Coercing between constraints of newtypes

This doesn't work

{-# Language ConstraintKinds #-}
{-# Language GADTs #-}

import Data.Coerce

newtype USD = USD Int

data Dict c where
  Dict :: c => Dict c

num :: Dict (Num Int) -> Dict (Num USD)
num = coerce

but this does

data NUM a = NUM (a -> a -> a)

num' :: NUM Int -> NUM USD
num' = coerce

is this a fundamental limitation?

Trac metadata

Trac field	Value
Version	8.2.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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