Incorrect behavior when let binding implicit CallStack object

I'm trying out ghc HEAD at 062feee4.

{-# LANGUAGE ImplicitParams #-}

import GHC.Types

f :: (?loc :: CallStack) => Int
f = let y = length $ getCallStack ?loc
    in if y < 2 then 3 else 4

f2 :: (?loc :: CallStack) => Int
f2 = if (length $ getCallStack ?loc) < 2 then 3 else 4

f3 :: (?loc :: [Int]) => Int
f3 = let y = length ?loc
    in if y < 2 then 3 else 4

produces:

[1 of 1] Compiling Main             ( Bug.hs, interpreted )

Bug.hs:5:6: warning:
    Redundant constraint: ?loc::CallStack
    In the type signature for:
         f :: (?loc::CallStack) => Int

f3 doesn't produce an error which makes me think it's related to CallStack handling. The warning seems accurate: the returned callstack only contains the entry at f.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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