GitLab

Apoorv Ingle pushed to branch wip/ani/T27156 at Glasgow Haskell Compiler / GHC

Commits:

85b10c00

by Alan Zimmerman at 2026-07-31T22:09:47+01:00

EPA: Remove LocatedP from OverlapMode

We have

  type LocatedP = GenLocated SrcSpanAnnP
  type SrcSpanAnnP = EpAnn AnnPragma

As the first step in removing this in favour of LocatedA which only
captures location, comments and trailing annotations, we remove it
from OverlapMode

We do this by moving the AnnPragma into the TTG extension point
instead.

c9a34a00
by Viktor Dukhovni at 2026-08-02T04:34:17-04:00
```
Fix note typo
```
4f2a21f7
by Andreas Klebinger at 2026-08-02T22:46:46-04:00
```
Apply oneShot Monad trick to STG LintM
```

21e4b89d

by Andreas Klebinger at 2026-08-02T22:46:46-04:00

stgLint: Use a single reader env for read only arguments.

d415f38a

by Alan Zimmerman at 2026-08-02T22:47:27-04:00

EPA: Remove LocatedP from CType

The next step of removing use of LocatedP by moving
the AnnPragma for CType into its TTG extension point
instead.

8fc6f882

by Simon Jakobi at 2026-08-05T14:53:41-04:00

testsuite: Don't crash on non-UTF-8 test output

read_stdout, read_stderr_for, read_comp_stderr and read_diff decoded
strictly (the first three with UTF-8, read_diff with the locale
encoding), so a test emitting invalid UTF-8 (binary output, or a crash
truncating a multi-byte character) raised UnicodeDecodeError and was
reported as a framework failure instead of its actual result.

Decode with errors='replace', like read_no_crs and safe_print.

Assisted-by: Claude Fable 5

56534866

by Simon Jakobi at 2026-08-05T14:53:41-04:00

testsuite: Colorize the test summary, also in CI

The summary headings were plain, and SUMMARY was colored unconditionally,
so the escapes also ended up in the file written by --summary-file.
Color is now decided per output sink via term_color.colored_if; see the
comments in term_color.

CI logs are not a tty, but GitLab's log viewer renders ANSI colors, so
add --force-colors and pass it in .gitlab/ci.sh.

Assisted-by: Claude Opus 5

bceb541a

by Simon Jakobi at 2026-08-05T14:53:42-04:00

testsuite: Repeat unexpected failure output in the summary

Finding out why a test failed meant scrolling back through a possibly
very long log to the point where the test ran. The summary now repeats
the captured output of unexpected failures, before the statistics, so
the most interesting part is at the end of the log (#16720).

Output mismatches report their diff instead of the mismatching stream
(see Note [Redundant output in test results]). The repeated output is
bounded per stream, and skipped altogether beyond
MAX_SUMMARY_OUTPUT_TESTS failure blocks. Tests failing identically in
several ways share one block.

Test results now report a source-relative directory, stable regardless
of where the run was started from.

Assisted-by: Claude Fable 5

2ab02c57

by Ben Gamari at 2026-08-05T14:54:24-04:00

base: Don't drop exception context in SomeException(toException)

For reasons that are lost to time, the implementation of [CLC #200]
that was merged inappropriately dropped `ExceptionContext` in the
`toException` implementation given to `SomeException`.

Fix this infelicity.

[CLC #200]: https://github.com/haskell/core-libraries-committee/issues/200

126ce574

by Vladislav Zavialov at 2026-08-05T14:55:05-04:00

Test case for #20902

Starting with GHC 9.14.1 (the first major release to include 51e3ec83),
and from point releases GHC 9.10.2 and GHC 9.12.3 (backports cc4470be68
and b30f25591e), all examples in this ticket are handled as expected.

b14d8d59

by Alan Zimmerman at 2026-08-05T14:55:46-04:00

EPA: Remove LocatedP, last use in WarningTxt

The last step of removing LocatedP, by moving the AnnPragma for
WarningTxt into its TTG extension point instead.

This also allows us to remove LocatedP and SrcSpanAnnP

70b58c8f

by Vladislav Zavialov at 2026-08-05T14:56:27-04:00

Test cases for #18725

Starting with GHC 9.4 (the first release to include 268efcc9a4), the program in
this ticket no longer panics. A standalone kind signature breaks the recursive
loop, so the type constructor can be used in a kind within its own group.

