[commit: ghc] master: desugar: Ensure that a module's dep_orphs doesn't contain itself (db3a8e1)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/db3a8e168ad81f54ec58eebc4c75a0eaad889daf/ghc

>---------------------------------------------------------------

commit db3a8e168ad81f54ec58eebc4c75a0eaad889daf
Author: Ben Gamari <bgamari.foss at gmail.com>
Date:   Sat Aug 26 16:17:18 2017 -0400

    desugar: Ensure that a module's dep_orphs doesn't contain itself
    
    Consider that we have two modules, A and B, both with hs-boot files,
    
      * A.hs contains a SOURCE import of B
      * B.hs-boot contains a SOURCE import of A
      * A.hs-boot declares an orphan instance
      * A.hs defines the orphan instance
    
    In this case, B's dep_orphs will contain A due to its SOURCE import of
    A.  Consequently, A will contain itself in its imp_orphs due to its
    import of B.  This fact would end up being recorded in A's interface
    file. This would then break the invariant asserted by calculateAvails
    that a module does not itself in its dep_orphs. This was the cause
    of #14128.
    
    The solution is to remove self-references from imp_orphs when
    constructing dep_orphs; we already did a similar thing for dep_mods. I
    believe we should do the same for dep_finsts, although I'm treating this
    as a separate bug.
    
    Reviewers: austin
    
    Subscribers: rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3892


>---------------------------------------------------------------

db3a8e168ad81f54ec58eebc4c75a0eaad889daf
 compiler/deSugar/DsUsage.hs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/compiler/deSugar/DsUsage.hs b/compiler/deSugar/DsUsage.hs
index 8158a8e..3f302fa 100644
--- a/compiler/deSugar/DsUsage.hs
+++ b/compiler/deSugar/DsUsage.hs
@@ -38,7 +38,7 @@ mkDependencies
       -- Template Haskell used?
       th_used <- readIORef th_var
       let dep_mods = modDepsElts (delFromUFM (imp_dep_mods imports)
-                                           (moduleName mod))
+                                             (moduleName mod))
                 -- M.hi-boot can be in the imp_dep_mods, but we must remove
                 -- it before recording the modules on which this one depends!
                 -- (We want to retain M.hi-boot in imp_dep_mods so that
@@ -46,6 +46,10 @@ mkDependencies
                 --  on M.hi-boot, and hence that we should do the hi-boot consistency
                 --  check.)
 
+          dep_orphs = filter (/= mod) (imp_orphs imports)
+                -- We must also remove self-references from imp_orphs. See
+                -- #14128.
+
           pkgs | th_used   = Set.insert (toInstalledUnitId thUnitId) (imp_dep_pkgs imports)
                | otherwise = imp_dep_pkgs imports
 
@@ -57,7 +61,7 @@ mkDependencies
 
       return Deps { dep_mods   = dep_mods,
                     dep_pkgs   = dep_pkgs',
-                    dep_orphs  = sortBy stableModuleCmp (imp_orphs  imports),
+                    dep_orphs  = dep_orphs,
                     dep_finsts = sortBy stableModuleCmp (imp_finsts imports) }
                     -- sort to get into canonical order
                     -- NB. remember to use lexicographic ordering