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reifyInstances doesn't appear to know how to deal with a constraint that is just an alias. For example, the following prints (True,False):

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Language.Haskell.TH.Syntax

class C a
instance C Int

type D a = C a

main = print $(
  do isCInst <- isInstance ''C [ConT ''Int]
     isDInst <- isInstance ''D [ConT ''Int]
     lift (isCInst,isDInst))

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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