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* Demand: Let `Boxed` win in `lubBoxity` (#21119)Sebastian Graf2022-03-161-1/+3
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Previously, we let `Unboxed` win in `lubBoxity`, which is unsoundly optimistic in terms ob Boxity analysis. "Unsoundly" in the sense that we sometimes unbox parameters that we better shouldn't unbox. Examples are #18907 and T19871.absent. Until now, we thought that this hack pulled its weight becuase it worked around some shortcomings of the phase separation between Boxity analysis and CPR analysis. But it is a gross hack which caused regressions itself that needed all kinds of fixes and workarounds. See for example #20767. It became impossible to work with in !7599, so I want to remove it. For example, at the moment, `lubDmd B dmd` will not unbox `dmd`, but `lubDmd A dmd` will. Given that `B` is supposed to be the bottom element of the lattice, it's hardly justifiable to get a better demand when `lub`bing with `A`. The consequence of letting `Boxed` win in `lubBoxity` is that we *would* regress #2387, #16040 and parts of #5075 and T19871.sumIO, until Boxity and CPR are able to communicate better. Fortunately, that is not the case since I could tweak the other source of optimism in Boxity analysis that is described in `Note [Unboxed demand on function bodies returning small products]` so that we *recursively* assume unboxed demands on function bodies returning small products. See the updated Note. `Note [Boxity for bottoming functions]` describes why we need bottoming functions to have signatures that say that they deeply unbox their arguments. In so doing, I had to tweak `finaliseArgBoxities` so that it will never unbox recursive data constructors. This is in line with our handling of them in CPR. I updated `Note [Which types are unboxed?]` to reflect that. In turn we fix #21119, #20767, #18907, T19871.absent and get a much simpler implementation (at least to think about). We can also drop the very ad-hoc definition of `deferAfterPreciseException` and its Note in favor of the simple, intuitive definition we used to have. Metric Decrease: T16875 T18223 T18698a T18698b hard_hole_fits Metric Increase: LargeRecord MultiComponentModulesRecomp T15703 T8095 T9872d Out of all the regresions, only the one in T9872d doesn't vanish in a perf build, where the compiler is bootstrapped with -O2 and thus SpecConstr. Reason for regressions: * T9872d is due to `ty_co_subst` taking its `LiftingContext` boxed. That is because the context is passed to a function argument, for example in `liftCoSubstTyVarBndrUsing`. * In T15703, LargeRecord and T8095, we get a bit more allocations in `expand_syn` and `piResultTys`, because a `TCvSubst` isn't unboxed. In both cases that guards against reboxing in some code paths. * The same is true for MultiComponentModulesRecomp, where we get less unboxing in `GHC.Unit.Finder.$wfindInstalledHomeModule`. In a perf build, allocations actually *improve* by over 4%! Results on NoFib: -------------------------------------------------------------------------------- Program Allocs Instrs -------------------------------------------------------------------------------- awards -0.4% +0.3% cacheprof -0.3% +2.4% fft -1.5% -5.1% fibheaps +1.2% +0.8% fluid -0.3% -0.1% ida +0.4% +0.9% k-nucleotide +0.4% -0.1% last-piece +10.5% +13.9% lift -4.4% +3.5% mandel2 -99.7% -99.8% mate -0.4% +3.6% parser -1.0% +0.1% puzzle -11.6% +6.5% reverse-complem -3.0% +2.0% scs -0.5% +0.1% sphere -0.4% -0.2% wave4main -8.2% -0.3% -------------------------------------------------------------------------------- Summary excludes mandel2 because of excessive bias Min -11.6% -5.1% Max +10.5% +13.9% Geometric Mean -0.2% +0.3% -------------------------------------------------------------------------------- Not bad for a bug fix. The regression in `last-piece` could become a win if SpecConstr would work on non-recursive functions. The regression in `fibheaps` is due to `Note [Reboxed crud for bottoming calls]`, e.g., #21128.
* Introduce ConcreteTv metavariablessheaf2022-03-021-1/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This patch introduces a new kind of metavariable, by adding the constructor `ConcreteTv` to `MetaInfo`. A metavariable with `ConcreteTv` `MetaInfo`, henceforth a concrete metavariable, can only be unified with a type that is concrete (that is, a type that answers `True` to `GHC.Core.Type.isConcrete`). This solves the problem of dangling metavariables in `Concrete#` constraints: instead of emitting `Concrete# ty`, which contains a secret existential metavariable, we simply emit a primitive equality constraint `ty ~# concrete_tv` where `concrete_tv` is a fresh concrete metavariable. This means we can avoid all the complexity of canonicalising `Concrete#` constraints, as we can just re-use the existing machinery for `~#`. To finish things up, this patch then removes the `Concrete#` special predicate, and instead introduces the special predicate `IsRefl#` which enforces that a coercion is reflexive. Such a constraint is needed because the canonicaliser is quite happy to rewrite an equality constraint such as `ty ~# concrete_tv`, but such a rewriting is not handled by the rest of the compiler currently, as we need to make use of the resulting coercion, as outlined in the FixedRuntimeRep plan. The big upside of this approach (on top of simplifying the code) is that we can now selectively implement PHASE 2 of FixedRuntimeRep, by changing individual calls of `hasFixedRuntimeRep_MustBeRefl` to `hasFixedRuntimeRep` and making use of the obtained coercion.
