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- Use extension suggestion hints instead of suggesting extensions in the
error message body for several FFI errors.
- Adds a test case for `TcRnForeignImportPrimExtNotSet`
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Converts all uses of 'TcRnUnknownMessage' to proper diagnostics.
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* Users can define their own (~) type operator
* Haddock can display documentation for the built-in (~)
* New transitional warnings implemented:
-Wtype-equality-out-of-scope
-Wtype-equality-requires-operators
Updates the haddock submodule.
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As #20837 pointed out, `isLiftedType_maybe` returned `Just False` in
many situations where it should return `Nothing`, because it didn't
take into account type families or type variables.
In this patch, we fix this issue. We rename `isLiftedType_maybe` to
`typeLevity_maybe`, which now returns a `Levity` instead of a boolean.
We now return `Nothing` for types with kinds of the form
`TYPE (F a1 ... an)` for a type family `F`, as well as
`TYPE (BoxedRep l)` where `l` is a type variable.
This fix caused several other problems, as other parts of the compiler
were relying on `isLiftedType_maybe` returning a `Just` value, and were
now panicking after the above fix. There were two main situations in
which panics occurred:
1. Issues involving the let/app invariant. To uphold that invariant,
we need to know whether something is lifted or not. If we get an
answer of `Nothing` from `isLiftedType_maybe`, then we don't know
what to do. As this invariant isn't particularly invariant, we
can change the affected functions to not panic, e.g. by behaving
the same in the `Just False` case and in the `Nothing` case
(meaning: no observable change in behaviour compared to before).
2. Typechecking of data (/newtype) constructor patterns. Some programs
involving patterns with unknown representations were accepted, such
as T20363. Now that we are stricter, this caused further issues,
culminating in Core Lint errors. However, the behaviour was
incorrect the whole time; the incorrectness only being revealed by
this change, not triggered by it.
This patch fixes this by overhauling where the representation
polymorphism involving pattern matching are done. Instead of doing
it in `tcMatches`, we instead ensure that the `matchExpected`
functions such as `matchExpectedFunTys`, `matchActualFunTySigma`,
`matchActualFunTysRho` allow return argument pattern types which
have a fixed RuntimeRep (as defined in Note [Fixed RuntimeRep]).
This ensures that the pattern matching code only ever handles types
with a known runtime representation. One exception was that
patterns with an unknown representation type could sneak in via
`tcConPat`, which points to a missing representation-polymorphism
check, which this patch now adds.
This means that we now reject the program in #20363, at least until
we implement PHASE 2 of FixedRuntimeRep (allowing type families in
RuntimeRep positions). The aforementioned refactoring, in which
checks have been moved to `matchExpected` functions, is a first
step in implementing PHASE 2 for patterns.
Fixes #20837
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AbsBinds and ABExport both depended on the typechecker, and were thus
removed from the main AST Expr.
CollectPass now has a new function `collectXXHsBindsLR` used for the new
HsBinds extension point
Bumped haddock submodule to work with AST changes.
The removed Notes from Language.Haskell.Syntax.Binds were duplicated
(and not referenced) and the copies in GHC.Hs.Binds are kept (and
referenced there). (See #19252)
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As the `hlint` executable is only available in the linters image.
Fixes #21146.
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Previously, there was an awful hack in `genInst` (now called `genInstBinds`
after this patch) where we had to return a continutation rather than directly
returning the bindings for a derived instance. This was done for staging
purposes, as we had to first infer the instance contexts for derived instances
and then feed these contexts into the continuations to ensure the generated
instance bindings had accurate instance contexts.
`Note [Staging of tcDeriving]` in `GHC.Tc.Deriving` described this confusing
state of affairs.
The root cause of this confusing design was the fact that `genInst` was trying
to generate instance bindings and associated type family instances for derived
instances simultaneously. This really isn't possible, however: as
`Note [Staging of tcDeriving]` explains, one needs to have access to the
associated type family instances before one can properly infer the instance
contexts for derived instances. The use of continuation-returning style was an
attempt to circumvent this dependency, but it did so in an awkward way.
