Add T12520 as a test

  #12520 was already fixed in HEAD. Adding a test to make sure it stays fixed.

testsuite/tests/ghci/scripts/T12520.hs

+{-# LANGUAGE MagicHash, UnboxedTuples #-}
+
+module Bug ( box, wrap, proxy ) where
+
+import GHC.Prim
+
+box :: (# Proxy# a, b #) -> b
+box (# x, y #) = y
+
+wrap :: b -> Proxy# a -> (# Proxy# a, b #)
+wrap x = \i# -> (# i#, x #)
+
+proxy :: () -> Proxy# a
+proxy () = proxy#

testsuite/tests/ghci/scripts/T12520.script

+:load T12520
+box (wrap "foo" (proxy ()))

testsuite/tests/ghci/scripts/T12520.stdout

+"foo"

testsuite/tests/ghci/scripts/all.T

@@ -258,3 +258,4 @@ test('T11376', normal, ghci_script, ['T11376.script'])
 test('T12007', normal, ghci_script, ['T12007.script'])
 test('T11975', normal, ghci_script, ['T11975.script'])
 test('T10963', normal, ghci_script, ['T10963.script'])
+test('T12520', normal, ghci_script, ['T12520.script'])