T18725a checks that this is accepted with the signature present, while T18725b
confirms it is still rejected without it.

b18c8dd5

by Zubin Duggal at 2026-08-06T15:51:01-04:00

hie files: Dump the type table when dumping with -ddump-hie

f839d0fb

by Zubin Duggal at 2026-08-06T15:51:01-04:00

hie files: Take evidence for quantified constraints into account when saving evidence terms to the hie ast

Fixes #25709

5753ebaa

by Simon Jakobi at 2026-08-06T15:51:43-04:00

testsuite: fix stale paths for the ghc-config build artifacts

ghc-config.hs moved from testsuite/mk/ to testsuite/ghc-config/ in
6c7a49139c, but the .gitignore entry and the clean rule still referred to
the old location. As a result the compiled ghc-config binary, which
boilerplate.mk rebuilds on every make-driven test run, showed up as an
untracked file and was never cleaned.

Assisted-by: Claude Opus 5

246d4d72

by Simon Peyton Jones at 2026-08-06T15:52:25-04:00

Documentation only

...driven by my investigation of #27591

be69e9df

by Alan Zimmerman at 2026-08-06T15:53:05-04:00

EPA: Replace AnnPragma with individual types

We introduced AnnPragma as a common type for all pragma usages wrapped
in LocatedP / SrcSpanAnnP. Now that those are gone, and the AnnPragma
moved into the TTG points for the given items, we can ensure that each
carries only the annotations it needs.

So we remove AnnPragma, and in its place bring in
  AnnCType
  AnnWarningTxt
  AnnOverlap
  AnnAnnDecl
  AnnPragSCC

0779e12c

by Simon Jakobi at 2026-08-07T12:36:11-04:00

Cmm: print unreachable blocks under -dppr-debug (#27368)

Unreachable blocks linger in a CmmGraph's block map for most of the Cmm
pipeline, but pprCmmGraph only ever printed the blocks reachable from the
entry, so dumps looked consistent while the graph was not. Issues like
 #27368 were hard to debug due to this.

pprCmmGraph now appends the stored-but-unreachable blocks under a
"// unreachable blocks:" heading when -dppr-debug is on.
See Note [unreachable blocks] in GHC.Cmm.Pipeline.

Assisted-by: Claude Opus 5

3a0f9a51

by Simon Peyton Jones at 2026-08-07T12:36:54-04:00

Fix three bugs related to required type args and INLINE pragmas

* `GHC.Core.Opt.Arity.mkEtaForAllMCo` got the visibility flags back to front,
  leading to a Lint error (#27557)

* The arity in an InlineSaturation is the VisArity not the Arity; the
  two can differ when we have "required" type arguments.  This made the
  INLINE pragma argument counting go wrong in `makeCorePair` (#27590).

* When a simple binding has a type signature, we take special path in `tcPolyCheck`,
  leading to an outer `AbsBinds` that has no dictionaries, even when the binding
  is in fact overloaded.  That confused the inline-arity computation in
  `makeCorePair` (#27589).

The latter two are fixed using the new function `GHC.HsToCore.Binds.findSatArity`.
That actually simplifies the API of `makeCorePair`, which is nice.

The first bug is fixed by swapping the visiblity flags in
`GHC.Core.Opt.Arity.mkEtaForAllMCo`

Getting the INLINE behaviour right led to some perf changes:
  * Runtime /halved/ on T7954 due to better specialisation
  * Compile time increased by 6% in T21839c because a bit more inlining
    happened, as it always should have done.
  * For some reason compile-time max-bytes-used dropped by 30% on
    T27336, but only on one build configuration; and it increased
    on LinkableUsage02 by 6% on another configuration

Geometric mean effect on our compile time benchmarks is +0.1%.

Metric Decrease:
    T27336
    T7954
Metric Increase:
    LinkableUsage02
    T21839c

4f985108

by Vladislav Zavialov at 2026-08-07T17:49:50-04:00

Discard type arguments in tcPatToExpr (#27440, #27583)

The builder expression of an implicitly bidirectional pattern synonym must not
mention types written in the RHS:

  * Invisible type arguments led to a panic (#27440)
  * Required type arguments failed with out-of-scope variables (#27583)

Both are now discarded, following the precedent established by pattern
signatures (#9867).

