1 files changed, 42 insertions, 0 deletions
diff --git a/testsuite/tests/linear/should_compile/Arity2.hs b/testsuite/tests/linear/should_compile/Arity2.hs
new file mode 100644
index 0000000000..d764d5111a
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+++ b/testsuite/tests/linear/should_compile/Arity2.hs
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+{-# LANGUAGE LinearTypes #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+module Arity2 where
+{-
+inplace/bin/ghc-stage1 -O2 -dcore-lint
+-}
+
+--import GHC.Base
+
+data Id a = Id a
+
+(<$>) :: (a -> b) -> Id a -> Id b
+(<$>) f (Id a) = Id (f a)
+
+(<*>) :: Id (a -> b) -> Id a -> Id b
+(<*>) (Id a) (Id b) = Id (a b)
+
+data Q = Q () ()
+data S = S ()
+
+-- Q only gets eta-expand once and then trapped
+foo = Q <$> Id () <*> Id ()
+
+-- This compiles fine
+foo2 = S <$> Id ()
+
+{-
+[1 of 1] Compiling Arity2           ( linear-tests/Arity2.hs, linear-tests/Arity2.o )
+
+linear-tests/Arity2.hs:21:7: error:
+    • Couldn't match type ‘() ⊸ Q’ with ‘() -> b’
+      Expected type: Id (() -> b)
+        Actual type: Id (() ⊸ Q)
+    • In the first argument of ‘(<*>)’, namely ‘Q <$> Id ()’
+      In the expression: Q <$> Id () <*> Id ()
+      In an equation for ‘foo’: foo = Q <$> Id () <*> Id ()
+    • Relevant bindings include
+        foo :: Id b (bound at linear-tests/Arity2.hs:21:1)
+   |
+21 | foo = Q <$> Id () <*> Id ()
+   |       ^^^^^^^^^^^
+-}