diff --git a/examples/cpp-libosmium/default.nix b/examples/cpp-libosmium/default.nix
index af2f01c..cbe29bd 100644
--- a/examples/cpp-libosmium/default.nix
+++ b/examples/cpp-libosmium/default.nix
@@ -11,7 +11,7 @@ with pkgs;
 
 stdenv.mkDerivation {
   name = "nix-cpp-demo";
-  nativeBuildInputs = [ cmake pkgconfig ];
+  nativeBuildInputs = [ cmake pkg-config ];
   src = pkgs.lib.cleanSource ./.;
 
   # tell Cmake location of all headers
diff --git a/examples/cpp-libosmium/nix/sources.json b/examples/cpp-libosmium/nix/sources.json
index 83c7eef..6d78290 100644
--- a/examples/cpp-libosmium/nix/sources.json
+++ b/examples/cpp-libosmium/nix/sources.json
@@ -5,34 +5,22 @@
         "homepage": "https://osmcode.org/libosmium/",
         "owner": "osmcode",
         "repo": "libosmium",
-        "rev": "a43ffb61cba34a07c240bcc591d88c9a863e9618",
-        "sha256": "0mlcvqrhp40bzj5r5j9nfc5vbis8hmzcq9xi8jylkciyydaynhz4",
+        "rev": "bf61abd59f3ded669da2bb98fd066ea05ec17c04",
+        "sha256": "0pj29hihlnpz1dnjxm87qlzxzlcvyhkv5zq3x3919i55f2xyjd5s",
         "type": "tarball",
-        "url": "https://github.com/osmcode/libosmium/archive/a43ffb61cba34a07c240bcc591d88c9a863e9618.tar.gz",
-        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
-    },
-    "niv": {
-        "branch": "master",
-        "description": "Easy dependency management for Nix projects",
-        "homepage": "https://github.com/nmattia/niv",
-        "owner": "nmattia",
-        "repo": "niv",
-        "rev": "983e7a049aea1d2033e619368b9a5d6d394d51db",
-        "sha256": "1d3p53q8anl6x9f8wg1wbznq4vwrp2ns044w0gc03drp8nh0dfdm",
-        "type": "tarball",
-        "url": "https://github.com/nmattia/niv/archive/983e7a049aea1d2033e619368b9a5d6d394d51db.tar.gz",
+        "url": "https://github.com/osmcode/libosmium/archive/bf61abd59f3ded669da2bb98fd066ea05ec17c04.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
-        "branch": "nixos-19.09",
+        "branch": "master",
         "description": "Nix Packages collection",
-        "homepage": "https://github.com/NixOS/nixpkgs",
+        "homepage": null,
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "d85e435b7bded2596d7b201bcd938c94d8a921c1",
-        "sha256": "1msjm4kx1z73v444i1iybvmc7z0kfkbn9nzr21rn5yc4ql1jwf99",
+        "rev": "2e4dec050ba7613b98043181d5ca82d1e943ef0a",
+        "sha256": "0bcnx3izyfzl8c0k468fgf3895zkmma9k6kjw0igzlak1k53rjvn",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/d85e435b7bded2596d7b201bcd938c94d8a921c1.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/2e4dec050ba7613b98043181d5ca82d1e943ef0a.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }
diff --git a/examples/cpp-libosmium/nix/sources.nix b/examples/cpp-libosmium/nix/sources.nix
index 6b2b47d..9e7db84 100644
--- a/examples/cpp-libosmium/nix/sources.nix
+++ b/examples/cpp-libosmium/nix/sources.nix
@@ -6,61 +6,91 @@ let
   # The fetchers. fetch_<type> fetches specs of type <type>.
   #
 
-  fetch_file = pkgs: spec:
+  fetch_file = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
     if spec.builtin or true then
-      builtins_fetchurl { inherit (spec) url sha256; }
+      builtins_fetchurl { inherit (spec) url sha256; name = name'; }
     else
-      pkgs.fetchurl { inherit (spec) url sha256; };
+      pkgs.fetchurl { inherit (spec) url sha256; name = name'; };
 
