing of types, esp in IfaceType, gets tidier, because we were already recording the (->) vs (=>) distinction in an ad-hoc way. Death to IfaceFunTy. * mkLamType needs to keep track of whether it is building (t1 -> t2) or (t1 => t2). See Type Note [mkLamType: dictionary arguments] Other minor stuff * Some tidy-up in validity checking involving constraints; Trac #16263
* Fix exprIsConApp_maybeSimon Peyton Jones2019-02-221-1/+1
| | | | | | | | | | | | | | | | | | | In this commit commit 7833cf407d1f608bebb1d38bb99d3035d8d735e6 Date: Thu Jan 24 17:58:50 2019 +0100 Look through newtype wrappers (Trac #16254) we made exprIsConApp_maybe quite a bit cleverer. But I had not paid enough attention to keeping exactly the correct substitution and in-scope set, which led to Trac #16348. There were several buglets (like applying the substitution twice in exprIsConApp_maybe, but the proximate source of the bug was that we were calling addNewInScopeIds, which deleted things from the substitution as well as adding them to the in-scope set. That's usually right, but not here! This was quite tricky to track down. But it is nicer now.
* Make constructor wrappers inline only during the final phaseArnaud Spiwack2019-02-191-2/+2
| | | | | | | For case-of-known constructor to continue triggering early, exprIsConApp_maybe is now capable of looking through lets and cases. See #15840
* 'forall' always a keyword, plus the dot type operatorVladislav Zavialov2019-02-151-4/+0
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* Fix #16293 by cleaning up Proxy# infelicitiesRyan Scott2019-02-121-5/+5
| | | | | | | | | | | | | | | | | | | | This bug fixes three problems related to `Proxy#`/`proxy#`: 1. Reifying it with TH claims that the `Proxy#` type constructor has two arguments, but that ought to be one for consistency with TH's treatment for other primitive type constructors like `(->)`. This was fixed by just returning the number of `tyConVisibleTyVars` instead of using `tyConArity` (which includes invisible arguments). 2. The role of `Proxy#`'s visible argument was hard-coded as nominal. Easily fixed by changing it to phantom. 3. The visibility of `proxy#`'s kind argument was specified, which is different from the `Proxy` constructor (which treats it as inferred). Some minor refactoring in `proxyHashId` fixed ths up. Along the way, I had to introduce a `mkSpecForAllTy` function, so I did some related Haddock cleanup in `Type`, where that function lives.
* Remove a few undefined prel namesÖmer Sinan Ağacan2019-02-081-29/+3
| | | | | | | | | | - breakpointAuto - breakpointJump - breakpointCondJump - breakpointAutoJump These Ids are never defined, but there were definitions about those in PrelNames. Those are now removed.
* Remove ExnStr and ThrowsExn businessSebastian Graf2019-02-011-11/+4
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* Use ByteString to represent Cmm string literals (#16198)Sylvain Henry2019-01-311-1/+1
| | | | Also used ByteString in some other relevant places
* Minor refactor [ci skip]Gabor Greif2019-01-232-5/+5
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* A few typofixesGabor Greif2019-01-231-1/+1
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* Error out of invalid Int/Word bit shiftsAlec Theriault2019-01-231-3/+2
| | | | | | | | | | | | | Although the Haddock's for `shiftL` and `shiftR` do require the number of bits to be non-negative, we should still check this before calling out to primitives (which also have undefined behaviour for negative bit shifts). If a user _really_ wants to bypass checks that the number of bits is sensible, they already have the aptly-named `unsafeShiftL`/`unsafeShiftR` at their disposal. See #16111.
* Prepare source-tree for base-4.13 MFP bumpHerbert Valerio Riedel2019-01-181-0/+2
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* Visible kind applicationmynguyen2019-01-031-17/+21
| | | | | | | | | | | | | | | | | | | | | | | Summary: This patch implements visible kind application (GHC Proposal 15/#12045), as well as #15360 and #15362. It also refactors unnamed wildcard handling, and requires that type equations in type families in Template Haskell be written with full type on lhs. PartialTypeSignatures are on and warnings are off automatically with visible kind application, just like in term-level. There are a few remaining issues with this patch, as documented in ticket #16082. Includes a submodule update for Haddock. Test Plan: Tests T12045a/b/c/TH1/TH2, T15362, T15592a Reviewers: simonpj, goldfire, bgamari, alanz, RyanGlScott, Iceland_jack Subscribers: ningning, Iceland_jack, RyanGlScott, int-index, rwbarton, mpickering, carter GHC Trac Issues: `#12045`, `#15362`, `#15592`, `#15788`, `#15793`, `#15795`, `#15797`, `#15799`, `#15801`, `#15807`, `#15816` Differential Revision: https://phabricator.haskell.org/D5229
* Careful tweaking to exprOkForSpeculationSimon Peyton Jones2018-12-072-56/+32
| | | | | | | | | | | | | | | | | | | | | | | | | | | This patch does several things: * Make exprOkForSpeculation ignore evaluatedness of variables See the Note [exprOkForSpeculation and evaluated variables] This means that the binder-swap transformation no longer invaliates the let/app invariant. * Make exprOkForSpeculation return False for DataToTagOp and SeqOp. See Note [exprOkForSpeculation and SeqOp/DataToTagOp] * Remove the 'can_fail' property from dataToTag#; it was always a hack (described in the old Note [dataToTag#] in primops.txt.pp), and now its not necessary because of the fixes above. * Make SetLevels use exprIsHNF, /not/ exprOkForSpeculation, when floating single-alternative cases. See SetLevels Note [Floating single-alternative cases] * Fix a buglet in FloatIn; probably never bites in practice See Note [Dead bindings] Collectively, these changes finally fix Trac #15696.
* Warn on all out-of-range literals in pats/exprsAlec Theriault2018-12-031-8/+8
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Summary: These changes were motivated by #13256. While poking around, I realized we weren't very consistent in our "-Woverflowed-literals" warnings. This patch fixes that by: * warning earlier on in the pipeline (ie. before we've desugared 'Int' patterns into 'I# Int#') * handling 'HsLit' as well as 'HsOverLit' (this covers unboxed literals) * covering more pattern / expression forms 4/6 of the warnings in the 'Overflow' test are due to this patch. The other two are mostly for completeness. Also fixed a missing empty-enumeration warning for 'Natural'. This warnings were tripped up by the 'Bounded Word' instance (see #9505), but the fix was obvious and simple: use unboxed word literals. Test Plan: make TEST=Overflow && make TEST=T10930 Reviewers: hvr, bgamari, RyanGlScott Reviewed By: RyanGlScott Subscribers: RyanGlScott, rwbarton, carter GHC Trac Issues: #13256, #10930 Differential Revision: https://phabricator.haskell.org/D5181
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