diff --git a/.travis.yml b/.travis.yml
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,6 @@
 # This Travis job script has been generated by a script via
 #
-#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-installed' 'cabal.project'
+#   runghc make_travis_yml_2.hs '-o' '.travis.yml' '--ghc-head' '--irc-channel=irc.freenode.org#haskell-lens' '--no-no-tests-no-bench' '--no-unconstrained' 'cabal.project'
 #
 # For more information, see https://github.com/hvr/multi-ghc-travis
 #
@@ -36,34 +36,37 @@
 
 matrix:
   include:
-    - compiler: "ghc-7.4.2"
+    - compiler: "ghc-8.6.1"
+      env: GHCHEAD=true
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-8.6.1], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.4.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.4.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.6.3"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.4.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.2.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.6.3], sources: [hvr-ghc]}}
-    - compiler: "ghc-7.8.4"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.2.2], sources: [hvr-ghc]}}
+    - compiler: "ghc-8.0.2"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.8.4], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-8.0.2], sources: [hvr-ghc]}}
     - compiler: "ghc-7.10.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-7.10.3], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.0.2"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.10.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.8.4"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.0.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.2.2"
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.8.4], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.6.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [*apt_packages,cabal-install-2.0,ghc-8.2.2], sources: [hvr-ghc]}}
-    - compiler: "ghc-8.4.1"
-      env: GHCHEAD=true
-      addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-8.4.1], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.6.3], sources: [hvr-ghc]}}
+    - compiler: "ghc-7.4.2"
+    # env: TEST=--disable-tests BENCH=--disable-benchmarks
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-2.2,ghc-7.4.2], sources: [hvr-ghc]}}
     - compiler: "ghc-head"
       env: GHCHEAD=true
-      addons: {apt: {packages: [*apt_packages,cabal-install-head,ghc-head], sources: [hvr-ghc]}}
+      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-head], sources: [hvr-ghc]}}
 
   allow_failures:
-    - compiler: "ghc-8.4.1"
     - compiler: "ghc-head"
+    - compiler: "ghc-8.6.1"
 
 before_install:
   - HC=${CC}
@@ -81,7 +84,8 @@
   - BENCH=${BENCH---enable-benchmarks}
   - TEST=${TEST---enable-tests}
   - HADDOCK=${HADDOCK-true}
-  - INSTALLED=${INSTALLED-true}
+  - UNCONSTRAINED=${UNCONSTRAINED-true}
+  - NOINSTALLEDCONSTRAINTS=${NOINSTALLEDCONSTRAINTS-false}
   - GHCHEAD=${GHCHEAD-false}
   - travis_retry cabal update -v
   - "sed -i.bak 's/^jobs:/-- jobs:/' ${HOME}/.cabal/config"
@@ -89,7 +93,8 @@
   # Overlay Hackage Package Index for GHC HEAD: https://github.com/hvr/head.hackage
   - |
     if $GHCHEAD; then
-      sed -i.bak 's/-- allow-newer:.*/allow-newer: *:base, *:template-haskell, *:ghc, *:Cabal/' ${HOME}/.cabal/config
+      sed -i 's/-- allow-newer: .*/allow-newer: *:base/' ${HOME}/.cabal/config
+      for pkg in $($HCPKG list --simple-output); do pkg=$(echo $pkg | sed 's/-[^-]*$//'); sed -i "s/allow-newer: /allow-newer: *:$pkg, /" ${HOME}/.cabal/config; done
 
       echo 'repository head.hackage'                                                        >> ${HOME}/.cabal/config
       echo '   url: http://head.hackage.haskell.org/'                                       >> ${HOME}/.cabal/config
@@ -99,17 +104,22 @@
       echo '              8f79fd2389ab2967354407ec852cbe73f2e8635793ac446d09461ffb99527f6e' >> ${HOME}/.cabal/config
       echo '   key-threshold: 3'                                                            >> ${HOME}/.cabal.config
 
+      grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
+
       cabal new-update head.hackage -v
     fi
   - grep -Ev -- '^\s*--' ${HOME}/.cabal/config | grep -Ev '^\s*$'
   - "printf 'packages: \".\"\\n' > cabal.project"
-  - cat cabal.project
+  - touch cabal.project.local
+  - "if ! $NOINSTALLEDCONSTRAINTS; then for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/^/constraints: /' | sed 's/-[^-]*$/ installed/' >> cabal.project.local; done; fi"
+  - cat cabal.project || true
+  - cat cabal.project.local || true
   - if [ -f "./configure.ac" ]; then
       (cd "." && autoreconf -i);
     fi
   - rm -f cabal.project.freeze
   - cabal new-build -w ${HC} ${TEST} ${BENCH} --project-file="cabal.project" --dep -j2 all
-  - rm -rf "."/.ghc.environment.* "."/dist
+  - rm -rf .ghc.environment.* "."/dist
   - DISTDIR=$(mktemp -d /tmp/dist-test.XXXX)
 
