[GHC] #14354: Unexpected type inference behavior with -XTypeApplications

[GHC]

#14354: Unexpected type inference behavior with -XTypeApplications
-------------------------------------+-------------------------------------
           Reporter:  mbw            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  POSIX
       Architecture:  x86_64         |   Type of failure:  None/Unknown
  (amd64)                            |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In the following snippet, type inference of the let bound identity
 function after applying a type unexpectedly fails:

 {{{#!hs
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 [...]
 *Main> :set -XTypeApplications
 *Main> :t id @Int
 id @Int :: Int -> Int
 *Main> let the = id
 *Main> :t the @Int

 <interactive>:1:1: error:
     • Cannot apply expression of type ‘a0 -> a0’
       to a visible type argument ‘Int’
     • In the expression: the @Int
 *Main> :t _ @Int
 }}}

 Note that it works with `id`, but not with `the`. I would expect to be
 able to write something like `the @Double 42`, for instance.

 For completeness' sake and since I don't know if it is related or
 relevant, with GHC 8.0.2, a panic is easily triggered:

 {{{#!hs
 Prelude> :set -XTypeApplications
 Prelude> :t _ @Int
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.2 for x86_64-unknown-linux):
         initTc: unsolved constraints
   WC {wc_insol = [W] __a15d :: t_a15c[tau:3] (CHoleCan: _)}

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 This however does not happen with GHC 8.2.1.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14354>
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