* Kill derived constraintsRichard Eisenberg2022-02-231-19/+11
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Co-authored by: Sam Derbyshire Previously, GHC had three flavours of constraint: Wanted, Given, and Derived. This removes Derived constraints. Though serving a number of purposes, the most important role of Derived constraints was to enable better error messages. This job has been taken over by the new RewriterSets, as explained in Note [Wanteds rewrite wanteds] in GHC.Tc.Types.Constraint. Other knock-on effects: - Various new Notes as I learned about under-described bits of GHC - A reshuffling around the AST for implicit-parameter bindings, with better integration with TTG. - Various improvements around fundeps. These were caused by the fact that, previously, fundep constraints were all Derived, and Derived constraints would get dropped. Thus, an unsolved Derived didn't stop compilation. Without Derived, this is no longer possible, and so we have to be considerably more careful around fundeps. - A nice little refactoring in GHC.Tc.Errors to center the work on a new datatype called ErrorItem. Constraints are converted into ErrorItems at the start of processing, and this allows for a little preprocessing before the main classification. - This commit also cleans up the behavior in generalisation around functional dependencies. Now, if a variable is determined by functional dependencies, it will not be quantified. This change is user facing, but it should trim down GHC's strange behavior around fundeps. - Previously, reportWanteds did quite a bit of work, even on an empty WantedConstraints. This commit adds a fast path. - Now, GHC will unconditionally re-simplify constraints during quantification. See Note [Unconditionally resimplify constraints when quantifying], in GHC.Tc.Solver. Close #18398. Close #18406. Solve the fundep-related non-confluence in #18851. Close #19131. Close #19137. Close #20922. Close #20668. Close #19665. ------------------------- Metric Decrease: LargeRecord T9872b T9872b_defer T9872d TcPlugin_RewritePerf -------------------------
* Fix a few Note inconsistenciesBen Gamari2022-02-011-2/+2
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* Consistently upper-case "Note ["Ben Gamari2022-02-011-1/+1
| | | | | | This was achieved with git ls-tree --name-only HEAD -r | xargs sed -i -e 's/note \[/Note \[/g'
* Rework the handling of SkolemInfoMatthew Pickering2022-01-291-1/+4
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The main purpose of this patch is to attach a SkolemInfo directly to each SkolemTv. This fixes the large number of bugs which have accumulated over the years where we failed to report errors due to having "no skolem info" for particular type variables. Now the origin of each type varible is stored on the type variable we can always report accurately where it cames from. Fixes #20969 #20732 #20680 #19482 #20232 #19752 #10946 #19760 #20063 #13499 #14040 The main changes of this patch are: * SkolemTv now contains a SkolemInfo field which tells us how the SkolemTv was created. Used when reporting errors. * Enforce invariants relating the SkolemInfoAnon and level of an implication (ic_info, ic_tclvl) to the SkolemInfo and level of the type variables in ic_skols. * All ic_skols are TcTyVars -- Check is currently disabled * All ic_skols are SkolemTv * The tv_lvl of the ic_skols agrees with the ic_tclvl * The ic_info agrees with the SkolInfo of the implication. These invariants are checked by a debug compiler by checkImplicationInvariants. * Completely refactor kcCheckDeclHeader_sig which kept doing my head in. Plus, it wasn't right because it wasn't skolemising the binders as it decomposed the kind signature. The new story is described in Note [kcCheckDeclHeader_sig]. The code is considerably shorter than before (roughly 240 lines turns into 150 lines). It still has the same awkward complexity around computing arity as before, but that is a language design issue. See Note [Arity inference in kcCheckDeclHeader_sig] * I added new type synonyms MonoTcTyCon and PolyTcTyCon, and used them to be clear which TcTyCons have "finished" kinds etc, and which are monomorphic. See Note [TcTyCon, MonoTcTyCon, and PolyTcTyCon] * I renamed etaExpandAlgTyCon to splitTyConKind, becuase that's a better name, and it is very useful in kcCheckDeclHeader_sig, where eta-expansion isn't an issue. * Kill off the nasty `ClassScopedTvEnv` entirely. Co-authored-by: Simon Peyton Jones <simon.peytonjones@gmail.com>
* Make RULE matching insensitive to eta-expansionSimon Peyton Jones2022-01-261-1/+1
| | | | | | | | | | This patch fixes #19790 by making the rule matcher do on-the-fly eta reduction. See Note [Eta reduction the target] in GHC.Core.Rules I found I also had to careful about casts when matching; see Note [Casts in the target] and Note [Casts in the template] Lots more comments and Notes in the rule matcher
* Ensure new Ct/evidence invariantChristiaan Baaij2021-11-251-1/+7
| | | | | | | | | | The `ctev_pred` field of a `CtEvidence` is a just a cache for the type of the evidence. More precisely: * For Givens, `ctev_pred` = `varType ctev_evar` * For Wanteds, `ctev_pred` = `evDestType ctev_dest` This new invariant is needed because evidence can become part of a type, via `Castty ty kco`.
* Add one line of comments (c.f. !5706)Simon Peyton Jones2021-11-181-0/+1
| | | | | | | Ticket #19815 suggested changing coToMCo to use isReflexiveCo rather than isReflCo. But perf results weren't encouraging. This patch just adds a comment to point to the data, such as it is.
* Compare FunTys as if they were TyConApps.Richard Eisenberg2021-09-291-1/+1
| | | | | | | | | | | See Note [Equality on FunTys] in TyCoRep. Close #17675. Close #17655, about documentation improvements included in this patch. Close #19677, about a further mistake around FunTy. test cases: typecheck/should_compile/T19677
* Use Reductions to keep track of rewritingssheaf2021-08-041-437/+50
| | | | | | | | | | | | | | | | | | | | | | | | | | | | We define Reduction = Reduction Coercion !Type. A reduction of the form 'Reduction co new_ty' witnesses an equality ty ~co~> new_ty. That is, the rewriting happens left-to-right: the right-hand-side type of the coercion is the rewritten type, and the left-hand-side type the original type. Sticking to this convention makes the codebase more consistent, helping to avoid certain applications of SymCo. This replaces the parts of the codebase which represented reductions as pairs, (Coercion,Type) or (Type,Coercion). Reduction being strict in the Type argument improves performance in some programs that rewrite many type families (such as T9872). Fixes #20161 ------------------------- Metric Decrease: T5321Fun T9872a T9872b T9872c T9872d -------------------------
* Improve performance of eta expansionSimon Peyton Jones2021-07-281-1/+10
| | | | | | | | | | | | | | | | | | | | | | | | | | | Eta expansion was taking ages on T18223. This patch * Aggressively squash reflexive casts in etaInfoApp. See Note [Check for reflexive casts in eta expansion] These changes decreased compile-time allocation by 80%! * Passes the Simplifier's in-scope set to etaExpandAT, so we don't need to recompute it. (This alone saved 10% of compile time.) Annoyingly several functions in the Simplifier (namely makeTrivialBinding and friends) need to get SimplEnv, rather than SimplMode, but that is no big deal. Lots of small changes in compile-time allocation, less than 1% and in both directions. A couple of bigger changes, including the rather delicate T18223 T12425(optasm) ghc/alloc 98448216.0 97121224.0 -1.3% GOOD T18223(normal) ghc/alloc 5454689676.0 1138238008.0 -79.1% GOOD Metric Decrease: T12425 T18223
* Simplify and improve the eta expansion mechanismSimon Peyton Jones2021-07-281-13/+43
| | | | | | | | | | | Previously the eta-expansion would return lambdas interspersed with casts; now the cast is just pushed to the outside: #20153. This actually simplifies the code. I also improved mkNthCo to account for SymCo, so that mkNthCo n (SymCo (TyConAppCo tc cos)) would work well.
