Why no multiple default method implementations?

[Bas van Dijk]

Hello,

Now that we have DefaultSignatures, why is it not allowed to have
multiple default method implementations, as in:

{-# LANGUAGE DefaultSignatures #-}

class Foo a where
    foo :: a
    foo = error "foo"

    default foo :: Num a => a
    foo = 1

GHC complains: "Conflicting definitions for `foo'"

The following use of multiple default signatures also gives the same error:

class Foo a where
    foo :: a

    default foo :: Fractional a => a
    foo = 0.5

    default foo :: Num a => a
    foo = 1

Couldn't GHC always pick the most specific default method, just as it
does with instances when OverlappingInstances is enabled?

Regards,

Bas