Discarding type arguments takes some care: a type pattern cannot be told from a
value pattern by syntax alone, as the `type` keyword may be omitted. Consider:

    data T a b c where
      MkT :: forall a. forall b c -> a -> T a b c

    pattern P :: x -> T x y z
    pattern P x = MkT @a (type b) c x

In P's right-hand side, `@a` and `type b` are clearly type arguments, but what
about `c` and `x`? We can only tell by matching the patterns against MkT's
type. So tcPatToExpr now runs in TcM and matches the arguments against the
constructor's TyVarBinders using zipPatsBndrs, which is made public for this
purpose. The resulting builder is $bP x = MkT _ _ x.

See Note [Discarding types in the builder expression].

Test cases: T27440a T27440b T27440c T27440d T27440e
            T27583a T27583b T27583c T27583d T27583e T27583f T27583g

Metric Increase: LinkableUsage02
Metric Decrease: T27336

Assisted-by: Claude Opus 5

eb1dcd4d

by sheaf at 2026-08-07T17:50:40-04:00

mkWpFun_FRR: fix ordering of coercion composition

When the subsumption machinery generates an eta-expansion, we must
perform a representation polymorphism check to ensure the lambda binder
it introduces has a fixed runtime representation.
This is done in GHC.Tc.Utils.mkWpFun_FRR.

This check involves composing quite a few coercions, arising from
representation-polymorphism checks on both the actual and expected
argument types. These coercions are then chained using HsWrapper
composition, <.>. The ordering of composition was incorrect, leading to
the Core Lint failure reported in #27639. This commit fixes that.

Fixes #27639

3a552476

by Simon Jakobi at 2026-08-09T15:20:06-04:00

Word64Map: add compareSize

compareSize m c compares the size of a map to an Int, but unlike
compare (size m) c it stops traversing the map once the outcome is
determined.

Based on https://github.com/haskell/containers/pull/1139

Assisted-by: Claude Opus 5

6e2c99d8

by Simon Jakobi at 2026-08-09T15:20:06-04:00

Use a pigeonhole sort for deterministic UniqDFM iteration

Deterministic UniqDFM iteration used a list mergesort, allocating O(n
log n) cons cells and contributing significantly to compiler allocations
(#27459).

Use a pigeonhole sort where appropriate, while retaining the mergesort
fallback. See Note [Sorting a UDFM] and Note [Cost of deterministic
iteration].

The peak_megabytes_allocated increase for LinkableUsage02 is probably
due to GC timing noise. See #27613.

-------------------------
Metric Decrease:
    InstanceMatching
    InstanceMatching1
    ManyAlternatives
    T12707
    T13379
    T13719
    T24471
    T27336
    T5321FD
    T5321Fun
    T783
Metric Increase 'peak_megabytes_allocated':
    LinkableUsage02
-------------------------

Assisted-by: gpt-5.6-sol via Codex CLI

a938ab12

by sheaf at 2026-08-09T15:20:48-04:00

Testsuite: don't measure max residency for T27336

We really care more about total allocations for this test, so this commit
removes the maximum residency measurement.

7d94bb78

by Alan Zimmerman at 2026-08-09T15:21:29-04:00

EPA: Remove LocatedE, replace with LocatedA

This gets rid of one more LocatedXXX occurrence

9df24b7e

by sheaf at 2026-08-10T14:28:30-04:00

hie.yaml: use a polyglot shell/batch script

This commit merges hie-bios and hie-bios.bat into a single polyglot
script. This avoids Windows users having to manually update hie.yaml
in order to be able to use HLS.

7f75c588

by Alan Zimmerman at 2026-08-10T14:29:11-04:00

EPA: Remove type parameter from AnnList

This is a step towards cutting AnnList down to its core for formatting
lists only

e8d1a0d6

by Bernhard M. Wiedemann at 2026-08-10T21:31:23-04:00

driver: Link object files in a deterministic order

The object files handed to the linker come from the HomePackageTable,
which is ordered by the order in which modules finished compiling.  With
-j1 that is the build plan order, with -jN it is whatever the scheduler
produced, so the same sources can link to different (but equivalent)
binaries.

The order reaches the output: .text and .rodata contributions are
concatenated in link order, so e.g. building the hdav executable of the
DAV package twice, once with -j1 and once with -j4, yields two binaries
that differ in ~100kB of section contents.

Sort the home modules by module before collecting their linkables,
guarded under `Opt_ObjectDeterminism` .

