Arrows: correctly query arrow methods (#17423)

Consider the following code:

    proc (C x y) -> ...

Before this patch, the evidence binding for the Arrow dictionary was
attached to the C pattern:

    proc (C x y) { $dArrow = ... } -> ...

But then when we desugar this, we use arrow operations ("arr", ">>>"...)
specialised for this arrow:

    let
        arr_xy = arr $dArrow -- <-- Not in scope!
        ...
    in
        arr_xy (\(C x y) { $dArrow = ... } -> ...)

This patch allows arrow operations to be type-checked before the proc
itself, avoiding this issue.

Fix #17423

2 files changed, 38 insertions, 0 deletions

diff --git a/testsuite/tests/arrows/should_compile/T17423.hs b/testsuite/tests/arrows/should_compile/T17423.hs
new file mode 100644
index 0000000000..35023f0612
--- /dev/null
+++ b/testsuite/tests/arrows/should_compile/T17423.hs
@@ -0,0 +1,37 @@
+{-# LANGUAGE Arrows, GADTs #-}
+
+module Main where
+
+import Control.Arrow
+import Control.Category
+import Prelude hiding (id, (.))
+
+data DecoType a where
+   -- | Icons and colours for @False@ and @True@ respectively.
+   DecoBool :: Maybe (String, String) -> Maybe (Int, Int) -> DecoType Bool
+   -- | Icons and colours for ranges within type @a@.
+   DecoRange :: String -> DecoType a
+
+-- Sub-dialog for designing decorated booleans.
+decoBoolDialog :: Gadget (DecoType Bool) (DecoType Bool)
+decoBoolDialog =
+      -- arr (\(DecoBool i c) -> (i, c)) >>> (icons *** colours) >>> arr (uncurry DecoBool)
+      proc (DecoBool i c) -> do   -- Compiler panic in GHC 8.6.5.
+         i1 <- id -< i
+         c1 <- id -< c
+         returnA -< DecoBool i1 c1
+
+
+
+data Gadget b c = Pure (b -> c)
+
+instance Category Gadget where
+   id = Pure id
+   Pure g1 . Pure g2 = Pure $ g1 . g2
+
+instance Arrow Gadget where
+   arr = Pure
+   first (Pure f) = Pure $ \(b, b1) -> (f b, b1)
+
+
+main = putStrLn "Hello world."
diff --git a/testsuite/tests/arrows/should_compile/all.T b/testsuite/tests/arrows/should_compile/all.T
index 279dd109dd..b47cea0538 100644
--- a/testsuite/tests/arrows/should_compile/all.T
+++ b/testsuite/tests/arrows/should_compile/all.T
@@ -16,3 +16,4 @@ test('T5283', normal, compile, [''])
 test('T5267', expect_broken(5267), compile, [''])
 test('T5022', normalise_fun(normalise_errmsg), compile, [''])
 test('T5333', normal, compile, [''])
+test('T17423', normal, compile, [''])