GHC mistakenly warns about type defaulting in the presence of -XTypeApplications

This code:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
module Main (main) where

theFloatDigits :: forall a. RealFloat a => Int
theFloatDigits = floatDigits (undefined @_ @a)

main :: IO ()
main = print (theFloatDigits @Double, theFloatDigits @Float)

erroneously produces this warning:

$ /opt/ghc/8.0.1/bin/runghc -Wall TheFloatDigits.hs

TheFloatDigits.hs:6:19: warning: [-Wtype-defaults]
    • Defaulting the following constraint to type ‘Double’ RealFloat a0
    • In the ambiguity check for ‘theFloatDigits’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature:
        theFloatDigits :: forall a. RealFloat a => Int
(53,24)

GHC's claim that a0 was defaulted to Double is clearly bogus, since theFloatDigits outputs different answers for Double and Float.

Trac metadata

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