Without the change the eval fails as:
$ nix build --no-link -f. defaultPkgConfigPackages.libsystemd.tests
error:
… while evaluating the attribute 'systemd-timesyncd'
… in the condition of the assert statement
at pkgs/os-specific/linux/systemd/default.nix:1023:11:
1022| name: value:
1023| assert lib.assertMsg (lib.isDerivation value) "${name} is not a derivation";
| ^
1024| value
… in the left operand of the OR (||) operator
at /home/slyfox/dev/git/nixpkgs-master/lib/asserts.nix:50:31:
49| # TODO(Profpatsch): add tests that check stderr
50| assertMsg = pred: msg: pred || throw msg;
| ^
51|
error: attribute 'systemd-timesyncd' missing
at pkgs/os-specific/linux/systemd/default.nix:1010:13:
1009| systemd-sysusers-password-option-override-ordering
1010| systemd-timesyncd
| ^
1011| systemd-timesyncd-nscd-dnssec
75114216e2