{-# LANGUAGE MultiParamTypeClasses #-}                                                                                                                 
module Amb where                                                                                                                                       
                                                                                                                                                       
class C a b where                                                                                                                                      
    f :: (a,b)                                                                                                                                         
                                                                                                                                                       
instance C Int Char where                                                                                                                              
    f = undefined                                                                                                                                      
                                                                                                                                                       
{-                                                                                                                                                     
x = fst f                                                                                                                                              
/home/saizan/snippets/Amb.hs:7:8:                                                                                                                      
    Ambiguous type variables `a', `b' in the constraint:                                                                                               
      `C a b'                                                                                                                                          
        arising from a use of `f' at /home/saizan/snippets/Amb.hs:7:8                                                                                  
    Possible cause: the monomorphism restriction applied to the following:                                                                             
      x :: a (bound at /home/saizan/snippets/Amb.hs:7:0)                                                                                               
    Probable fix: give these definition(s) an explicit type signature                                                                                  
                  or use -XNoMonomorphismRestriction                                                                                                   
Failed, modules loaded: none.                                                                                                                          
-}                                                                                                                                                     
                                                                                                                                                       
{-                                                                                                                                                     
y = fst f :: Int                                                                                                                                       
                                                                                                                                                       
/home/saizan/snippets/Amb.hs:21:8:                                                                                                                     
    No instance for (C Int b)                                                                                                                          
      arising from a use of `f' at /home/saizan/snippets/Amb.hs:21:8                                                                                   
    Possible fix: add an instance declaration for (C Int b)                                                                                            
    In the first argument of `fst', namely `f'                                                                                                         
    In the expression: fst f :: Int                                                                                                                    
    In the definition of `y': y = fst f :: Int                                                                                                         
Failed, modules loaded: none.                                                                                                                          
-}

Both x and y have the same problem, there isn't enough type information to let the typechecker decide on an instance, so it seems they should produce similar error messages.

In particular, the error for y is quite confusing since it can be reasonably interpreted as saying there's no type b for which there's an instance C Int b, which in fact is not true, so i think explicitly mentioning the ambiguity like in the first message would help many to understand the problem better.

I can see though that an "instance C Int b" could make sense, more often than C a b, so maybe "Possible fix: add an instance declaration for (C Int b)" should be conserved, even if it still has the problem of expressing that the second argument needs to be a variable.