[GHC] #16059: checkValidType is defeated by a type synonym

[GHC]

#16059: checkValidType is defeated by a type synonym
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:  16140
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 For the sake of gathering more examples, this also manifests when checking
 unboxed tuples. For instance, GHC compiles `B.hs` below successfully, even
 though it shouldn't (since the `UnboxedTuples` extension isn't enabled in
 `B`):

 {{{#!hs
 {-# LANGUAGE UnboxedTuples #-}
 module A where

 type Foo = (# #)
 }}}
 {{{#!hs
 -- B.hs
 module B where

 import A

 type Bar = Foo
 }}}

 On the other hand, in `C.hs` below:

 {{{#!hs
 -- C.hs
 {-# LANGUAGE TemplateHaskell #-}
 module C where

 import Language.Haskell.TH (conT, unboxedTupleTypeName)

 type Baz = $(conT (unboxedTupleTypeName 0))
 }}}

 GHC correctly rejects this, since there's no intermediate type synonym:

 {{{
 C.hs:7:1: error:
     • Illegal unboxed tuple type as function argument: (# #)
       Perhaps you intended to use UnboxedTuples
     • In the type synonym declaration for ‘Baz’
   |
 7 | type Baz = $(conT (unboxedTupleTypeName 0))
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 -----

 This also affects `RankNTypes`, since `B.hs` is erroneously accepted:

 {{{#!hs
 -- A.hs
 {-# LANGUAGE RankNTypes #-}
 module A where

 type Foo = forall a. a
 }}}
 {{{#!hs
 -- B.hs
 module B where

 import A

 f :: Foo -> b -> b
 f g x = g x
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16059#comment:11>
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