Fix #1879 (#2644)

Co-authored-by: Sandy Maguire <sandy@sandymaguire.me>

plugins/hls-tactics-plugin/src/Wingman/KnownStrategies/QuickCheck.hs

@@ -43,8 +43,9 @@ deriveArbitrary = do
               mempty
               mempty
               mempty
-          $ noLoc $
-              let' [valBind (fromString "terminal") $ list $ fmap genExpr terminal] $
+          $ noLoc $ case terminal of
+            [onlyCon] -> genExpr onlyCon -- See #1879
+            _ -> let' [valBind (fromString "terminal") $ list $ fmap genExpr terminal] $
                 appDollar (mkFunc "sized") $ lambda [bvar' (mkVarOcc "n")] $
                   case' (infixCall "<=" (mkVal "n") (int 1))
                     [ match [conP (fromString "True") []] $

plugins/hls-tactics-plugin/test/CodeAction/AutoSpec.hs

@@ -72,6 +72,7 @@ spec = do
 
   describe "known" $ do
     autoTest 25 13 "GoldenArbitrary"
+    autoTest  6 13 "GoldenArbitrarySingleConstructor"
     autoTestNoWhitespace
               6 10 "KnownBigSemigroup"
     autoTest  4 10 "KnownThetaSemigroup"

plugins/hls-tactics-plugin/test/golden/GoldenArbitrarySingleConstructor.expected.hs

+data Gen a
+
+data Obj = Obj Int Bool Char String
+
+arbitrary :: Gen Obj
+arbitrary
+  = (((Obj <$> arbitrary) <*> arbitrary) <*> arbitrary) <*> arbitrary
\ No newline at end of file

plugins/hls-tactics-plugin/test/golden/GoldenArbitrarySingleConstructor.hs

+data Gen a
+
+data Obj = Obj Int Bool Char String
+
+arbitrary :: Gen Obj
+arbitrary = _
\ No newline at end of file