{-# LANGUAGE TypeSynonymInstances #-}

allows to use type synonyms as instances

example:

code:

main :: IO ()
main = print x1
 
class MyClass x where
    fMy :: x -> x -> x
 
type Speed = Double
 
instance MyClass Speed where
    fMy xA xB = xA + xB
 
x1 :: Speed
x1 = fMy 1.3 2.5

compiler error:

app\Main.hs:9:10: error:
    * Illegal instance declaration for `MyClass Speed'
        (All instance types must be of the form (T t1 ... tn)
         where T is not a synonym.
         Use TypeSynonymInstances if you want to disable this.)
    * In the instance declaration for `MyClass Speed'
  |
9 | instance MyClass Speed where
  |          ^^^^^^^^^^^^^

example, with GHC option:

code:

{-# LANGUAGE TypeSynonymInstances #-}
 
main :: IO ()
main = print x1
 
class MyClass x where
    fMy :: x -> x -> x
 
type Speed = Double
 
instance MyClass Speed where
    fMy xA xB = xA + xB
 
x1 :: Speed
x1 = fMy 1.3 2.5

compiles, error and warning free, with compiler: GHC 8.10.4, using compiler option -Wall
executes, with output:
```
3.8
```

{-# LANGUAGE TypeSynonymInstances #-}

✎