Fixes #27612

Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>

556db2f3

by sheaf at 2026-08-10T21:32:06-04:00

Reduce SpecConstr threshold in GHC.Tc.Solver.Rewrite

As remarked in #27628, this module currently sits on a knife's edge: if
the body of 'simplifyArgsWorker' is made even a tiny bit smaller, then
SpecConstr suddenly kicks in and causes disastrous reboxing of the
LiftingContext argument.

To make this less likely to happen, this commit lowers the SpecConstr
threshold.

c77d88fc

by sheaf at 2026-08-13T10:15:22-04:00

Allow rewriting in RuntimeReps for newtype ConPats

This commit implements PHASE 2 of the FixedRuntimeRep plan described in
Note [The Concrete mechanism] in GHC.Tc.Utils.Concrete for newtype
constructor patterns.

In short, GHC now accepts programs of the form

  f (MkN x) = ...

in which the argument 'x' of the newtype constructor pattern 'MkN x'
has a representation that is not syntactically concrete, e.g. it can be
'Id IntRep' reducing to 'IntRep'. See T20363{,b,c} for examples.

There are two main parts to the implementation:

  1. Typechecking, in GHC.Tc.Gen.Pat.tcDataConPat.
     See Note [Typechecking newtype constructor patterns] in GHC.Tc.Gen.Pat.

  2. Desugaring. We restructure the code for desugaring pattern matches
     by allowing the scrutinised match variable to be casted. This allows
     us to accumulate coercions and avoids creating binders at intermediate
     types tha don't have a fixed RuntimeRep.
     See the revamped Note [Match Ids] in GHC.HsToCore.Monad.

Fixes #20363

-------------------------
Metric Increase:
    InstanceMatching
-------------------------

6ba9dd41

by Wolfgang Jeltsch at 2026-08-13T10:16:07-04:00

Improve the documentation of `--show-iface`

This change in particular gets rid of the claim that `--show-iface`
writes *the* contents of the interface file in question. It doesn’t do
that; it only writes those parts that are likely of interest to a human
reader.

4bd193fa

by Sylvain Henry at 2026-08-13T10:17:00-04:00

RTS: fix LDV profiler's slop skipping (#27585)

processHeapForDead was the one heap scanner not updated for the slop
marker encoding introduced in #19048. It still assumed slop is zeroed:

    while (p < bd->free && !*p) p++;   // skip slop

so it stopped at the (StgWord)(-1) sentinel and passed it to
processHeapClosureForDead. IS_FORWARDING_PTR(-1) holds, hence a garbage
size was read out of LDVW and the scan ran off the block, tripping
ASSERT(p == bd->free) on a debug RTS and silently corrupting the census
otherwise.

The loop was hand-copied in four places, so factor it out into skipSlop
in ClosureMacros.h and use it in ProfHeap.c, Sanity.c, Printer.c and
LdvProfile.c.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

1446bb03

by Zubin Duggal at 2026-08-13T10:17:48-04:00

hadrian: Fix links to remaining doc sites to not use the package hash for haddock links

In 07267f79d91169f474cacc8bcd38d76a6e97887d we changed hadrian to not include the package hash in the haddock
directory. This patch takes care of a  few remaining links that were missed in that patch

Fixes #27671

257c3ed7

by Simon Jakobi at 2026-08-13T10:18:29-04:00

Testsuite: widen InstanceMatching acceptance window to 5% (#27673)

...in order to unblock CI.

Assisted-by: Claude Fable 5

e4cfaaa0

by Simon Peyton Jones at 2026-08-14T01:09:32+02:00

Major patch to re-engineer known-key names

This big patch implements the New Plan for known-key names,
described in #27013.

Read the big Note [Overview of known-key names] in GHC.Types.Name

Some things had to be reworked slightly to accomodate the new known-keys
design. A significant one was the generation of auxiliary KindRep
bindings, which was greatly simplified. Note [Grand plan for Typeable]
was updated accordingly. Another example: GHC.Internal.CString was
merged into GHC.Internal.Types.

