The -package flag should select match from right-most package db.

The shadowing and default behavior (in the absence of
-hide-all-packages) prefers packages that come from "later" package
databases.  So for example if tmp1.d and tmp2.d both expose p-1.0, then

    ghc -package-db tmp1.d -package-db tmp2.d

brings the p-1.0 from tmp2.d into scope (and if they have the same IPID,
tmp2.d shadows tmp1.d).  HOWEVER, -package flags do NOT respect this
behavior.

    ghc -package-db tmp1.d -package-db tmp2.d -package p-1.0

this will force the p-1.0 from tmp1.d to be exposed!  This is
confusing, so this patch makes the behavior of -package flags
consistent.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1709

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/main/Packages.hs b/compiler/main/Packages.hs
index 2849b7e70a..f7bff71f1f 100644
--- a/compiler/main/Packages.hs
+++ b/compiler/main/Packages.hs
@@ -593,7 +593,9 @@ selectPackages matches pkgs unusable
   = let (ps,rest) = partition matches pkgs
     in if null ps
         then Left (filter (matches.fst) (Map.elems unusable))
-        else Right (sortByVersion ps, rest)
+        -- NB: packages from later package databases are LATER
+        -- in the list.  We want to prefer the latest package.
+        else Right (sortByVersion (reverse ps), rest)
 
 -- A package named on the command line can either include the
 -- version, or just the name if it is unambiguous.