GHC.Stack.HasCallStack not compatible with constraint synonyms

The programs Good and Bad are the same, except that Bad uses a constraint synonym including GHC.Stack.HasCallStack whereas Good inlines the constraints. I expect them to produce the same output, but they don't:

$ ./Good
["callStack","f"]

$ ./Bad
["f"]

Here is the source for Good.hs:

{-# LANGUAGE MultiParamTypeClasses #-} -- For nullary 'Trivial' class
module Main where

import qualified GHC.Stack as Ghc

class Trivial where
instance Trivial where

-- | Print the functions on the call stack.
callStack :: (Ghc.HasCallStack, Trivial) => IO ()
callStack = print $ map fst (Ghc.getCallStack Ghc.callStack)

f :: (Ghc.HasCallStack, Trivial) => IO ()
f = callStack

main :: IO ()
main = f -- Should print @["callStack", "f"]@.

Here is the source for Bad.hs:

{-# LANGUAGE ConstraintKinds #-}       -- For 'C'
{-# LANGUAGE MultiParamTypeClasses #-} -- For nullary 'Trivial' class
module Main where

import qualified GHC.Stack as Ghc

class Trivial where
instance Trivial where

type C = (Ghc.HasCallStack, Trivial)

-- | Print the functions on the call stack.
callStack :: C => IO ()
callStack = print $ map fst (Ghc.getCallStack Ghc.callStack)

f :: C => IO ()
f = callStack

main :: IO ()
main = f -- Should print @["callStack", "f"]@.

Tested compiled and interpreted with GHC 8.2.1 and 8.0.2.

Trac metadata

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