DeriveAnyClass fails on multi-parameter type classes

When I say

{-# LANGUAGE DeriveAnyClass, MultiParamTypeClasses #-}

module Bug where

class C a b
data X
  deriving (C Int)

and load that into GHCi, I see this:

*Main> :load "/Users/rae/temp/DeriveAny.hs"
[1 of 1] Compiling Bug              ( /Users/rae/temp/DeriveAny.hs, interpreted )
Var/Type length mismatch: 
  [b_a2YW]
  []
Var/Type length mismatch: 
  [b_a2YW]
  []
Ok, modules loaded: Bug.
*Bug> :info C
class C a b 	-- Defined at /Users/rae/temp/DeriveAny.hs:5:1
instance C X -- Defined at /Users/rae/temp/DeriveAny.hs:7:13

It seems my Int parameter to C is being dropped. I don't believe I have a debug build in this example, so that debugging output is shipping.

Trac metadata

Trac field	Value
Version	7.10.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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