[GHC] #15043: Expand type synonym

[GHC]

#15043: Expand type synonym
-------------------------------------+-------------------------------------
        Reporter:  domenkozar        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by domenkozar:

Old description:

> Using GHC 8.2.2 and `stack build --ghc-options=-fprint-expanded-synonyms`
> I have the following type synonym for servant:
>
> {{{#!hs
> type Get302 (cts :: [*]) (hs :: [*]) = Verb 'GET 302 cts
>   (Headers (Header "Location" String ': hs) NoContent)
> }}}
>
> and get the following error message when `String` is mismatched for
> `Text`:
>
> {{{
>         • Couldn't match type ‘Text’ with ‘[Char]’
>           Expected type: AsServerT App
>                          :- ("login"
>                              :> ("callback"
>                                  :> (QueryParam "code" Text
>                                      :> (QueryParam "state" Text
>                                          :> MyApp.Types.Servant.Get302
>                                               '[PlainText]
>                                               '[Header "Set-Cookie"
> SetCookie,
>                                                 Header "Set-Cookie"
> SetCookie]))))
>             Actual type: Maybe Code
>                          -> Maybe Text
>                          -> App
>                               (Headers
>                                  '[Header "Location" Text, Header "Set-
> Cookie" SetCookie,
>                                    Header "Set-Cookie" SetCookie]
>                                  NoContent)
> }}}
>
> The error is confusing as type synonym is not expanded and offending
> `Header` is missing from the output.

New description:

 Using GHC 8.2.2 and `stack build --ghc-options=-fprint-expanded-synonyms`
 I have the following type synonym for servant:

 {{{#!hs
 type Get302 (cts :: [*]) (hs :: [*]) = Verb 'GET 302 cts
   (Headers (Header "Location" String ': hs) NoContent)
 }}}

 and get the following error message when `String` is mismatched for
 `Text`:

 {{{
         • Couldn't match type ‘Text’ with ‘[Char]’
           Expected type: AsServerT App
                          :- ("login"
                              :> ("callback"
                                  :> (QueryParam "code" Text
                                      :> (QueryParam "state" Text
                                          :> MyApp.Types.Servant.Get302
                                               '[PlainText]
                                               '[Header "Set-Cookie"
 SetCookie,
                                                 Header "Set-Cookie"
 SetCookie]))))
             Actual type: Maybe Code
                          -> Maybe Text
                          -> App
                               (Headers
                                  '[Header "Location" Text, Header "Set-
 Cookie" SetCookie,
                                    Header "Set-Cookie" SetCookie]
                                  NoContent)
 }}}

 The error is confusing as type synonym is not expanded and offending
 `Header` is missing from the output in the expected type.

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15043#comment:1>
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