Fix build-depends also.

Author: angerman

cabal-install-solver/src/Distribution/Solver/Modular/Preference.hs

@@ -353,8 +353,13 @@ avoidReinstalls p = go
 pruneHostFromSetup :: EndoTreeTrav d c
 pruneHostFromSetup = go
   where
+    -- for Setup(.hs) and build-depends, we want to force Build packages.
     go (PChoiceF qpn rdm gr cs) | (Q (PackagePath _ (QualSetup _)) _) <- qpn =
       PChoiceF qpn rdm gr (W.filterKey (not . isHost) cs)
+    -- QualExe are build-depends. Structure is QualExe (comp) (build-depend).
+    go (PChoiceF qpn rdm gr cs) | (Q (PackagePath _ (QualExe _ _)) _) <- qpn =
+      PChoiceF qpn rdm gr (W.filterKey (not . isHost) cs)
+    -- everything else use Host packages.
     go (PChoiceF qpn rdm gr cs) | (Q (PackagePath _ _) _) <- qpn =
       PChoiceF qpn rdm gr (W.filterKey isHost cs)
     go x = x

cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs

@@ -115,6 +115,7 @@ solve sc toolchains idx pkgConfigDB userPrefs userConstraints userGoals =
   traceTree "pruned.json" id .
   trav prunePhase   .
   trav P.pruneHostFromSetup .
+  -- stageBuildDeps "build: " .
   traceTree "build.json" id $
   buildPhase
   where
@@ -153,9 +154,15 @@ solve sc toolchains idx pkgConfigDB userPrefs userConstraints userGoals =
 
     stageBuildDeps prefix = go
       where go :: Tree d c -> Tree d c
+            -- For Setup we must use the build compiler, as the host compiler
+            -- may not be able to produce code that runs on the build machine.
             go (PChoice qpn rdm gr cs) | (Q (PackagePath _ (QualSetup _)) _) <- qpn =
               (PChoice qpn rdm gr (trace (prefix ++ show qpn ++ '\n':unlines (map (" - " ++) candidates)) (go <$> cs)))
               where candidates = map show . filter (\(I _s _v l) -> l /= InRepo) . map (\(_w, (POption i _), _v) -> i) $ W.toList cs
+            -- Same for build-depends. These show up as QualExe (component) (build-depends).
+            go (PChoice qpn rdm gr cs) | (Q (PackagePath _ (QualExe _ _)) _) <- qpn =
+              (PChoice qpn rdm gr (trace (prefix ++ show qpn ++ '\n':unlines (map (" - " ++) candidates)) (go <$> cs)))
+              where candidates = map show . filter (\(I _s _v l) -> l /= InRepo) . map (\(_w, (POption i _), _v) -> i) $ W.toList cs
             go (PChoice qpn rdm gr cs) =
               (PChoice qpn rdm gr (go <$> cs))
             go (FChoice qfn rdm gr t b d cs) =