Add regression test for #12760

The bug seems to be fixed in the meantime, make sure it stays fixed.

Closes #12760

2 files changed, 37 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T12760.hs b/testsuite/tests/typecheck/should_compile/T12760.hs
new file mode 100644
index 0000000000..40db2d500a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T12760.hs
@@ -0,0 +1,36 @@
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+module A where
+
+import Data.List (minimumBy)
+import Data.Ord (comparing)
+
+data A a = A Int
+
+newtype B = B Double
+  deriving (Eq,Ord,Num,Real,Fractional,RealFrac,Floating,RealFloat)
+
+class C a where
+    _c :: [a] -> D a
+
+instance C B where
+    _c = f2 u
+
+data D x = D [(x,Double)] [ x ]
+
+u = undefined
+
+f1 :: RealFloat a => A a -> a -> [a] -> D a
+f1 (A a1) m ps0 = D (zip tickvs []) labelvs
+  where
+    range _ | m == m = if m==0 then (-1,1) else (m, m)
+    labelvs   = map fromRational $ f3 (fromIntegral a1) (range ps0)
+    tickvs    = map fromRational $ f3 (fromIntegral a1) (head labelvs, head labelvs)
+
+f2 :: RealFloat a => A a -> [a] -> D a
+f2 lap ps = f1 u (minimum ps) ps
+
+f3 :: RealFloat a => a -> (a,a) -> [Rational]
+f3 k rs@(m,_ ) = map ((s*) . fromIntegral) [floor m .. ]
+  where
+    s = minimumBy (comparing ((+ k) . realToFrac)) [0]
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index 0f61b57faf..215b57d87b 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -693,3 +693,4 @@ test('T15839a', normal, compile, [''])
 test('T15839b', normal, compile, [''])
 test('T17343', exit_code(1), compile_and_run, [''])
 test('T17566', [extra_files(['T17566a.hs'])], makefile_test, [])
+test('T12760', unless(compiler_debugged(), skip), compile, ['-O'])