0test.nix

with import <nixpkgs> {};
with stdenv;

let
  a = runCommand "a" {} "mkdir -p $out; touch $out/a";
  b = runCommand "b" {} "mkdir -p $out; touch $out/b";
  c = symlinkJoin { name = "c"; paths = [ a b ]; };
in
runCommand "z" { input = c; } ''                                                                                                                                                                            
  echo "All inputs:"                                                                                                                                                                                        
  ls -l $input                                                                                                                                                                                              
''

1error.txt

$ nix-build --show-trace 0test.nix
error: while evaluating the attribute ‘input’ of the derivation ‘z’ at /nix/store/mqk7i2wcx521gacigcadsiq5i4lm1rvr-nixos-16.03.1349.dda40aa8d1/nixos/pkgs/build-support/trivial-builders.nix:10:14:
cannot coerce a function to a string, at /nix/store/mqk7i2wcx521gacigcadsiq5i4lm1rvr-nixos-16.03.1349.dda40aa8d1/nixos/pkgs/build-support/trivial-builders.nix:10:14