This patch detangles this awkwardness by splitting up `genInst` into two
functions: `genFamInsts` (for associated type family instances) and
`genInstBinds` (for instance bindings). Now, the `tcDeriving` function calls
`genFamInsts` and brings all the family instances into scope before calling
`genInstBinds`. This removes the need for the awkward continuation-returning
style seen in the previous version of `genInst`, making the code easier to
understand.
There are some knock-on changes as well:
1. `hasStockDeriving` now needs to return two separate functions: one that
describes how to generate family instances for a stock-derived instance,
and another that describes how to generate the instance bindings. I factored
out this pattern into a new `StockGenFns` data type.
2. While documenting `StockGenFns`, I realized that there was some
inconsistency regarding which `StockGenFns` functions needed which
arguments. In particular, the function in `GHC.Tc.Deriv.Generics` which
generates `Rep(1)` instances did not take a `SrcSpan` like other `gen_*`
functions did, and it included an extra `[Type]` argument that was entirely
redundant. As a consequence, I refactored the code in
`GHC.Tc.Deriv.Generics` to more closely resemble other `gen_*` functions.
A happy result of all this is that all `StockGenFns` functions now take
exactly the same arguments, which makes everything more uniform.
This is purely a refactoring that should not have any effect on user-observable
behavior. The new design paves the way for an eventual fix for #20719.
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When deriving a `Generic1` instance, we need to know what the last type
variable of a data type is. Previously, there were two mechanisms to determine
this information:
* `GenericKind_`, where `Gen1_` stored the last type variable of a data type
constructor (i.e., the `tyConTyVars`).
* `GenericKind_DC`, where `Gen1_DC` stored the last universally quantified
type variable in a data constructor (i.e., the `dataConUnivTyVars`).
These had different use cases, as `GenericKind_` was used for generating
`Rep(1)` instances, while `GenericKind_DC` was used for generating `from(1)`
and `to(1)` implementations. This was already a bit confusing, but things went
from confusing to outright wrong after !6976. This is because after !6976,
the `deriving` machinery stopped using `tyConTyVars` in favor of
`dataConUnivTyVars`. Well, everywhere with the sole exception of
`GenericKind_`, which still continued to use `tyConTyVars`. This lead to
disaster when deriving a `Generic1` instance for a GADT family instance, as
the `tyConTyVars` do not match the `dataConUnivTyVars`. (See #21185.)
The fix is to stop using `GenericKind_` and replace it with `GenericKind_DC`.
For the most part, this proves relatively straightforward. Some highlights:
* The `forgetArgVar` function was deleted entirely, as it no longer proved
necessary after `GenericKind_`'s demise.
* The substitution that maps from the last type variable to `Any` (see
`Note [Generating a correctly typed Rep instance]`) had to be moved from
`tc_mkRepTy` to `tc_mkRepFamInsts`, as `tc_mkRepTy` no longer has access to
the last type variable.
Fixes #21185.
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'no_instance_msg' and 'no_deduce_msg' were omitting the first wanted.
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Fix #21023 by always generalising top-level binding; change
the documentation of -XMonoLocalBinds to match.
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This patch does the following two things:
1. Fix the check in Core Lint to properly throw an error when it
comes across Float#/Double# literal patterns. The check
was incorrect before, because it expected the type to be
Float/Double instead of Float#/Double#.
2. Add an error in the parser when the user writes a floating-point
literal pattern such as `case x of { 2.0## -> ... }`.
Fixes #21115
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Close #20231.
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Don't instantiate type variables for :type in
`GHC.Tc.Gen.App.tcInstFun`, to avoid inconsistently instantianting
`r1` but not `r2` in the type
forall {r1} (a :: TYPE r1) {r2} (b :: TYPE r2). ...
This fixes #21088.
This patch also changes the primop pretty-printer to ensure
that we put all the inferred type variables first. For example,
the type of reallyUnsafePtrEquality# is now
forall {l :: Levity} {k :: Levity}
(a :: TYPE (BoxedRep l))
(b :: TYPE (BoxedRep k)).
a -> b -> Int#
This means we avoid running into issue #21088 entirely with
the types of primops. Users can still write a type signature where
the inferred type variables don't come first, however.
