Add Travis job to build cabal with solver debug flags enabled.

.travis.yml

@@ -43,6 +43,9 @@ matrix:
    - env: GHCVER=8.0.1 SCRIPT=script DEPLOY_DOCS=YES
      sudo: required
      os: linux
+   - env: GHCVER=8.0.1 SCRIPT=solver-debug-flags
+     sudo: required
+     os: linux
    - env: GHCVER=8.0.1 SCRIPT=bootstrap
      sudo: required
      os: linux

travis-solver-debug-flags.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+# Build cabal with solver debug flags enabled.
+#
+# We use a sandbox, because cabal-install-1.24.0.0's new-build command tries to
+# build tracetree's dependencies with the inplace Cabal, which leads to compile
+# errors. We also need to skip the tests, because debug-tracetree prints the
+# whole solver tree as JSON.
+
+cabal update
+cd cabal-install
+cabal sandbox init
+cabal sandbox add-source ../Cabal
+cabal install --dependencies-only --constraint "cabal-install +debug-tracetree +debug-conflict-sets"
+cabal configure --ghc-option=-Werror --constraint "cabal-install +debug-tracetree +debug-conflict-sets"
+cabal build