Associated types without type families - why can't this work?

indexable.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}

class Indexable a where


instance Indexable (String, Int, Char) where
instance Indexable ([(String, a)], String, a) where

test1 :: Indexable (a,b,c) => a -> b -> c
test1 x y = undefined

t1 = test1 "hi" (3 :: Int)

main = return ()

-- Doesn't compile, with:
-------------------------------------------------------------------------
-- indexable.hs:13:6:                                                  --
--     No instance for (Indexable ([Char], Int, c0))                   --
--       arising from a use of ‘test1’                                 --
--     The type variable ‘c0’ is ambiguous                             --
--     Relevant bindings include t1 :: c0 (bound at indexable.hs:13:1) --
--     Note: there is a potential instance available:                  --
--       instance Indexable (String, Int, Char)                        --
--         -- Defined at indexable.hs:7:10                             --
--     In the expression: test1 "hi" (3 :: Int)                        --
--     In an equation for ‘t1’: t1 = test1 "hi" (3 :: Int)             --
-------------------------------------------------------------------------