* Remove useless {-# LANGUAGE CPP #-} pragmasSylvain Henry2021-05-121-1/+0
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* Fully remove HsVersions.hSylvain Henry2021-05-121-2/+0
| | | | | | | | | | Replace uses of WARN macro with calls to: warnPprTrace :: Bool -> SDoc -> a -> a Remove the now unused HsVersions.h Bump haddock submodule
* Replace CPP assertions with Haskell functionsSylvain Henry2021-05-121-52/+53
| | | | | | | | | | | | | | | There is no reason to use CPP. __LINE__ and __FILE__ macros are now better replaced with GHC's CallStack. As a bonus, assert error messages now contain more information (function name, column). Here is the mapping table (HasCallStack omitted): * ASSERT: assert :: Bool -> a -> a * MASSERT: massert :: Bool -> m () * ASSERTM: assertM :: m Bool -> m () * ASSERT2: assertPpr :: Bool -> SDoc -> a -> a * MASSERT2: massertPpr :: Bool -> SDoc -> m () * ASSERTM2: assertPprM :: m Bool -> SDoc -> m ()
* Persist CorePrepProv into IfaceUnivCoProvSimon Peyton Jones2021-05-041-5/+5
| | | | | | | | | | | | | | | | CorePrepProv is only created in CorePrep, so I thought it wouldn't be needed in IfaceUnivCoProv. But actually IfaceSyn is used during pretty-printing, and we can certainly pretty-print things after CorePrep as #19768 showed. So the simplest thing is to represent CorePrepProv in IfaceSyn. To improve what Lint can do I also added a boolean to CorePrepProv, to record whether it is homogeneously kinded or not. It is introduced in two distinct ways (see Note [Unsafe coercions] in GHC.CoreToStg.Prep), one of which may be hetero-kinded (e.g. Int ~ Int#) beause it is casting a divergent expression; but the other is not. The boolean keeps track.
* Eliminate unsafeEqualityProof in CorePrepSimon Peyton Jones2021-04-261-0/+3
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The main idea here is to avoid treating * case e of {} * case unsafeEqualityProof of UnsafeRefl co -> blah specially in CoreToStg. Instead, nail them in CorePrep, by converting case e of {} ==> e |> unsafe-co case unsafeEqualityProof of UnsafeRefl cv -> blah ==> blah[unsafe-co/cv] in GHC.Core.Prep. Now expressions that we want to treat as trivial really are trivial. We can get rid of cpExprIsTrivial. And we fix #19700. A downside is that, at least under unsafeEqualityProof, we substitute in types and coercions, which is more work. But a big advantage is that it's all very simple and principled: CorePrep really gets rid of the unsafeCoerce stuff, as it does empty case, runRW#, lazyId etc. I've updated the overview in GHC.Core.Prep, and added Note [Unsafe coercions] in GHC.Core.Prep Note [Implementing unsafeCoerce] in base:Unsafe.Coerce We get 3% fewer bytes allocated when compiling perf/compiler/T5631, which uses a lot of unsafeCoerces. (It's a happy-generated parser.) Metric Decrease: T5631
* Rename the flattener to become the rewriter.Richard Eisenberg2020-12-011-43/+43
| | | | | | | | Now that flattening doesn't produce flattening variables, it's not really flattening anything: it's rewriting. This change also means that the rewriter can no longer be confused the core flattener (in GHC.Core.Unify), which is sometimes used during type-checking.
* Remove flattening variablesRichard Eisenberg2020-12-011-27/+73
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This patch redesigns the flattener to simplify type family applications directly instead of using flattening meta-variables and skolems. The key new innovation is the CanEqLHS type and the new CEqCan constraint (Ct). A CanEqLHS is either a type variable or exactly-saturated type family application; either can now be rewritten using a CEqCan constraint in the inert set. Because the flattener no longer reduces all type family applications to variables, there was some performance degradation if a lengthy type family application is now flattened over and over (not making progress). To compensate, this patch contains some extra optimizations in the flattener, leading to a number of performance improvements. Close #18875. Close #18910. There are many extra parts of the compiler that had to be affected in writing this patch: * The family-application cache (formerly the flat-cache) sometimes stores coercions built from Given inerts. When these inerts get kicked out, we must kick out from the cache as well. (This was, I believe, true previously, but somehow never caused trouble.) Kicking out from the cache requires adding a filterTM function to TrieMap. * This patch obviates the need to distinguish "blocking" coercion holes from non-blocking ones (which, previously, arose from CFunEqCans). There is thus some simplification around coercion holes. * Extra commentary throughout parts of the code I read through, to preserve the knowledge I gained while working. * A change in the pure unifier around unifying skolems with other types. Unifying a skolem now leads to SurelyApart, not MaybeApart, as documented in Note [Binding when looking up instances] in GHC.Core.InstEnv. * Some more use of MCoercion where appropriate. * Previously, class-instance lookup automatically noticed that e.g. C Int was a "unifier" to a target [W] C (F Bool), because the F Bool was flattened to a variable. Now, a little more care must be taken around checking for unifying instances. * Previously, tcSplitTyConApp_maybe would split (Eq a => a). This is silly, because (=>) is not a tycon in Haskell. Fixed now, but there are some knock-on changes in e.g. TrieMap code and in the canonicaliser. * New function anyFreeVarsOf{Type,Co} to check whether a free variable satisfies a certain predicate. * Type synonyms now remember whether or not they are "forgetful"; a forgetful synonym drops at least one argument. This is useful when flattening; see flattenView. * The pattern-match completeness checker invokes the solver. This invocation might need to look through newtypes when checking representational equality. Thus, the desugarer needs to keep track of the in-scope variables to know what newtype constructors are in scope. I bet this bug was around before but never noticed. * Extra-constraints wildcards are no longer simplified before printing. See Note [Do not simplify ConstraintHoles] in GHC.Tc.Solver. * Whether or not there are Given equalities has become slightly subtler. See the new HasGivenEqs datatype. * Note [Type variable cycles in Givens] in GHC.Tc.Solver.Canonical explains a significant new wrinkle in the new approach. * See Note [What might match later?] in GHC.Tc.Solver.Interact, which explains the fix to #18910. * The inert_count field of InertCans wasn't actually used, so I removed it. Though I (Richard) did the implementation, Simon PJ was very involved in design and review. This updates the Haddock submodule to avoid #18932 by adding a type signature. ------------------------- Metric Decrease: T12227 T5030 T9872a T9872b T9872c Metric Increase: T9872d -------------------------
