[GHC] #10895: "no output will be generated" should be an error, not a warning.

[GHC]

#10895: "no output will be generated" should  be an error, not a warning.
-------------------------------------+-------------------------------------
              Reporter:  gershomb    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Consider a file "Test.hs" which has a "main" function but is declared as
 "module Test where..."

 If we attempt to compile it, we get the following warning:

 {{{
 Warning: output was redirected with -o, but no output will be generated
 because there is no Main module.
 }}}

 This should be upgraded from a warning to an error. This would help e.g.
 Cabal, because such warnings do not lead to a failure of the wrapping
 Cabal process, which is confusing to end-users.

 For gory details, see e.g. discussion at
 https://github.com/haskell/cabal/issues/1847

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10895>
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