Be mindful about the default arguments passed to nixpkgs in nix-shell

tests/nix-shell.sh

@@ -52,6 +52,18 @@ output=$(NIX_PATH=nixpkgs="$shellDotNix" nix-shell --pure -p foo bar --run 'echo
 output=$(NIX_PATH=nixpkgs="$shellDotNix" nix-shell --pure -p foo --argstr fooContents baz --run 'echo "$(foo)"')
 [ "$output" = "baz" ]
 
+# Test that we are compatible with an old nixpkgs that doesn't accept the
+# inNixShell attribute
+output=$(NIX_PATH=nixpkgs="$PWD/shell-no-in-nix-shell.nix" nix-shell --pure -p foo)
+[ "$output" = "noInNixShell" ]
+
+# Test that passing arguments that aren't part of the formals (of the outer
+# shell expression) are still passed into the expression while *not* passing
+# inNixShell
+output=$(NIX_PATH=nixpkgs="$PWD/shell-no-in-nix-shell.nix" nix-shell --pure -p foo --argstr fooContents zes)
+[ "$output" = "zes" ]
+
+
 # Test nix-shell shebang mode
 sed -e "s|@ENV_PROG@|$(type -P env)|" shell.shebang.sh > $TEST_ROOT/shell.shebang.sh
 chmod a+rx $TEST_ROOT/shell.shebang.sh

tests/shell-no-in-nix-shell.nix

@@ -0,0 +1,3 @@
+# This tries to replicate old nixpkgs behavior where `inNixShell` wasn't a supported attribute
+args@{ ... }:
+({ fooContents ? "noInNixShell" }: import ./shell.sh { inherit fooContents; }) args

tests/shell.nix

@@ -1,6 +1,12 @@
-{ inNixShell ? false, contentAddressed ? false, fooContents ? "foo" }:
-
-let cfg = import ./config.nix; in
+# This file should resemble the function signature of Nixpkgs as it is used to
+# simulate nixpkgs within the nix-shell tests.
+{ ... }@args:
+let
+  cfg = import ./config.nix;
+  inNixShell = args.inNixShell or false;
+  contentAddressed = args.contentAddressed or false;
+  fooContents = args.fooContents or "foo";
+in
 with cfg;
 
 let