Change:
# opentoonz requires its own modified version of libtiff.  We still build it as # a separate package #  1. For visibility for tools like vulnix, and <del> #  2. To avoid a diamond-dependency problem with qt linking the normal libtiff #     and opentoonz linking qt and this modified libtiff, we build a qt against #     this modified libtiff as well. </del> <ins> #  2. To avoid a diamond-dependency problem with opencv linking the normal libtiff #     and opentoonz linking opencv and this modified libtiff, we build an opencv #     against this modified libtiff as well. # # We use a separate mkDerivation rather than a minimal libtiff.overrideAttrs # because the main libtiff builds with cmake and this version of libtiff was # forked before libtiff gained CMake build capability (added in libtiff-4.0.5). { lib, fetchFromGitHub, stdenv, pkg-config, zlib, libjpeg, xz, libtiff, }: </ins> <del> { fetchFromGitHub, libtiff }: </del> let source = import ./source.nix { inherit fetchFromGitHub; }; <del> in libtiff.overrideAttrs (old: { inherit (source) src; </del> <ins> in stdenv.mkDerivation { pname = "libtiff"; </ins> version = source.versions.libtiff + "-opentoonz"; <del> postUnpack = (old.postUnpack or "") + '' </del> <ins> inherit (source) src; outputs = [ "bin" "dev" "out" "man" "doc" ]; nativeBuildInputs = [ pkg-config ]; propagatedBuildInputs = [ zlib libjpeg xz ]; postUnpack = '' </ins> sourceRoot="$sourceRoot/thirdparty/tiff-${source.versions.libtiff}" ''; # opentoonz uses internal libtiff headers <del> postInstall = (old.postInstall or "") + '' </del> <ins> postInstall = '' </ins> cp libtiff/{tif_config,tif_dir,tiffiop}.h $dev/include ''; <del> }) </del> <ins> meta = libtiff.meta // { knownVulnerabilities = ['' Do not open untrusted files with Opentoonz: Opentoonz uses an old custom fork of tibtiff from 2012 that is known to be affected by at least these 50 vulnerabilities: CVE-2012-4564 CVE-2013-4232 CVE-2013-4243 CVE-2013-4244 CVE-2014-8127 CVE-2014-8128 CVE-2014-8129 CVE-2014-8130 CVE-2014-9330 CVE-2015-1547 CVE-2015-8781 CVE-2015-8782 CVE-2015-8783 CVE-2015-8784 CVE-2015-8870 CVE-2016-3620 CVE-2016-3621 CVE-2016-3623 CVE-2016-3624 CVE-2016-3625 CVE-2016-3631 CVE-2016-3632 CVE-2016-3633 CVE-2016-3634 CVE-2016-3658 CVE-2016-3945 CVE-2016-3990 CVE-2016-3991 CVE-2016-5102 CVE-2016-5314 CVE-2016-5315 CVE-2016-5316 CVE-2016-5318 CVE-2016-5319 CVE-2016-5321 CVE-2016-5322 CVE-2016-5323 CVE-2016-6223 CVE-2016-9453 CVE-2016-9532 CVE-2017-9935 CVE-2017-9937 CVE-2018-10963 CVE-2018-5360 CVE-2019-14973 CVE-2019-17546 CVE-2020-35521 CVE-2020-35522 CVE-2020-35523 CVE-2020-35524 More info at https://github.com/opentoonz/opentoonz/issues/4193 '']; maintainers = with lib.maintainers; [ chkno ]; }; } </ins>

Discussion:
Thought I'd try out Opentoonz because it looks cool, however I can't get the derivation for libtiff-4.0.3-opentoonz (patched version of libtiff for opentoonz) to build. Steps to reproduce the behavior: on NixOS 21.05 -4.0.3-opentoonz derivation fails to build to successfully evaluate and drop me to a shell prompt from whence I can run and try out Opentoonz. This is the relevant output from :
the build of opentoonz is also failing on master but for a different reason