Enable other GHCs with Parsec (for Backpack coverage.)

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

.travis.yml

@@ -46,7 +46,7 @@ matrix:
    - env: GHCVER=8.0.1 SCRIPT=solver-debug-flags
      sudo: required
      os: linux
-   - env: GHCVER=8.0.1 SCRIPT=script PARSEC=YES
+   - env: GHCVER=8.0.1 SCRIPT=script PARSEC=YES TEST_OTHER_VERSIONS=YES
      os: linux
      sudo: required
    - env: GHCVER=8.0.1 SCRIPT=bootstrap

Cabal/tests/PackageTests.hs

@@ -177,7 +177,7 @@ main = do
     -- as, unlike in the case of the GHC used to build Cabal, we don't
     -- expect htere to be a Cabal available.
     with_ghc_db_stack_env :: Maybe String
-        <- lookupEnv "CABAL_PACKAGETESTS_WITH_GHC_DB_STACK"
+        <- lookupEnv "CABAL_PACKAGETESTS_WITH_DB_STACK"
     let withGhcDBStack0 :: PackageDBStack
         withGhcDBStack0 =
               interpretPackageDbFlags True

travis-script.sh

@@ -75,11 +75,12 @@ fi
 
 # Redo the package tests with different versions of GHC
 if [ "x$TEST_OTHER_VERSIONS" = "xYES" ]; then
-    (export CABAL_PACKAGETESTS_WITH_GHC="/opt/ghc/7.0.4/bin/ghc"; \
+    (export CABAL_PACKAGETESTS_DB_STACK="clear:global:${CABAL_STORE_DB}:${CABAL_LOCAL_DB}"; \
         cd Cabal && timed ${CABAL_BDIR}/build/package-tests/package-tests $TEST_OPTIONS)
-    (export CABAL_PACKAGETESTS_WITH_GHC="/opt/ghc/7.2.2/bin/ghc"; \
+    (export CABAL_PACKAGETESTS_DB_STACK="clear:global:${CABAL_STORE_DB}:${CABAL_LOCAL_DB}"; \
         cd Cabal && timed ${CABAL_BDIR}/build/package-tests/package-tests $TEST_OPTIONS)
-    (export CABAL_PACKAGETESTS_WITH_GHC="/opt/ghc/head/bin/ghc"; \
+    GHC_HEAD_VER=$(ghc-head --version 8 | cut -d' ' -f 8)
+    (export CABAL_PACKAGETESTS_DB_STACK="clear:global:${CABAL_STORE_DB}:${CABAL_LOCAL_DB}"; \
         cd Cabal && timed ${CABAL_BDIR}/build/package-tests/package-tests $TEST_OPTIONS)
 fi