use ~ in KeyValue instances

Author: phadej

changelog.md

@@ -3,6 +3,7 @@ For the latest version of this document, please see [https://github.com/haskell/
 ### 2.1.0.0
 
 - Change time instances of types with year (`Day`, `UTCTime`) to require years with at least 4 digits.
+- Change `KeyValue` instances to be more general (and use equality to constraint them) instead of being more lax flexible instances.
 
 ### 2.0.3.0
 

src/Data/Aeson/Types/ToJSON.hs

@@ -334,14 +334,14 @@ instance KeyValue Series where
     name .= value = E.pair name (toEncoding value)
     {-# INLINE (.=) #-}
 
-instance KeyValue Pair where
+instance (key ~ Key, value ~ Value) => KeyValue (key, value) where
     name .= value = (name, toJSON value)
     {-# INLINE (.=) #-}
 
 -- | Constructs a singleton 'KM.KeyMap'. For calling functions that
 --   demand an 'Object' for constructing objects. To be used in
 --   conjunction with 'mconcat'. Prefer to use 'object' where possible.
-instance KeyValue Object where
+instance value ~ Value => KeyValue (KM.KeyMap value) where
     name .= value = KM.singleton name (toJSON value)
     {-# INLINE (.=) #-}