Co-authored-by: Rodrigo Mesquita <rodrigo.m.mesquita@gmail.com>

The couple hundreds of hours spent here by Rodrigo were sponsored by Well-Typed

Metrics: compile_time/bytes allocated
-------------------------------------
                                                       Baseline
                                  Test    Metric          value      New value Change
------------------------------------------------------------------------------------------
      MultiComponentModules100(normal) ghc/alloc 24,312,779,672 24,990,470,432  +2.8%  BAD
   MultiComponentModulesRecomp(normal) ghc/alloc    601,924,960    621,884,888  +3.3%  BAD
MultiComponentModulesRecomp100(normal) ghc/alloc 11,884,065,432 12,531,373,704  +5.4%  BAD
             MultiLayerModules(normal) ghc/alloc  3,861,537,072  3,706,919,512  -4.0% GOOD
                        T13701(normal) ghc/alloc  3,517,246,392  3,237,179,616  -8.0% GOOD
                        T13820(normal) ghc/alloc     28,961,056     29,663,208  +2.4%  BAD
                        T14697(normal) ghc/alloc    472,044,184    443,550,048  -6.0% GOOD
                        T18140(normal) ghc/alloc     47,905,664     49,115,808  +2.5%  BAD
                         T4801(normal) ghc/alloc    269,339,096    263,432,040  -2.2% GOOD
                          T783(normal) ghc/alloc    341,112,672    333,339,952  -2.3% GOOD
                hard_hole_fits(normal) ghc/alloc    222,164,728    213,433,808  -3.9% GOOD
                      mhu-perf(normal) ghc/alloc     49,011,440     46,706,280  -4.7% GOOD

                             geo. mean                                          +0.1%
                             minimum                                            -8.0%
                             maximum                                            +5.4%

All performance regressions were investigated in depth. The surviving
ones:

- MultiComponentModules100, MultiComponentModulesRecomp100,
  MultiComponentModulesRecomp regresses because existing bugs that make
  an additional implicit edge do too much redundant work: #27053 and #27461
- T13820, T18140, T10547, T13035 regress because we load an additional
  interface and associated Names for GHC.Essentials.

-------------------------
Metric Decrease:
    MultiLayerModules
    T13379
    T13701
    T14697
    T26989
    T4801
    T783
    T9961
    hard_hole_fits
    mhu-perf
    size_hello_artifact
    size_hello_obj
    size_hello_unicode
Metric Increase:
    LinkableUsage01
    LinkableUsage02
    MultiComponentModules100
    MultiComponentModulesRecomp
    MultiComponentModulesRecomp100
    T10547
    T13035
    T13820
    T18140
    T18698a
    T18698b
    T20049
-------------------------

Bumps submodule binary

Closes #27013

61665e79

by sheaf at 2026-08-14T01:09:33+02:00

Allow GHC.Essentials to be hidden

This commit overhauls several aspects of the known entity handling,
in order to allow GHC.Essentials to be hidden without a proliferation
of special cases in the compiler.

The main contribution is to introduce the 'UnresolvedImport' datatype
which cleans up a lot of ad-hoc handling relating to 'ModSummary',
fixing #27603. This allows us to reduce duplication, e.g. by having
Backpack reuse 'mkUnresolvedImports' instead of replicating the
"add implicit imports" logic. It also makes it easier to avoid
undesirable edge cases (such as making sure that the Template Haskell
'reifyModule' function does not leak the implicit GHC.Essentials import).
In particular, the infamous 'findImportedModuleWithIsBoot' is now simply
'resolveImport', taking a single 'UnresolvedImport' and resolving it
to a 'FindResult' (usually a 'Module').

Other changes:

  - Cache the result of looking up GHC.Essentials (in TcM and DsM
    environments) to avoid redundant work.
    This reduces allocations on LinkableUsage01 and hard_hole_fits.
  - Properly look up known entities for StaticPointers like we do for
    other known entities everywhere else. This allows e.g. modules in
    ghc-internal to use -XStaticPointers.
  - When using multiple home units, we are now careful to handle the
    situation in which we may have multiple GHC.Essentials modules
    around. See the new tests under 'driver/multipleHomeUnits'.

b19fcc1c

by Vladislav Zavialov at 2026-08-14T06:26:11-04:00

Increase test coverage of diagnostics, batch 2

Add test cases for the previously untested diagnostics:
  [GHC-26133] TcRnForeignImportPrimSafeAnn
  [GHC-68444] SumAltArityExceeded
  [GHC-63966] IllegalSumAlt
  [GHC-23882] IllegalDeclaration
  [GHC-60220] InvalidCCallImpent
  [GHC-18816] RecGadtNoCons
  [GHC-38140] GadtNoCons
  [GHC-37056] InvalidTypeInstanceHeader
  [GHC-78486] InvalidTyFamInstLHS
  [GHC-39639] DefaultDataInstDecl
  [GHC-78822] AssocDefaultNotAssoc
  [GHC-43510] NotSimpleUnliftedType
  [GHC-41843] IOResultExpected
  [GHC-07641] AtLeastOneArgExpected
  [GHC-52886] InvalidTopDecl