This change to primops had a knock-on consequence, revealing that
we were sometimes performing eta reduction on keepAlive#.
This patch updates tryEtaReduce to avoid eta reducing functions
with no binding, bringing it in line with tryEtaReducePrep,
and thus fixing #21090.
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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.
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This patch adds a check to Core Lint, checkCanEtaExpand,
which ensures that primops and other wired-in functions with
no binding such as unsafeCoerce#, oneShot, rightSection...
can always be eta-expanded, by checking that the remaining
argument types have a fixed RuntimeRep.
Two subtleties came up:
- the notion of arity in Core looks through newtypes, so we may
need to unwrap newtypes in this check,
- we want to avoid calling hasNoBinding on something whose unfolding
we are in the process of linting, as this would cause a loop;
to avoid this we add some information to the Core Lint environment
that holds this information.
Fixes #20480
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The code in tcAnonWildCardOcc assumed that it could never encounter
anonymous wildcards in illegal positions, because the renamer would
have ruled them out. However, it's possible to sneak past the checks
in the renamer by using Template Haskell. It isn't possible to simply
pass on additional information when renaming Template Haskell
brackets, because we don't know in advance in what context the bracket
will be spliced in (see test case T15433b). So we accept that we might
encounter these bogus wildcards in the typechecker and throw the
appropriate error.
This patch also migrates the error messages for illegal wildcards in
types to use the diagnostic infrastructure.
Fixes #15433
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The old logic was unecessarily strict in loading unfoldings because when
reading the unfolding we would case on the result of attempting to load
the template before commiting to which type of unfolding we were
producing. Hence trying to inspect any of the information about an
unfolding would force the template to be loaded.
This also removes a potentially hard to discover bug where if the
template failed to be typechecked for some reason then we would just not
return an unfolding. Instead we now panic so these bad situations which
should never arise can be identified.
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This patch skips performing type normalisation when we haven't
fully instantiated the type. That is, in tcRnExpr
(used only for :type in GHCi), skip normalisation if
the result type responds True to isSigmaTy.
Fixes #20974
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It turns out this job hasn't been running for quite a while (perhaps
ever) so there are quite a few failures when running the linter locally.
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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
-------------------------
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Update manual; explain ticks as optional disambiguation
rather than the preferred default.
This is a part of #20531.
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`check_special_inst_head` includes logic that disallows hand-written
instances for built-in classes such as Typeable, KnownNat
and KnownSymbol.
However, it also allowed standalone deriving declarations. This was
because we do want to allow standalone deriving instances with
Typeable as they are harmless, but we certainly don't want to allow
instances for e.g. KnownNat.
This patch ensures that we don't allow derived instances for
KnownNat, KnownSymbol (and also KnownChar, which was previously
omitted entirely).
Fixes #21087
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This patch makes the "missing signature" errors from
"GHC.Rename.Names" use the diagnostic infrastructure.
This encompasses missing type signatures for top-level bindings
and pattern synonyms, as well as missing kind signatures for
type constructors.
This patch also renames TcReportMsg to TcSolverReportMsg,
and adds a few convenience functions to compute whether such a
TcSolverReportMsg is an expected/actual message.
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`hscCompileCoreExprHook` is changed to return a list of `Module`s required
by a splice. These modules are accumulated in the TcGblEnv (tcg_th_needed_mods).
Dependencies on the object files of these modules are recording in the
interface.
The data structures in `LoaderState` are replaced with more efficient versions
to keep track of all the information required. The
MultiLayerModulesTH_Make allocations increase slightly but runtime is
faster.
Fixes #20604
-------------------------
Metric Increase:
MultiLayerModulesTH_Make
-------------------------
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This does three major things:
* Enforce the invariant that all strict fields must contain tagged
pointers.
* Try to predict the tag on bindings in order to omit tag checks.
* Allows functions to pass arguments unlifted (call-by-value).
The former is "simply" achieved by wrapping any constructor allocations with
a case which will evaluate the respective strict bindings.
The prediction is done by a new data flow analysis based on the STG
representation of a program. This also helps us to avoid generating
redudant cases for the above invariant.
