testsuite: Assert that testsuite ways are known
[ghc.git] / testsuite / tests / typecheck / should_compile / Tc173a.hs
1 {-# OPTIONS_GHC -fno-warn-redundant-constraints #-}
2 {-# LANGUAGE FlexibleInstances, TypeSynonymInstances, UndecidableInstances #-}
3
4 module Tc173a where
5
6 class FormValue value where
7 isFormValue :: value -> ()
8 isFormValue _ = ()
9
10 class FormTextField value
11
12 instance FormTextField String
13
14 instance {-# OVERLAPPABLE #-} FormTextField value => FormTextFieldIO value
15
16 class FormTextFieldIO value
17
18 instance FormTextFieldIO value => FormValue value
19
20 instance {-# OVERLAPPING #-} FormTextFieldIO value => FormTextFieldIO (Maybe value)