-  fetch_tarball = pkgs: spec:
+  fetch_tarball = pkgs: name: spec:
+    let
+      name' = sanitizeName name + "-src";
+    in
     if spec.builtin or true then
-      builtins_fetchTarball { inherit (spec) url sha256; }
+      builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
     else
-      pkgs.fetchzip { inherit (spec) url sha256; };
-
-  fetch_git = spec:
-    builtins.fetchGit { url = spec.repo; inherit (spec) rev ref; };
-
-  fetch_builtin-tarball = spec:
-    builtins.trace
-      ''
-        WARNING:
-          The niv type "builtin-tarball" will soon be deprecated. You should
-          instead use `builtin = true`.
-
-          $ niv modify <package> -a type=tarball -a builtin=true
-      ''
-      builtins_fetchTarball
-      { inherit (spec) url sha256; };
-
-  fetch_builtin-url = spec:
-    builtins.trace
-      ''
-        WARNING:
-          The niv type "builtin-url" will soon be deprecated. You should
-          instead use `builtin = true`.
-
-          $ niv modify <package> -a type=file -a builtin=true
-      ''
-      (builtins_fetchurl { inherit (spec) url sha256; });
+      pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
+
+  fetch_git = name: spec:
+    let
+      ref =
+        spec.ref or (
+          if spec ? branch then "refs/heads/${spec.branch}" else
+          if spec ? tag then "refs/tags/${spec.tag}" else
+          abort "In git source '${name}': Please specify `ref`, `tag` or `branch`!"
+        );
+      submodules = spec.submodules or false;
+      submoduleArg =
+        let
+          nixSupportsSubmodules = builtins.compareVersions builtins.nixVersion "2.4" >= 0;
+          emptyArgWithWarning =
+            if submodules
+            then
+              builtins.trace
+                (
+                  "The niv input \"${name}\" uses submodules "
+                  + "but your nix's (${builtins.nixVersion}) builtins.fetchGit "
+                  + "does not support them"
+                )
+                { }
+            else { };
+        in
+        if nixSupportsSubmodules
+        then { inherit submodules; }
+        else emptyArgWithWarning;
+    in
+    builtins.fetchGit
+      ({ url = spec.repo; inherit (spec) rev; inherit ref; } // submoduleArg);
+
+  fetch_local = spec: spec.path;
+
+  fetch_builtin-tarball = name: throw
+    ''[${name}] The niv type "builtin-tarball" is deprecated. You should instead use `builtin = true`.
+        $ niv modify ${name} -a type=tarball -a builtin=true'';
+
+  fetch_builtin-url = name: throw
+    ''[${name}] The niv type "builtin-url" will soon be deprecated. You should instead use `builtin = true`.
+        $ niv modify ${name} -a type=file -a builtin=true'';
 
   #
   # Various helpers
   #
 
-  # The set of packages used when specs are fetched using non-builtins.
-  mkPkgs = sources:
-    if hasNixpkgsPath
-    then
-      if hasThisAsNixpkgsPath
-      then import (builtins_fetchTarball { inherit (mkNixpkgs sources) url sha256; }) { }
-      else import <nixpkgs> { }
-    else
-      import (builtins_fetchTarball { inherit (mkNixpkgs sources) url sha256; }) { };
+  # https://github.com/NixOS/nixpkgs/pull/83241/files#diff-c6f540a4f3bfa4b0e8b6bafd4cd54e8bR695
+  sanitizeName = name:
+    (
+      concatMapStrings (s: if builtins.isList s then "-" else s)
+        (
+          builtins.split "[^[:alnum:]+._?=-]+"
+            ((x: builtins.elemAt (builtins.match "\\.*(.*)" x) 0) name)
+        )
+    );
 
-  mkNixpkgs = sources:
+  # The set of packages used when specs are fetched using non-builtins.
+  mkPkgs = sources: system:
+    let
+      sourcesNixpkgs =
+        import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) { inherit system; };
+      hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
+      hasThisAsNixpkgsPath = <nixpkgs> == ./.;
+    in
     if builtins.hasAttr "nixpkgs" sources
-    then sources.nixpkgs
+    then sourcesNixpkgs
+    else if hasNixpkgsPath && ! hasThisAsNixpkgsPath then
+      import <nixpkgs> { }
     else
       abort
         ''
@@ -68,23 +98,32 @@ let
           add a package called "nixpkgs" to your sources.json.
         '';
 
-  hasNixpkgsPath = (builtins.tryEval <nixpkgs>).success;
-  hasThisAsNixpkgsPath =
-    (builtins.tryEval <nixpkgs>).success && <nixpkgs> == ./.;
-
   # The actual fetching function.
   fetch = pkgs: name: spec:
 
     if ! builtins.hasAttr "type" spec then
       abort "ERROR: niv spec ${name} does not have a 'type' attribute"
-    else if spec.type == "file" then fetch_file pkgs spec
-    else if spec.type == "tarball" then fetch_tarball pkgs spec
-    else if spec.type == "git" then fetch_git spec
-    else if spec.type == "builtin-tarball" then fetch_builtin-tarball spec
-    else if spec.type == "builtin-url" then fetch_builtin-url spec
+    else if spec.type == "file" then fetch_file pkgs name spec
+    else if spec.type == "tarball" then fetch_tarball pkgs name spec
+    else if spec.type == "git" then fetch_git name spec
+    else if spec.type == "local" then fetch_local spec
+    else if spec.type == "builtin-tarball" then fetch_builtin-tarball name
+    else if spec.type == "builtin-url" then fetch_builtin-url name
     else
       abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
 