StrictWorkers are created by W/W directly and SpecConstr indirectly.
See the Note [Strict Worker Ids]
Other minor changes:
* Add StgUtil module containing a few functions needed by, but
not specific to the tag analysis.
-------------------------
Metric Decrease:
T12545
T18698b
T18140
T18923
LargeRecord
Metric Increase:
LargeRecord
ManyAlternatives
ManyConstructors
T10421
T12425
T12707
T13035
T13056
T13253
T13253-spj
T13379
T15164
T18282
T18304
T18698a
T1969
T20049
T3294
T4801
T5321FD
T5321Fun
T783
T9233
T9675
T9961
T19695
WWRec
-------------------------
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ghc-prim doesn't depend on base so can't have any Monoid or Semigroup
instances. However, attempting to load these definitions ran into issues
when the interface for `GHC.Base` did exist as that would try and load
the interface for `GHC.Types` (which is the module we are trying to
compile and has no interface).
The fix is to just not do this check when we are compiling a module in
ghc-prim.
Fixes #21069
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Previously, surrounding a head expression with a TH splice would defeat
`tcInferAppHead_maybe`, preventing some expressions from typechecking that
used to typecheck in previous GHC versions (see #21038 for examples). This is
simple enough to fix: just look through `HsSpliceE`s in `tcInferAppHead_maybe`.
I've added some additional prose to `Note [Application chains and heads]` in
`GHC.Tc.Gen.App` to accompany this change.
Fixes #21038.
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The 'bad_newtype' assertion in GHC.Tc.Solver.Canonical.canEqCanLHSFinish
failed to account for the possibility that the newtype constructor
might not be in scope, in which case we don't provide any guarantees
about canonicalising away a newtype on the RHS of a representational
equality.
Fixes #21010
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We perform validity checking on user-written HasField instances,
for example to disallow:
data Foo a = Foo { fld :: Int }
instance HasField "fld" (Foo a) Bool
However, these checks were also being made on quantified constraints,
e.g.
data Bar where
Bar :: (forall a. HasField s (Foo a) Int) => Proxy s -> Bar
This patch simply skips validity checking for quantified constraints,
in line with what we already do for equality constraints such as
Coercible.
Fixes #20989
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Note [Tidying multiple names at once] indicates that if multiple
variables have the same name then we shouldn't prioritise one of them
and instead rename them all to a1, a2, a3... etc
This patch implements that change, some error message changes as
expected.
Closes #20932
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This patch fixes #17469, by improving matters when you use
non-existent field names in a record construction:
data T = MkT { x :: Int }
f v = MkT { y = 3 }
The check is now made in the renamer, in GHC.Rename.Env.lookupRecFieldOcc.
That in turn led to a spurious error in T9975a, which is fixed by
making GHC.Rename.Names.extendGlobalRdrEnvRn fail fast if it finds
duplicate bindings. See Note [Fail fast on duplicate definitions]
in that module for more details.
This patch was originated and worked on by Alex D (@nineonine)
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The commit
commit 18df4013f6eaee0e1de8ebd533f7e96c4ee0ff04
Date: Sat Jan 22 01:12:30 2022 +0000
Define and use restoreLclEnv
omitted to change one setLclEnv to restoreLclEnv, namely
the one in GHC.Tc.Errors.warnRedundantConstraints.
This new commit fixes the omission.
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Here we introduce a new data structure, RoughMap, inspired by the
previous `RoughTc` matching mechanism for checking instance matches.
This allows [Fam]InstEnv to be implemented as a trie indexed by these
RoughTc signatures, reducing the complexity of instance lookup and
FamInstEnv merging (done during the family instance conflict test)
from O(n) to O(log n).
The critical performance improvement currently realised by this patch is
in instance matching. In particular the RoughMap mechanism allows us to
discount many potential instances which will never match for constraints
involving type variables (see Note [Matching a RoughMap]). In realistic
code bases matchInstEnv was accounting for 50% of typechecker time due
to redundant work checking instances when simplifying instance contexts
when deriving instances. With this patch the cost is significantly
reduced.
