Inferred CallStacks expose implicit parameter

It was noticed (in a thread on ghc-devs) that introducing a CallStack-carrying term into the RHS of a let binding in GHCi produces a type which itself carries a callstack.

I could be wrong but as far as I know this is a bug: the solver should never introduce a constraint that the user didn't write; it should merely provide an empty callstack in the event that a callstack constraint can't be solved.

Prelude> let myList = [1, 2, 3 :: Integer]
Prelude> let myList' = myList ++ undefined
Prelude> :t myList
myList :: [Integer]
Prelude> :t myList'
myList' :: (?callStack::GHC.Stack.Types.CallStack) => [Integer]

Trac metadata

Trac field	Value
Version	8.0.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	gridaphobe
Operating system
Architecture