Remove unused error constructors:
  [GHC-92057] ImportLookupAmbiguous

7b27f25a

by Simon Jakobi at 2026-08-14T06:26:54-04:00

testsuite: Drop peak_megabytes_allocated from LinkableUsage tests

LinkableUsage01/02 collected all metrics with a 2% tolerance. For
peak_megabytes_allocated, whose granularity is 1 MB, that window is
under 0.7 MB at this test's ~34 MB peak, so any 1 MB step failed the
test (#27613, #27489). Drop that metric: max_bytes_used guards the
Linkable-retention property with byte granularity, at a tolerance
that still comfortably exceeds the noise observed in CI.

Assisted-by: Claude Fable 5

e5de423b

by Simon Jakobi at 2026-08-14T06:26:54-04:00

testsuite: Don't truncate fractional baselines when computing bounds

RelativeMetricAcceptanceWindow.get_bounds truncated the baseline with
int() before applying the tolerance. Baselines can be fractional (they
are averaged over several measurements), so this skewed the acceptance
window downwards: in #27613, a baseline of 33.67 at 2% tolerance
yielded bounds (32, 34) instead of (32, 35), rejecting a measurement
that was within tolerance.

Assisted-by: Claude Fable 5

db959f83

by Simon Jakobi at 2026-08-14T15:16:44-04:00

testsuite: Expect length001 failure in nonmoving_thr_sanity

length001 relies on an optimization rule to avoid excessive stack use.
The nonmoving_thr_sanity way does not enable optimization, so classify
its stack overflow as an expected failure, as is already done for the
other unoptimized nonmoving ways.

Assisted-by: gpt-5.6-sol via Codex CLI

4f2b7d90

by Simon Jakobi at 2026-08-14T15:16:44-04:00

testsuite: Omit T22859 in nonmoving threaded ways

T22859 checks allocation-limit handlers with output that depends on
precise allocation behaviour. The nonmoving threaded ways change where
these limits are reached, just as the already-omitted LLVM ways do.

Omit these ways instead of treating their incidental output differences
as test failures.

Assisted-by: gpt-5.6-sol via Codex CLI

c4abddcb

by Simon Jakobi at 2026-08-14T15:16:44-04:00

rts/js: Implement eq_thread, and test Eq/Ord ThreadId (#16761)

Since d1f3c63701, Eq ThreadId is implemented via the RTS function
eq_thread, but the JS RTS never provided it, so comparing ThreadIds
for equality on the JS backend crashed with

    ReferenceError: h$eq_thread is not defined

Like the C implementation, h$eq_thread uses pointer equality: The JS
RTS has exactly one thread object per thread.

Since previously no test exercised eq_thread directly, this commit
adds a test covering equality, its stability across GC, and agreement
with Ord.

Assisted-by: Claude Fable 5

4a7defa1

by Simon Jakobi at 2026-08-14T15:16:44-04:00

testsuite: Make listThreads1 insensitive to the RTS's own threads

listThreads1 expected `listThreads` to return exactly [ThreadId 1]. That
holds only under a non-threaded RTS. Under a threaded RTS however there
are more threads present, so we change the test to simply check that
`myThreadId` is present in the list.

Assisted-by: Claude Opus 5

b757727a

by Vladislav Zavialov at 2026-08-14T15:17:27-04:00

Fix tcLookupId panic with RequiredTypeArguments and PatternSynonyms (#27586)

The arguments declared on the left-hand side of a pattern synonym are looked up
as term variables bound by its right-hand side. Prior to this patch, that lookup
panicked with RequiredTypeArguments:

    data T a where
      MkT :: forall a -> T a

    pattern P :: Int -> T Int
    pattern P x = MkT x

On the RHS, `x` looks like a term argument, so the renamer binds it in the term
namespace. Only during type checking does it turn out to be a type variable, so
the lookup on the LHS finds an ATyVar rather than an ATcId. As the lookup was
done with tcLookupId, it resulted in a panic.

Now the arguments are looked up with tcLookupPatSynArg, which reports an illegal
term-level use of `x`, just as an ordinary function definition `f (MkT x) = x`
does.

Test cases: T27586a T27586b T27586c

Assisted-by: Claude Opus 5

c130188d