* Name (tc)SplitForAll- functions more consistentlyRyan Scott2020-11-151-7/+7
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | There is a zoo of `splitForAll-` functions in `GHC.Core.Type` (as well as `tcSplitForAll-` functions in `GHC.Tc.Utils.TcType`) that all do very similar things, but vary in the particular form of type variable that they return. To make things worse, the names of these functions are often quite misleading. Some particularly egregious examples: * `splitForAllTys` returns `TyCoVar`s, but `splitSomeForAllTys` returns `VarBndr`s. * `splitSomeForAllTys` returns `VarBndr`s, but `tcSplitSomeForAllTys` returns `TyVar`s. * `splitForAllTys` returns `TyCoVar`s, but `splitForAllTysInvis` returns `InvisTVBinder`s. (This in particular arose in the context of #18939, and this finally motivated me to bite the bullet and improve the status quo vis-à-vis how we name these functions.) In an attempt to bring some sanity to how these functions are named, I have opted to rename most of these functions en masse to use consistent suffixes that describe the particular form of type variable that each function returns. In concrete terms, this amounts to: * Functions that return a `TyVar` now use the suffix `-TyVar`. This caused the following functions to be renamed: * `splitTyVarForAllTys` -> `splitForAllTyVars` * `splitForAllTy_ty_maybe` -> `splitForAllTyVar_maybe` * `tcSplitForAllTys` -> `tcSplitForAllTyVars` * `tcSplitSomeForAllTys` -> `tcSplitSomeForAllTyVars` * Functions that return a `CoVar` now use the suffix `-CoVar`. This caused the following functions to be renamed: * `splitForAllTy_co_maybe` -> `splitForAllCoVar_maybe` * Functions that return a `TyCoVar` now use the suffix `-TyCoVar`. This caused the following functions to be renamed: * `splitForAllTy` -> `splitForAllTyCoVar` * `splitForAllTys` -> `splitForAllTyCoVars` * `splitForAllTys'` -> `splitForAllTyCoVars'` * `splitForAllTy_maybe` -> `splitForAllTyCoVar_maybe` * Functions that return a `VarBndr` now use the suffix corresponding to the most relevant type synonym. This caused the following functions to be renamed: * `splitForAllVarBndrs` -> `splitForAllTyCoVarBinders` * `splitForAllTysInvis` -> `splitForAllInvisTVBinders` * `splitForAllTysReq` -> `splitForAllReqTVBinders` * `splitSomeForAllTys` -> `splitSomeForAllTyCoVarBndrs` * `tcSplitForAllVarBndrs` -> `tcSplitForAllTyVarBinders` * `tcSplitForAllTysInvis` -> `tcSplitForAllInvisTVBinders` * `tcSplitForAllTysReq` -> `tcSplitForAllReqTVBinders` * `tcSplitForAllTy_maybe` -> `tcSplitForAllTyVarBinder_maybe` Note that I left the following functions alone: * Functions that split apart things besides `ForAllTy`s, such as `splitFunTys` or `splitPiTys`. Thankfully, there are far fewer of these functions than there are functions that split apart `ForAllTy`s, so there isn't much of a pressing need to apply the new naming convention elsewhere. * Functions that split apart `ForAllCo`s in `Coercion`s, such as `GHC.Core.Coercion.splitForAllCo_maybe`. We could theoretically apply the new naming convention here, but then we'd have to figure out how to disambiguate `Type`-splitting functions from `Coercion`-splitting functions. Ultimately, the `Coercion`-splitting functions aren't used nearly as much as the `Type`-splitting functions, so I decided to leave the former alone. This is purely refactoring and should cause no change in behavior.
* Replace HsImplicitBndrs with HsOuterTyVarBndrsRyan Scott2020-11-061-2/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This refactors the GHC AST to remove `HsImplicitBndrs` and replace it with `HsOuterTyVarBndrs`, a type which records whether the outermost quantification in a type is explicit (i.e., with an outermost, invisible `forall`) or implicit. As a result of this refactoring, it is now evident in the AST where the `forall`-or-nothing rule applies: it's all the places that use `HsOuterTyVarBndrs`. See the revamped `Note [forall-or-nothing rule]` in `GHC.Hs.Type` (previously in `GHC.Rename.HsType`). Moreover, the places where `ScopedTypeVariables` brings lexically scoped type variables into scope are a subset of the places that adhere to the `forall`-or-nothing rule, so this also makes places that interact with `ScopedTypeVariables` easier to find. See the revamped `Note [Lexically scoped type variables]` in `GHC.Hs.Type` (previously in `GHC.Tc.Gen.Sig`). `HsOuterTyVarBndrs` are used in type signatures (see `HsOuterSigTyVarBndrs`) and type family equations (see `HsOuterFamEqnTyVarBndrs`). The main difference between the former and the latter is that the former cares about specificity but the latter does not. There are a number of knock-on consequences: * There is now a dedicated `HsSigType` type, which is the combination of `HsOuterSigTyVarBndrs` and `HsType`. `LHsSigType` is now an alias for an `XRec` of `HsSigType`. * Working out the details led us to a substantial refactoring of the handling of explicit (user-written) and implicit type-variable bindings in `GHC.Tc.Gen.HsType`. Instead of a confusing family of higher order functions, we now have a local data type, `SkolemInfo`, that controls how these binders are kind-checked. It remains very fiddly, not fully satisfying. But it's better than it was. Fixes #16762. Bumps the Haddock submodule. Co-authored-by: Simon Peyton Jones <simonpj@microsoft.com> Co-authored-by: Richard Eisenberg <rae@richarde.dev> Co-authored-by: Zubin Duggal <zubin@cmi.ac.in>
* Fix two constraint solving problemsSimon Peyton Jones2020-10-271-11/+49
| | | | | | | | | | | | | | | | | | | | | This patch fixes two problems in the constraint solver. * An actual bug #18555: we were floating out a constraint to eagerly, and that was ultimately fatal. It's explained in Note [Do not float blocked constraints] in GHC.Core.Constraint. This is all very delicate, but it's all going to become irrelevant when we stop floating constraints (#17656). * A major performance infelicity in the flattener. When flattening (ty |> co) we *never* generated Refl, even when there was nothing at all to do. Result: we would gratuitously rewrite the constraint to exactly the same thing, wasting work. Described in #18413, and came up again in #18855. Solution: exploit the special case by calling the new function castCoercionKind1. See Note [castCoercionKind1] in GHC.Core.Coercion
* Lint the compiler for extraneous LANGUAGE pragmasHécate2020-10-101-3/+5
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* Add TyCon Set/Env and use them in a few places.Andreas Klebinger2020-10-091-0/+1
| | | | | | | | | | | Firstly this improves code clarity. But it also has performance benefits as we no longer go through the name of the TyCon to get at it's unique. In order to make this work the recursion check for TyCon has been moved into it's own module in order to avoid import cycles.
