Report class constraints when reporting type errors.

Summary

GHC does not report classes when it reports type errors, making it hard to fix bugs.

Steps to reproduce

Take a function like e.g. Hedgehog's Hedgehog.Gen.sample

sample :: MonadIO m => Gen a -> m a

write

sample :: ()

You will get an error such as:

    • Couldn't match expected type ‘()’      
                  with actual type ‘Gen a0 -> m0 a0’
    • Probable cause: ‘Gen.sample’ is applied to too few arguments
      In the expression: Gen.sample :: ()    
      In an equation for ‘x’: x = Gen.sample :: ()
      In the expression:                     
        do putStrLn "Hello, Test!"           
           let x = ...                       
           return ()                         
   |                                         
16 |   let x = Gen.sample :: ()              
   |           ^^^^^^^^^^                    

Expected behavior

I'd like to have GHC report that m0 above is a MonadIO. That would have saved me a lot of trouble by tipping me off that it's just something I can use at the top level of the do block in main.

I understand the code that had an error is a little funny, it's just a minimal case I could come up with to report here that is still somewhat practical.

I feel like I've encountered this many times before, but it only now occurred to me that it would be much better if GHC actually reported the classes.

Environment

• GHC version used: 8.6.3