 # Here starts the actual work to be performed for the package under test;
@@ -121,8 +131,10 @@
   - cd ${DISTDIR} || false
   - find . -maxdepth 1 -name '*.tar.gz' -exec tar -xvf '{}' \;
   - "printf 'packages: keys-*/*.cabal\\n' > cabal.project"
-  - cat cabal.project
-
+  - touch cabal.project.local
+  - "if ! $NOINSTALLEDCONSTRAINTS; then for pkg in $($HCPKG list --simple-output); do echo $pkg | sed 's/^/constraints: /' | sed 's/-[^-]*$/ installed/' >> cabal.project.local; done; fi"
+  - cat cabal.project || true
+  - cat cabal.project.local || true
 
   # build & run tests, build benchmarks
   - cabal new-build -w ${HC} ${TEST} ${BENCH} all
@@ -134,5 +146,5 @@
   - rm -rf ./dist-newstyle
   - if $HADDOCK; then cabal new-haddock -w ${HC} ${TEST} ${BENCH} all; else echo "Skipping haddock generation";fi
 
-# REGENDATA ["-o",".travis.yml","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-installed","cabal.project"]
+# REGENDATA ["-o",".travis.yml","--ghc-head","--irc-channel=irc.freenode.org#haskell-lens","--no-no-tests-no-bench","--no-unconstrained","cabal.project"]
 # EOF
diff --git a/CHANGELOG.markdown b/CHANGELOG.markdown
--- a/CHANGELOG.markdown
+++ b/CHANGELOG.markdown
@@ -1,3 +1,8 @@
+3.12.1 [2018.07.03]
+-------------------
+* Allow building with `containers-0.6`.
+* Avoid the use of deprecated functions from `containers`.
+
 3.12 [2018.01.28]
 -----------------
 * Add instances for data types in `GHC.Generics`. Change the existing instances
@@ -5,7 +10,7 @@
 * Add instances for `Proxy` and `Tagged`.
 * Add instances for `ZipList`.
 * Add `MINIMAL` sets for `Zip` and `FoldableWithKey`.
-* Allow `free-6`.
+* Allow `free-5`.
 
 3.11
 ----
diff --git a/keys.cabal b/keys.cabal
--- a/keys.cabal
+++ b/keys.cabal
@@ -1,6 +1,6 @@
 name:          keys
 category:      Data Structures, Containers
-version:       3.12
+version:       3.12.1
 license:       BSD3
 cabal-version: >= 1.10
 license-file:  LICENSE
@@ -20,7 +20,14 @@
 
 build-type:    Simple
 extra-source-files: .travis.yml CHANGELOG.markdown README.markdown cabal.project
-tested-with:   GHC == 8.4.1, GHC == 8.2.2, GHC == 8.0.2, GHC == 7.10.3, GHC == 7.8.4, GHC == 7.6.3, GHC == 7.4.2
+tested-with:   GHC == 8.6.1
+             , GHC == 8.4.3
+             , GHC == 8.2.2
+             , GHC == 8.0.2
+             , GHC == 7.10.3
+             , GHC == 7.8.4
+             , GHC == 7.6.3
+             , GHC == 7.4.2
 
 source-repository head
   type: git
@@ -34,7 +41,7 @@
     array                >= 0.3.0.2 && < 0.6,
     base                 >= 4.5     && < 5,
     comonad              >= 4       && < 6,
-    containers           >= 0.3     && < 0.6,
+    containers           >= 0.3     && < 0.7,
     free                 >= 4       && < 6,
     hashable             >= 1.1.2.3 && < 1.3,
     semigroupoids        >= 5.2     && < 6,
@@ -44,15 +51,15 @@
     transformers-compat  >= 0.3     && < 1,
     unordered-containers >= 0.2.4   && < 0.3
 
-  if impl(ghc < 7.10)
+  if !impl(ghc >= 7.10)
     build-depends: void >= 0.4 && < 0.8
 
   if impl(ghc < 7.6)
     -- GHC.Generics lived in ghc-prim initially
     build-depends: ghc-prim == 0.2.*
 
-  if impl(ghc < 8)
-    build-depends: base-orphans >= 0.5.4 && < 0.6
+  if !impl(ghc >= 8.0)
+    build-depends: base-orphans >= 0.5.4 && < 0.9
 
   exposed-modules:
     Data.Key
diff --git a/src/Data/Key.hs b/src/Data/Key.hs
--- a/src/Data/Key.hs
+++ b/src/Data/Key.hs
@@ -947,7 +947,11 @@
   mapWithKey = IntMap.mapWithKey
 
 instance FoldableWithKey IntMap where
+#if MIN_VERSION_containers(0,5,0)
+  foldrWithKey = IntMap.foldrWithKey
+#else
   foldrWithKey = IntMap.foldWithKey
+#endif
 
 instance TraversableWithKey IntMap where
   traverseWithKey f = fmap IntMap.fromDistinctAscList . traverse (\(k, v) -> (,) k <$> f k v) . IntMap.toAscList