[GHC] #15708: Cross-module SPECIALZE pragmas aren't typechecked in -O0

[GHC]

#15708: Cross-module SPECIALZE pragmas aren't typechecked in -O0
-------------------------------------+-------------------------------------
           Reporter:  regnat         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 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

 {{{#!hs
 -- 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

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15708>
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