backpack: Be more careful when adding together ImportAvails

There was some code in the signature merging logic which added together
the ImportAvails of the signature and the signature which was merged
into it. This had the side-effect of making the merged signature depend
on the signature (via a normal module dependency). The intention was to
propagate orphan instances through the merge but this also messed up
recompilation logic because we shouldn't be attempting to load B.hi when
mergeing it.

The fix is to just combine the part of ImportAvails that we intended to
(transitive info, orphan instances and type family instances) rather
than the whole thing.

2 files changed, 22 insertions, 11 deletions

diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index db923a0982..886bc12192 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -589,7 +589,7 @@ checkDependencies hsc_env summary iface
         liftIO $
           check_mods (sort hs) prev_dep_mods
           `recompThen`
-            let allPkgDeps = sortBy (comparing snd) $ nubOrdOn snd (ps ++ implicit_deps ++ bkpk_units)
+            let allPkgDeps = sortBy (comparing snd) $ nubOrdOn snd (ps ++ implicit_deps)
             in check_packages allPkgDeps prev_dep_pkgs
  where
 
@@ -613,7 +613,6 @@ checkDependencies hsc_env summary iface
    prev_dep_mods = map (second gwib_mod) $ Set.toAscList $ dep_direct_mods (mi_deps iface)
    prev_dep_pkgs = Set.toAscList (Set.union (dep_direct_pkgs (mi_deps iface))
                                             (dep_plugin_pkgs (mi_deps iface)))
-   bkpk_units    = map ((fsLit "Signature",) . instUnitInstanceOf . moduleUnit) (requirementMerges units (moduleName (mi_module iface)))
 
    implicit_deps = map (fsLit "Implicit",) (implicitPackageDeps dflags)
 
diff --git a/compiler/GHC/Tc/Utils/Backpack.hs b/compiler/GHC/Tc/Utils/Backpack.hs
index 7e5f339d3f..a73c01c90f 100644
--- a/compiler/GHC/Tc/Utils/Backpack.hs
+++ b/compiler/GHC/Tc/Utils/Backpack.hs
@@ -856,7 +856,6 @@ mergeSignatures
         -- we hope that we get lucky / the overlapping instances never
         -- get used, but it is not a very good situation to be in.
         --
-        hsc_env <- getTopEnv
         let merge_inst (insts, inst_env) inst
                 | memberInstEnv inst_env inst -- test DFun Type equality
                 = (insts, inst_env)
@@ -867,18 +866,17 @@ mergeSignatures
             (insts, inst_env) = foldl' merge_inst
                                     (tcg_insts tcg_env, tcg_inst_env tcg_env)
                                     (instEnvElts $ md_insts details)
-            -- This is a HACK to prevent calculateAvails from including imp_mod
-            -- in the listing.  We don't want it because a module is NOT
+            -- Use mi_deps directly rather than calculateAvails.
+            -- because a module is NOT
             -- supposed to include itself in its dep_orphs/dep_finsts.  See #13214
-            iface' = iface { mi_final_exts = (mi_final_exts iface){ mi_orphan = False, mi_finsts = False } }
-            home_unit = hsc_home_unit hsc_env
-            other_home_units = hsc_all_home_unit_ids hsc_env
-            avails = plusImportAvails (tcg_imports tcg_env) $
-                        calculateAvails home_unit other_home_units iface' False NotBoot ImportedBySystem
+            avails = tcg_imports tcg_env
+            deps = mi_deps iface
+            avails_with_trans = addTransitiveDepInfo avails deps
+
         return tcg_env {
             tcg_inst_env = inst_env,
             tcg_insts    = insts,
-            tcg_imports  = avails,
+            tcg_imports  = avails_with_trans,
             tcg_merged   =
                 if outer_mod == mi_module iface
                     -- Don't add ourselves!
@@ -912,6 +910,20 @@ mergeSignatures
 
     return tcg_env
 
+-- | Add on the necessary transitive information from the merged signature to
+-- the 'ImportAvails' of the result of merging. This propagates the orphan instances
+-- which were in the transitive closure of the signature through the merge.
+addTransitiveDepInfo :: ImportAvails -- ^ From the signature resulting from the merge
+                     -> Dependencies -- ^ From the original signature
+                     -> ImportAvails
+addTransitiveDepInfo avails deps =
+  -- Avails for the merged in signature
+  -- Add on transitive information from the signature but nothing else..
+  -- because we do not "import" the signature.
+  avails { imp_orphs = imp_orphs avails ++ dep_orphs deps
+         , imp_finsts = imp_finsts avails ++ dep_finsts deps
+         , imp_sig_mods = imp_sig_mods avails ++ dep_sig_mods deps }
+
 -- | Top-level driver for signature instantiation (run when compiling
 -- an @hsig@ file.)
 tcRnInstantiateSignature ::