* Fix desugaring of record updates on data familiesSimon Peyton Jones2020-10-091-0/+22
| | | | | | | | | | | | This fixes a long-standing bug in the desugaring of record updates for data families, when the latter involves a GADT. It's all explained in Note [Update for GADTs] in GHC.HsToCore.Expr. Building the correct cast is surprisingly tricky, as that Note explains. Fixes #18809. The test case (in indexed-types/should_compile/T18809) contains several examples that exercise the dark corners.
* Optimize NthCo (FunCo ...) in coercion optRichard Eisenberg2020-09-291-19/+25
| | | | | | | | | | We were missing this case previously. Close #18528. Metric Decrease: T18223 T5321Fun
* Fix handling of function coercions (#18747)Krzysztof Gogolewski2020-09-261-1/+1
| | | | This was broken when we added multiplicity to the function type.
* Better eta-expansion (again) and don't specilise DFunsSimon Peyton Jones2020-09-221-26/+41
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This patch fixes #18223, which made GHC generate an exponential amount of code. There are three quite separate changes in here 1. Re-engineer eta-expansion (again). The eta-expander was generating lots of intermediate stuff, which could be optimised away, but which choked the simplifier meanwhile. Relatively easy to kill it off at source. See Note [The EtaInfo mechanism] in GHC.Core.Opt.Arity. The main new thing is the use of pushCoArg in getArg_maybe. 2. Stop Specialise specalising DFuns. This is the cause of a huge (and utterly unnecessary) blowup in program size in #18223. See Note [Do not specialise DFuns] in GHC.Core.Opt.Specialise. I also refactored the Specialise monad a bit... it was silly, because it passed on unchanging values as if they were mutable state. 3. Do an extra Simplifer run, after SpecConstra and before late-Specialise. I found (investigating perf/compiler/T16473) that failing to do this was crippling *both* SpecConstr *and* Specialise. See Note [Simplify after SpecConstr] in GHC.Core.Opt.Pipeline. This change does mean an extra run of the Simplifier, but only with -O2, and I think that's acceptable. T16473 allocates *three* times less with this change. (I changed it to check runtime rather than compile time.) Some smaller consequences * I moved pushCoercion, pushCoArg and friends from SimpleOpt to Arity, because it was needed by the new etaInfoApp. And pushCoValArg now returns a MCoercion rather than Coercion for the argument Coercion. * A minor, incidental improvement to Core pretty-printing This does fix #18223, (which was otherwise uncompilable. Hooray. But there is still a big intermediate because there are some very deeply nested types in that program. Modest reductions in compile-time allocation on a couple of benchmarks T12425 -2.0% T13253 -10.3% Metric increase with -O2, due to extra simplifier run T9233 +5.8% T12227 +1.8% T15630 +5.0% There is a spurious apparent increase on heap residency on T9630, on some architectures at least. I tried it with -G1 and the residency is essentially unchanged. Metric Increase T9233 T12227 T9630 Metric Decrease T12425 T13253
* DynFlags: disentangle OutputableSylvain Henry2020-08-121-2/+4
| | | | | | | | | - put panic related functions into GHC.Utils.Panic - put trace related functions using DynFlags in GHC.Driver.Ppr One step closer making Outputable fully independent of DynFlags. Bump haddock submodule
* Add two bangs to improve perf of flatteningSimon Peyton Jones2020-07-301-4/+6
| | | | | | | | | | | | | | | | | | | | This tiny patch improves the compile time of flatten-heavy programs by 1-2%, by adding two bangs. Addresses (somewhat) #18502 This reduces allocation by T9872b -1.1% T9872d -3.3% T5321Fun -0.2% T5631 -0.2% T5837 +0.1% T6048 +0.1% Metric Decrease: T9872b T9872d
* This patch addresses the exponential blow-up in the simplifier.Simon Peyton Jones2020-07-281-1/+1
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Specifically: #13253 exponential inlining #10421 ditto #18140 strict constructors #18282 another nested-function call case This patch makes one really significant changes: change the way that mkDupableCont handles StrictArg. The details are explained in GHC.Core.Opt.Simplify Note [Duplicating StrictArg]. Specific changes * In mkDupableCont, when making auxiliary bindings for the other arguments of a call, add extra plumbing so that we don't forget the demand on them. Otherwise we haev to wait for another round of strictness analysis. But actually all the info is to hand. This change affects: - Make the strictness list in ArgInfo be [Demand] instead of [Bool], and rename it to ai_dmds. - Add as_dmd to ValArg - Simplify.makeTrivial takes a Demand - mkDupableContWithDmds takes a [Demand] There are a number of other small changes 1. For Ids that are used at most once in each branch of a case, make the occurrence analyser record the total number of syntactic occurrences. Previously we recorded just OneBranch or MultipleBranches. I thought this was going to be useful, but I ended up barely using it; see Note [Note [Suppress exponential blowup] in GHC.Core.Opt.Simplify.Utils Actual changes: * See the occ_n_br field of OneOcc. * postInlineUnconditionally 2. I found a small perf buglet in SetLevels; see the new function GHC.Core.Opt.SetLevels.hasFreeJoin 3. Remove the sc_cci field of StrictArg. I found I could get its information from the sc_fun field instead. Less to get wrong! 4. In ArgInfo, arrange that ai_dmds and ai_discs have a simpler invariant: they line up with the value arguments beyond ai_args This allowed a bit of nice refactoring; see isStrictArgInfo, lazyArgcontext, strictArgContext There is virtually no difference in nofib. (The runtime numbers are bogus -- I tried a few manually.) Program Size Allocs Runtime Elapsed TotalMem -------------------------------------------------------------------------------- fft +0.0% -2.0% -48.3% -49.4% 0.0% multiplier +0.0% -2.2% -50.3% -50.9% 0.0% -------------------------------------------------------------------------------- Min -0.4% -2.2% -59.2% -60.4% 0.0% Max +0.0% +0.1% +3.3% +4.9% 0.0% Geometric Mean +0.0% -0.0% -33.2% -34.3% -0.0% Test T18282 is an existing example of these deeply-nested strict calls. We get a big decrease in compile time (-85%) because so much less inlining takes place. Metric Decrease: T18282
