[GHC] #11197: Overeager deferred type errors

[GHC]

#11197: Overeager deferred type errors
-------------------------------------+-------------------------------------
           Reporter:  goldfire       |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.11
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 With `TypeInType`, the solver now works in terms of unlifted equality. The
 only place this matters is in deferred type errors, which now are more
 eager than they were before. For example:

 {{{
 {-# OPTIONS_GHC -fdefer-type-errors #-}

 module Main where

 main = do
   putStrLn "Hi there."
   putStrLn True
 }}}

 This used to print

 {{{
 Hi there.
 Defer: Defer.hs:7:12: error:
    ...
 }}}

 Now it prints

 {{{
 Defer: Defer.hs:7:12: error:
    ...
 }}}

 No more `Hi there.`.

 Thinking about Core, this change should be expected. GHC puts all evidence
 bindings for `main` at the top, and with unlifted equality in the solver,
 these bindings now contain `case typeError ... of ...`, which forces the
 type error eagerly. Previously, there was a lazy binding for a lifted
 equality witness, unpacked only at the last moment by magic in the
 desugarer, thus the lazier warning.

 Simon has proposed customizing the !FloatIn pass to deal with this
 scenario. I do believe this would work nicely, but I have not yet
 implemented it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11197>
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