Added new regresion test for #18036 from ticket #19865.

2 files changed, 34 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T18036b.hs b/testsuite/tests/typecheck/should_compile/T18036b.hs
new file mode 100644
index 0000000000..0d4e78ead4
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T18036b.hs
@@ -0,0 +1,33 @@
+-- https://gitlab.haskell.org/ghc/ghc/-/issues/19865
+{-# LANGUAGE ConstrainedClassMethods #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE TypeFamilies #-}
+
+{-# OPTIONS_GHC -Wredundant-constraints #-}
+
+module Main where
+
+-- declare class
+class Mode t where
+  type Scalar t
+  myf :: Fractional (Scalar t) => t -> Scalar t -> t
+
+
+-- declare data type + instance
+data ReverseDouble a
+
+instance Mode (ReverseDouble s) where
+  type Scalar (ReverseDouble s) = Double
+  myf = undefined
+
+
+-- declare a newtype and derive the instance from the above data type
+newtype GDouble s a = GDouble (ReverseDouble s)
+  deriving newtype Mode
+
+
+
+main :: IO ()
+main = pure ()
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 868771542e..7ebb9ae65c 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -720,6 +720,7 @@ test('T18005', normal, compile, [''])
 test('T18023', normal, compile, [''])
 test('T18036', normal, compile, [''])
 test('T18036a', normal, compile, [''])
+test('T18036b', normal, compile, [''])
 test('T17873', normal, compile, [''])
 test('T18129', expect_broken(18129), compile, [''])
 test('T18185', normal, compile, [''])