+  # If the environment variable NIV_OVERRIDE_${name} is set, then use
+  # the path directly as opposed to the fetched source.
+  replace = name: drv:
+    let
+      saneName = stringAsChars (c: if (builtins.match "[a-zA-Z0-9]" c) == null then "_" else c) name;
+      ersatz = builtins.getEnv "NIV_OVERRIDE_${saneName}";
+    in
+    if ersatz == "" then drv else
+      # this turns the string into an actual Nix path (for both absolute and
+      # relative paths)
+    if builtins.substring 0 1 ersatz == "/" then /. + ersatz else /. + builtins.getEnv "PWD" + "/${ersatz}";
+
   # Ports of functions for older nix versions
 
   # a Nix version of mapAttrs if the built-in doesn't exist
@@ -93,23 +132,37 @@ let
     listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
   );
 
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/lists.nix#L295
+  range = first: last: if first > last then [ ] else builtins.genList (n: first + n) (last - first + 1);
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L257
+  stringToCharacters = s: map (p: builtins.substring p 1 s) (range 0 (builtins.stringLength s - 1));
+
+  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L269
+  stringAsChars = f: s: concatStrings (map f (stringToCharacters s));
+  concatMapStrings = f: list: concatStrings (map f list);
+  concatStrings = builtins.concatStringsSep "";
+
+  # https://github.com/NixOS/nixpkgs/blob/8a9f58a375c401b96da862d969f66429def1d118/lib/attrsets.nix#L331
+  optionalAttrs = cond: as: if cond then as else { };
+
   # fetchTarball version that is compatible between all the versions of Nix
-  builtins_fetchTarball = { url, sha256 }@attrs:
+  builtins_fetchTarball = { url, name ? null, sha256 }@attrs:
     let
       inherit (builtins) lessThan nixVersion fetchTarball;
     in
     if lessThan nixVersion "1.12" then
-      fetchTarball { inherit url; }
+      fetchTarball ({ inherit url; } // (optionalAttrs (name != null) { inherit name; }))
     else
       fetchTarball attrs;
 
   # fetchurl version that is compatible between all the versions of Nix
-  builtins_fetchurl = { url, sha256 }@attrs:
+  builtins_fetchurl = { url, name ? null, sha256 }@attrs:
     let
       inherit (builtins) lessThan nixVersion fetchurl;
     in
     if lessThan nixVersion "1.12" then
-      fetchurl { inherit url; }
+      fetchurl ({ inherit url; } // (optionalAttrs (name != null) { inherit name; }))
     else
       fetchurl attrs;
 
@@ -123,15 +176,16 @@ let
             abort
               "The values in sources.json should not have an 'outPath' attribute"
           else
-            spec // { outPath = fetch config.pkgs name spec; }
+            spec // { outPath = replace name (fetch config.pkgs name spec); }
       )
       config.sources;
 
   # The "config" used by the fetchers
   mkConfig =
-    { sourcesFile ? ./sources.json
-    , sources ? builtins.fromJSON (builtins.readFile sourcesFile)
-    , pkgs ? mkPkgs sources
+    { sourcesFile ? if builtins.pathExists ./sources.json then ./sources.json else null
+    , sources ? if sourcesFile == null then { } else builtins.fromJSON (builtins.readFile sourcesFile)
+    , system ? builtins.currentSystem
+    , pkgs ? mkPkgs sources system
     }: {
       # The sources, i.e. the attribute set of spec name to spec
       inherit sources;
@@ -139,5 +193,6 @@ let
       # The "pkgs" (evaluated nixpkgs) to use for e.g. non-builtin fetchers
       inherit pkgs;
     };
+
 in
 mkSources (mkConfig { }) // { __functor = _: settings: mkSources (mkConfig settings); }
diff --git a/examples/cpp-libosmium/overlay/libosmium.nix b/examples/cpp-libosmium/overlay/libosmium.nix
index 261f808..9430b2a 100644
--- a/examples/cpp-libosmium/overlay/libosmium.nix
+++ b/examples/cpp-libosmium/overlay/libosmium.nix
@@ -1,4 +1,4 @@
-{ stdenv, cmake, protozero, expat, zlib, bzip2, boost, sources }:
+{ stdenv, cmake, protozero, expat, zlib, lz4, bzip2, boost, sources }:
 
 stdenv.mkDerivation {
   name = "libosmium";
@@ -16,6 +16,7 @@ stdenv.mkDerivation {
     protozero
     expat
     zlib
+    lz4
     bzip2
     boost
   ];