The larger constants in InstEnv creation do mean that a few small
tests regress in allocations slightly. However, the runtime of T19703 is
reduced by a factor of 4. Moreover, the compilation time of the Cabal
library is slightly improved.
A couple of test cases are included which demonstrate significant
improvements in compile time with this patch.
This unfortunately does not fix the testcase provided in #19703 but does
fix #20933
-------------------------
Metric Decrease:
T12425
Metric Increase:
T13719
T9872a
T9872d
hard_hole_fits
-------------------------
Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>
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As #20929 pointed out, we were in-elegantly checking for escaping
kinds in `checkValidType`, even though that check was guaranteed
to succeed for type signatures -- it's part of kind-checking a type.
But for /data constructors/ we kind-check the pieces separately,
so we still need the check.
This MR is a pure refactor, moving the test from `checkValidType` to
`checkValidDataCon`.
No new tests; external behaviour doesn't change.
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This was achieved with
git ls-tree --name-only HEAD -r | xargs sed -i -e 's/note \[/Note \[/g'
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Fixes #21011
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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>
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Fix a simple omission in GHC.Tc.Solver.Canonical.solveForAll,
where we ended up with the wrong TcLclEnv captured in an implication.
Result: unhelpful error message (#21006)
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The second component is supposed to be "insoluble equalities arising
from givens". But we were getting wanteds too; and that led to an
outright duplication of constraints. It's not harmful, but it's not
right either.
I came across this when debugging something else. Easily fixed.
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This fixes #20981. See Note [restoreLclEnv vs setLclEnv]
in GHC.Tc.Utils.Monad.
I also use updLclEnv rather than get/set when I can, because it's
then much clearer that it's an update rather than an entirely new
TcLclEnv coming from who-knows-where.
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`markNominal` is repsonsible for setting the roles of type variables
that appear underneath an `AppTy` to be nominal. However, `markNominal`
previously did not expand type synonyms, so in a data type like this:
```hs
data M f a = MkM (f (T a))
type T a = Int
```
The `a` in `M f a` would be marked nominal, even though `T a` would simply
expand to `Int`. The fix is simple: call `coreView` as appropriate in
`markNominal`. This is much like the fix for #14101, but in a different spot.
Fixes #20999.
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As #20921 showed, with partial signatures, it is helpful to use the
same algorithm (namely findInferredDiff) for
* picking the constraints to retain for the /group/
in Solver.decideQuantification
* picking the contraints to retain for the /individual function/
in Bind.chooseInferredQuantifiers
This is still regrettably declicate, but it's a step forward.
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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
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Fixes #20995
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This patch makes the following types levity-polymorphic in their
last argument:
- Array# a, SmallArray# a, Weak# b, StablePtr# a, StableName# a
- MutableArray# s a, SmallMutableArray# s a,
MutVar# s a, TVar# s a, MVar# s a, IOPort# s a
The corresponding primops are also made levity-polymorphic, e.g.
`newArray#`, `readArray#`, `writeMutVar#`, `writeIOPort#`, etc.
Additionally, exception handling functions such as `catch#`, `raise#`,
`maskAsyncExceptions#`,... are made levity/representation-polymorphic.
Now that Array# and MutableArray# also work with unlifted types,
we can simply re-define ArrayArray# and MutableArrayArray# in terms
of them. This means that ArrayArray# and MutableArrayArray# are no
longer primitive types, but simply unlifted newtypes around Array# and
MutableArrayArray#.
This completes the implementation of the Pointer Rep proposal
https://github.com/ghc-proposals/ghc-proposals/pull/203
Fixes #20911
-------------------------
Metric Increase:
T12545
-------------------------
-------------------------
Metric Decrease:
T12545
-------------------------
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Previously, `DeriveGeneric` would look up the fixity of a data constructor
using `getFixityEnv`, but this is subtly incorrect for data constructors
defined in external modules. This sort of situation can happen with
`StandaloneDeriving`, as noticed in #20994. In fact, the same bug has occurred
in the past in #9830, and while that bug was fixed for `deriving Read` and
`deriving Show`, the fix was never extended to `DeriveGeneric` due to an
oversight. This patch corrects that oversight.
Fixes #20994.
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