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

1 files changed, 6 insertions, 2 deletions

diff --git a/compiler/deSugar/DsUsage.hs b/compiler/deSugar/DsUsage.hs
index 8158a8e122..3f302fadf6 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