Cross-module SPECIALZE pragmas aren't typechecked in -O0

If a module defines a SPECIALIZE pragma for a value defined in another module, then the signature of this pragma won't be typecheck by ghc -O0 (but it will be if the SPECIALIZE pragma is in the same module as the value).

For example, given

-- Foo.hs
module Foo where

foo :: a -> a
foo = id


----------
-- Bar.hs
module Bar where

import Foo

{-# SPECIALIZE foo :: Int -> Bool #-}

running ghc --make Bar.hs will run fine, while ghc --make -O2 Bar.hs will complain:

Bar.hs:5:1: error:
    • Couldn't match type ‘Bool’ with ‘Int’
      Expected type: Int -> Int
        Actual type: Int -> Bool
    • In the SPECIALISE pragma {-# SPECIALIZE foo :: Int -> Bool #-}
  |
5 | {-# SPECIALIZE foo :: Int -> Bool #-}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Tested on ghc 8.6.1 and 8.4.3

Trac metadata

Trac field	Value
Version	8.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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