Core Lint: collect args through floatable ticks

We were not looking through floatable ticks when collecting arguments in
Core Lint, which caused `checkCanEtaExpand` to fail on something like:

```haskell
reallyUnsafePtrEquality
  = \ @a ->
      (src<loc> reallyUnsafePtrEquality#)
        @Lifted @a @Lifted @a
```

We fix this by using `collectArgsTicks tickishFloatable` instead of
`collectArgs`, to be consistent with the behaviour of eta expansion
outlined in Note [Eta expansion and source notes] in GHC.Core.Opt.Arity.

Fixes #21152.

3 files changed, 20 insertions, 1 deletions

diff --git a/compiler/GHC/Core/Lint.hs b/compiler/GHC/Core/Lint.hs
index 688b63ff55..32d109b2f1 100644
--- a/compiler/GHC/Core/Lint.hs
+++ b/compiler/GHC/Core/Lint.hs
@@ -945,7 +945,17 @@ lintCoreExpr e@(App _ _)
        ; checkCanEtaExpand fun args app_ty
        ; return app_pair}
   where
-    (fun, args) = collectArgs e
+    (fun, args, _source_ticks) = collectArgsTicks tickishFloatable e
+      -- We must look through source ticks to avoid #21152, for example:
+      --
+      -- reallyUnsafePtrEquality
+      --   = \ @a ->
+      --       (src<loc> reallyUnsafePtrEquality#)
+      --         @Lifted @a @Lifted @a
+      --
+      -- To do this, we use `collectArgsTicks tickishFloatable` to match
+      -- the eta expansion behaviour, as per Note [Eta expansion and source notes]
+      -- in GHC.Core.Opt.Arity.
 
 lintCoreExpr (Lam var expr)
   = markAllJoinsBad $
diff --git a/testsuite/tests/corelint/T21152.hs b/testsuite/tests/corelint/T21152.hs
new file mode 100644
index 0000000000..6722d95267
--- /dev/null
+++ b/testsuite/tests/corelint/T21152.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE MagicHash #-}
+
+module T21152 where
+
+import GHC.Exts ( Int#, reallyUnsafePtrEquality# )
+
+reallyUnsafePtrEquality :: a -> a -> Int#
+reallyUnsafePtrEquality = reallyUnsafePtrEquality#
diff --git a/testsuite/tests/corelint/all.T b/testsuite/tests/corelint/all.T
index 2f8b9070ec..1b53c188be 100644
--- a/testsuite/tests/corelint/all.T
+++ b/testsuite/tests/corelint/all.T
@@ -2,3 +2,4 @@ setTestOpts(extra_hc_opts('-package ghc'))
 setTestOpts(extra_run_opts('"' + config.libdir + '"'))
 
 test('LintEtaExpand', normal, compile_and_run, [''])
+test('T21152', normal, compile, ['-g3'])