* Clean up haddock hyperlinks of GHC.* (part1)Takenobu Tani2020-06-251-3/+3
| | | | | | | | | | | | | | | | | | | This updates haddock comments only. This patch focuses to update for hyperlinks in GHC API's haddock comments, because broken links especially discourage newcomers. This includes the following hierarchies: - GHC.Hs.* - GHC.Core.* - GHC.Stg.* - GHC.Cmm.* - GHC.Types.* - GHC.Data.* - GHC.Builtin.* - GHC.Parser.* - GHC.Driver.* - GHC top
* Two small tweaks to Coercion.simplifyArgsWorkerSimon Peyton Jones2020-06-231-1/+5
| | | | | | | | | | | These tweaks affect the inner loop of simplifyArgsWorker, which in turn is called from the flattener in Flatten.hs. This is a key perf bottleneck to T9872{a,b,c,d}. These two small changes have a modest but useful benefit. No change in functionality whatsoever. Relates to #18354
* Various performance improvementsKrzysztof Gogolewski2020-06-171-3/+3
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This implements several general performance improvements to GHC, to offset the effect of the linear types change. General optimisations: - Add a `coreFullView` function which iterates `coreView` on the head. This avoids making function recursive solely because the iterate `coreView` themselves. As a consequence, this functions can be inlined, and trigger case-of-known constructor (_e.g._ `kindRep_maybe`, `isLiftedRuntimeRep`, `isMultiplicityTy`, `getTyVar_maybe`, `splitAppTy_maybe`, `splitFunType_maybe`, `tyConAppTyCon_maybe`). The common pattern about all these functions is that they are almost always used as views, and immediately consumed by a case expression. This commit also mark them asx `INLINE`. - In `subst_ty` add a special case for nullary `TyConApp`, which avoid allocations altogether. - Use `mkTyConApp` in `subst_ty` for the general `TyConApp`. This required quite a bit of module shuffling. case. `myTyConApp` enforces crucial sharing, which was lost during substitution. See also !2952 . - Make `subst_ty` stricter. - In `eqType` (specifically, in `nonDetCmpType`), add a special case, tested first, for the very common case of nullary `TyConApp`. `nonDetCmpType` has been made `INLINE` otherwise it is actually a regression. This is similar to the optimisations in !2952. Linear-type specific optimisations: - Use `tyConAppTyCon_maybe` instead of the more complex `eqType` in the definition of the pattern synonyms `One` and `Many`. - Break the `hs-boot` cycles between `Multiplicity.hs` and `Type.hs`: `Multiplicity` now import `Type` normally, rather than from the `hs-boot`. This way `tyConAppTyCon_maybe` can inline properly in the `One` and `Many` pattern synonyms. - Make `updateIdTypeAndMult` strict in its type and multiplicity - The `scaleIdBy` gets a specialised definition rather than being an alias to `scaleVarBy` - `splitFunTy_maybe` is given the type `Type -> Maybe (Mult, Type, Type)` instead of `Type -> Maybe (Scaled Type, Type)` - Remove the `MultMul` pattern synonym in favour of a view `isMultMul` because pattern synonyms appear not to inline well. - in `eqType`, in a `FunTy`, compare multiplicities last: they are almost always both `Many`, so it helps failing faster. - Cache `manyDataConTy` in `mkTyConApp`, to make sure that all the instances of `TyConApp ManyDataConTy []` are physically the same. This commit has been authored by * Richard Eisenberg * Krzysztof Gogolewski * Arnaud Spiwack Metric Decrease: haddock.base T12227 T12545 T12990 T1969 T3064 T5030 T9872b Metric Increase: haddock.base haddock.Cabal haddock.compiler T12150 T12234 T12425 T12707 T13035 T13056 T15164 T16190 T18304 T1969 T3064 T3294 T5631 T5642 T5837 T6048 T9020 T9233 T9675 T9872a T9961 WWRec
* Linear types (#15981)Krzysztof Gogolewski2020-06-171-40/+52
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This is the first step towards implementation of the linear types proposal (https://github.com/ghc-proposals/ghc-proposals/pull/111). It features * A language extension -XLinearTypes * Syntax for linear functions in the surface language * Linearity checking in Core Lint, enabled with -dlinear-core-lint * Core-to-core passes are mostly compatible with linearity * Fields in a data type can be linear or unrestricted; linear fields have multiplicity-polymorphic constructors. If -XLinearTypes is disabled, the GADT syntax defaults to linear fields The following items are not yet supported: * a # m -> b syntax (only prefix FUN is supported for now) * Full multiplicity inference (multiplicities are really only checked) * Decent linearity error messages * Linear let, where, and case expressions in the surface language (each of these currently introduce the unrestricted variant) * Multiplicity-parametric fields * Syntax for annotating lambda-bound or let-bound with a multiplicity * Syntax for non-linear/multiple-field-multiplicity records * Linear projections for records with a single linear field * Linear pattern synonyms * Multiplicity coercions (test LinearPolyType) A high-level description can be found at https://ghc.haskell.org/trac/ghc/wiki/LinearTypes/Implementation Following the link above you will find a description of the changes made to Core. This commit has been authored by * Richard Eisenberg * Krzysztof Gogolewski * Matthew Pickering * Arnaud Spiwack With contributions from: * Mark Barbone * Alexander Vershilov Updates haddock submodule.
