Error message for deriving type synonym is better and different with standalone deriving

Using standalone deriving

type My x = (Show x, Read x)
data Test = Test
deriving instance My Test

Error is:

<interactive>:19:25: error: [GHC-53946]
    • Illegal instance for a type synonym
      A class instance must be for a class
    • In the stand-alone deriving instance for ‘My Test’

But with inline deriving data Test = Test deriving (My) I am getting:

<interactive>:16:28: error: [GHC-11913] • Illegal deriving item ‘My’ • In the data declaration for ‘Test’

All same with explicit stock strategy.

By the way: why type synonyms deriving is prohibited at all? Cannot GHC just try to expand it to regular constraints by rewriting type family?

• GHC version used: 9.6.1