[GHC] #13727: `-Wmissing-home-modules` doesn't properly recognize filepath-qualified modules

[GHC]

#13727: `-Wmissing-home-modules` doesn't properly recognize filepath-qualified
modules
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.2.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The `-Wmissing-home-modules` feature was implemented in order to be used
 by Cabal (see also #13129), however it turns out that it currently has a
 minor bug which makes it much less useful for `cabal` as we can't enable
 it for executables this way, as Cabal needs to provide the path to the
 Main module directly.

 Consider the following two files:

 - `src-lib/M1.hs`
 {{{#!hs
 module M1 where
 }}}

 - `src-exe/Main.hs`
 {{{#!hs
 import M1

 main = return ()
 }}}

 And now consider the following inconsistent results:

 Expected good behaviour:

 {{{
 ghc-8.2.1 -fforce-recomp -Wmissing-home-modules -isrc-lib -isrc-exe Main

 <no location info>: warning: [-Wmissing-home-modules]
     Modules are not listed in command line: M1
 [1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
 [2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
 Linking src-exe/Main ...
 }}}

 {{{
 ghc-8.2.1 -fforce-recomp -Wmissing-home-modules -isrc-lib -isrc-exe M1
 Main
 [1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
 [2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
 Linking src-exe/Main ...
 }}}

 Unexpected bad behaviour:

 {{{
 ghc-8.2.1 -fforce-recomp -Wmissing-home-modules src-lib/M1.hs src-
 exe/Main.hs

 <no location info>: warning: [-Wmissing-home-modules]
     Modules are not listed in command line: M1 Main
 [1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
 [2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
 Linking src-exe/Main ...
 }}}

 {{{
 ghc-8.2.1 -fforce-recomp -Wmissing-home-modules -isrc-lib M1 src-exe/Main

 <no location info>: warning: [-Wmissing-home-modules]
     Modules are not listed in command line: Main
 [1 of 2] Compiling M1               ( src-lib/M1.hs, src-lib/M1.o )
 [2 of 2] Compiling Main             ( src-exe/Main.hs, src-exe/Main.o )
 Linking src-exe/Main ...
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13727>
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