* Implement cast worker/wrapper properlySimon Peyton Jones2020-06-101-2/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | The cast worker/wrapper transformation transforms x = e |> co into y = e x = y |> co This is done by the simplifier, but we were being careless about transferring IdInfo from x to y, and about what to do if x is a NOINLNE function. This resulted in a series of bugs: #17673, #18093, #18078. This patch fixes all that: * Main change is in GHC.Core.Opt.Simplify, and the new prepareBinding function, which does this cast worker/wrapper transform. See Note [Cast worker/wrappers]. * There is quite a bit of refactoring around prepareRhs, makeTrivial etc. It's nicer now. * Some wrappers from strictness and cast w/w, notably those for a function with a NOINLINE, should inline very late. There wasn't really a mechanism for that, which was an existing bug really; so I invented a new finalPhase = Phase (-1). It's used for all simplifier runs after the user-visible phase 2,1,0 have run. (No new runs of the simplifier are introduced thereby.) See new Note [Compiler phases] in GHC.Types.Basic; the main changes are in GHC.Core.Opt.Driver * Doing this made me trip over two places where the AnonArgFlag on a FunTy was being lost so we could end up with (Num a -> ty) rather than (Num a => ty) - In coercionLKind/coercionRKind - In contHoleType in the Simplifier I fixed the former by defining mkFunctionType and using it in coercionLKind/RKind. I could have done the same for the latter, but the information is almost to hand. So I fixed the latter by - adding sc_hole_ty to ApplyToVal (like ApplyToTy), - adding as_hole_ty to ValArg (like TyArg) - adding sc_fun_ty to StrictArg Turned out I could then remove ai_type from ArgInfo. This is just moving the deck chairs around, but it worked out nicely. See the new Note [AnonArgFlag] in GHC.Types.Var * When looking at the 'arity decrease' thing (#18093) I discovered that stable unfoldings had a much lower arity than the actual optimised function. That's what led to the arity-decrease message. Simple solution: eta-expand. It's described in Note [Eta-expand stable unfoldings] in GHC.Core.Opt.Simplify * I also discovered that unsafeCoerce wasn't being inlined if the context was boring. So (\x. f (unsafeCoerce x)) would create a thunk -- yikes! I fixed that by making inlineBoringOK a bit cleverer: see Note [Inline unsafeCoerce] in GHC.Core.Unfold. I also found that unsafeCoerceName was unused, so I removed it. I made a test case for #18078, and a very similar one for #17673. The net effect of all this on nofib is very modest, but positive: -------------------------------------------------------------------------------- Program Size Allocs Runtime Elapsed TotalMem -------------------------------------------------------------------------------- anna -0.4% -0.1% -3.1% -3.1% 0.0% fannkuch-redux -0.4% -0.3% -0.1% -0.1% 0.0% maillist -0.4% -0.1% -7.8% -1.0% -14.3% primetest -0.4% -15.6% -7.1% -6.6% 0.0% -------------------------------------------------------------------------------- Min -0.9% -15.6% -13.3% -14.2% -14.3% Max -0.3% 0.0% +12.1% +12.4% 0.0% Geometric Mean -0.4% -0.2% -2.3% -2.2% -0.1% All following metric decreases are compile-time allocation decreases between -1% and -3%: Metric Decrease: T5631 T13701 T14697 T15164
* Simple subsumptionwip/T17775Simon Peyton Jones2020-06-051-0/+1
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This patch simplifies GHC to use simple subsumption. Ticket #17775 Implements GHC proposal #287 https://github.com/ghc-proposals/ghc-proposals/blob/master/ proposals/0287-simplify-subsumption.rst All the motivation is described there; I will not repeat it here. The implementation payload: * tcSubType and friends become noticably simpler, because it no longer uses eta-expansion when checking subsumption. * No deeplyInstantiate or deeplySkolemise That in turn means that some tests fail, by design; they can all be fixed by eta expansion. There is a list of such changes below. Implementing the patch led me into a variety of sticky corners, so the patch includes several othe changes, some quite significant: * I made String wired-in, so that "foo" :: String rather than "foo" :: [Char] This improves error messages, and fixes #15679 * The pattern match checker relies on knowing about in-scope equality constraints, andd adds them to the desugarer's environment using addTyCsDs. But the co_fn in a FunBind was missed, and for some reason simple-subsumption ends up with dictionaries there. So I added a call to addTyCsDs. This is really part of #18049. * I moved the ic_telescope field out of Implication and into ForAllSkol instead. This is a nice win; just expresses the code much better. * There was a bug in GHC.Tc.TyCl.Instance.tcDataFamInstHeader. We called checkDataKindSig inside tc_kind_sig, /before/ solveEqualities and zonking. Obviously wrong, easily fixed. * solveLocalEqualitiesX: there was a whole mess in here, around failing fast enough. I discovered a bad latent bug where we could successfully kind-check a type signature, and use it, but have unsolved constraints that could fill in coercion holes in that signature -- aargh. It's all explained in Note [Failure in local type signatures] in GHC.Tc.Solver. Much better now. * I fixed a serious bug in anonymous type holes. IN f :: Int -> (forall a. a -> _) -> Int that "_" should be a unification variable at the /outer/ level; it cannot be instantiated to 'a'. This was plain wrong. New fields mode_lvl and mode_holes in TcTyMode, and auxiliary data type GHC.Tc.Gen.HsType.HoleMode. This fixes #16292, but makes no progress towards the more ambitious #16082 * I got sucked into an enormous refactoring of the reporting of equality errors in GHC.Tc.Errors, especially in mkEqErr1 mkTyVarEqErr misMatchMsg misMatchMsgOrCND In particular, the very tricky mkExpectedActualMsg function is gone. It took me a full day. But the result is far easier to understand. (Still not easy!) This led to various minor improvements in error output, and an