Tickle a bug in impredicative polymorphism

testsuite/tests/ghc-regress/typecheck/should_compile/all.T

@@ -217,6 +217,7 @@ test('tc204', normal, compile, [''])
 test('tc205', normal, compile, [''])
 test('tc206', normal, compile, [''])
 test('tc207', normal, compile, [''])
+test('tc208', normal, compile, [''])
 
 # Omitting temporarily
 # test('syn-perf', normal, compile, ['-fcontext-stack=30'])

testsuite/tests/ghc-regress/typecheck/should_compile/tc208.hs

+{-# OPTIONS -fglasgow-exts -fimplicit-params #-}
+
+-- This program failed to typecheck in an early version of
+-- GHC with impredicative polymorphism, but it was fixed by
+-- doing pre-subsumption in the subsumption check.
+-- Trac bug #821
+
+module ShouldCompile where
+
+type PPDoc = (?env :: Int) => Char
+
+f :: Char -> PPDoc
+f  = succ