UNPACK is paranoid about a phantom type argument

Attempting to use code with a phantom type like the following:

newtype Id a = Id Int

inlines fine into

data Foo = Foo {-# UNPACK #-} !(Id Foo) String

but when I go to inline THAT

data Bar = Bar {-# UNPACK #-} !Foo

GHC tells me no, even though the recursive use of Foo is only through a phantom type.

Distilled.hs:3:12:
    Warning: Ignoring unusable UNPACK pragma on the
             first argument of `Bar'
    In the definition of data constructor `Bar'
    In the data type declaration for `Bar'

Trac metadata

Trac field	Value
Version	7.0.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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