enormous number of test-case error wibbles. One particular point: for occurs-check errors I now just say Can't match 'a' against '[a]' rather than using the intimidating language of "occurs check". * Pretty-printing AbsBinds Tests review * Eta expansions T11305: one eta expansion T12082: one eta expansion (undefined) T13585a: one eta expansion T3102: one eta expansion T3692: two eta expansions (tricky) T2239: two eta expansions T16473: one eta determ004: two eta expansions (undefined) annfail06: two eta (undefined) T17923: four eta expansions (a strange program indeed!) tcrun035: one eta expansion * Ambiguity check at higher rank. Now that we have simple subsumption, a type like f :: (forall a. Eq a => Int) -> Int is no longer ambiguous, because we could write g :: (forall a. Eq a => Int) -> Int g = f and it'd typecheck just fine. But f's type is a bit suspicious, and we might want to consider making the ambiguity check do a check on each sub-term. Meanwhile, these tests are accepted, whereas they were previously rejected as ambiguous: T7220a T15438 T10503 T9222 * Some more interesting error message wibbles T13381: Fine: one error (Int ~ Exp Int) rather than two (Int ~ Exp Int, Exp Int ~ Int) T9834: Small change in error (improvement) T10619: Improved T2414: Small change, due to order of unification, fine T2534: A very simple case in which a change of unification order means we get tow unsolved constraints instead of one tc211: bizarre impredicative tests; just accept this for now Updates Cabal and haddock submodules. Metric Increase: T12150 T12234 T5837 haddock.base Metric Decrease: haddock.compiler haddock.Cabal haddock.base Merge note: This appears to break the `UnliftedNewtypesDifficultUnification` test. It has been marked as broken in the interest of merging. (cherry picked from commit 66b7b195cb3dce93ed5078b80bf568efae904cc5)
* Explicit SpecificityGert-Jan Bottu2020-05-211-1/+1
| | | | | | | | | | | | | | | | | | | | Implementation for Ticket #16393. Explicit specificity allows users to manually create inferred type variables, by marking them with braces. This way, the user determines which variables can be instantiated through visible type application. The additional syntax is included in the parser, allowing users to write braces in type variable binders (type signatures, data constructors etc). This information is passed along through the renamer and verified in the type checker. The AST for type variable binders, data constructors, pattern synonyms, partial signatures and Template Haskell has been updated to include the specificity of type variables. Minor notes: - Bumps haddock submodule - Disables pattern match checking in GHC.Iface.Type with GHC 8.8
* Document (->) using inferred quantification for its runtime representations.Baldur Blöndal2020-05-131-1/+3
| | | | Fixes #18142.
* Modules: Utils and Data (#13009)Sylvain Henry2020-04-261-6/+6
| | | | | | | Update Haddock submodule Metric Increase: haddock.compiler
* Modules (#13009)Sylvain Henry2020-04-181-2/+2
| | | | | | | | | | | | | | * SysTools * Parser * GHC.Builtin * GHC.Iface.Recomp * Settings Update Haddock submodule Metric Decrease: Naperian parsing001
* GHC.Core.Opt renamingSylvain Henry2020-04-181-1/+1
| | | | | | | | | | | * GHC.Core.Op => GHC.Core.Opt * GHC.Core.Opt.Simplify.Driver => GHC.Core.Opt.Driver * GHC.Core.Opt.Tidy => GHC.Core.Tidy * GHC.Core.Opt.WorkWrap.Lib => GHC.Core.Opt.WorkWrap.Utils As discussed in: * https://mail.haskell.org/pipermail/ghc-devs/2020-April/018758.html * https://gitlab.haskell.org/ghc/ghc/issues/13009#note_264650
* Modules: type-checker (#13009)Sylvain Henry2020-04-071-4/+4
| | | | Update Haddock submodule
* Clean up "Eta reduction for data families" NotesRyan Scott2020-04-011-1/+1
| | | | | | | | | | | | Before, there were two distinct Notes named "Eta reduction for data families". This renames one of them to "Implementing eta reduction for data families" to disambiguate the two and fixes references in other parts of the codebase to ensure that they are pointing to the right place. Fixes #17313. [ci skip]
* Modules: Types (#13009)Sylvain Henry2020-03-291-8/+8
| | | | | | | Update Haddock submodule Metric Increase: haddock.compiler
* Update core spec to reflect changes to Core.Richard Eisenberg2020-03-201-2/+2
| | | | | | | | | | | | Key changes: * Adds a new rule for forall-coercions over coercion variables, which was implemented but conspicuously missing from the spec. * Adds treatment for FunCo. * Adds treatment for ForAllTy over coercion variables. * Improves commentary (including restoring a Note lost in 03d4852658e1b7407abb4da84b1b03bfa6f6db3b) in the source. No changes to running code.
* Simplify treatment of heterogeneous equalityRichard Eisenberg2020-03-201-2/+43
| | | | | | | | | | | | | | | | | | | | | | | | | | Previously, if we had a [W] (a :: k1) ~ (rhs :: k2), we would spit out a [D] k1 ~ k2 and part the W as irreducible, hoping for a unification. But we needn't do this. Instead, we now spit out a [W] co :: k2 ~ k1 and then use co to cast the rhs of the original Wanted. This means that we retain the connection between the spat-out constraint and the original. The problem with this new approach is that we cannot use the casted equality for substitution; it's too like wanteds-rewriting- wanteds. So, we forbid CTyEqCans that mention coercion holes. All the details are in Note [Equalities with incompatible kinds] in TcCanonical. There are a few knock-on effects, documented where they occur. While debugging an error in this patch, Simon and I ran into infelicities in how patterns and matches are printed; we made small improvements. This patch includes mitigations for #17828, which causes spurious pattern-match warnings. When #17828 is fixed, these lines should be removed.
* Modules: Core operations (#13009)Sylvain Henry2020-03-181-1/+1
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* Modules: Core (#13009)Sylvain Henry2020-03-161-0/+2906
Update submodule: haddock
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