From 787cc66cd15159ab80b3a1b7644aa86cd3467bff Mon Sep 17 00:00:00 2001
From: Scott Worley <scottworley@scottworley.com>
Date: Mon, 23 Aug 2021 12:51:27 -0700
Subject: [PATCH 1/1] Make --arg work

---
 default.nix | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/default.nix b/default.nix
index 95c7a7e..15034d1 100644
--- a/default.nix
+++ b/default.nix
@@ -1,4 +1,33 @@
-args@{ ... }:
-let actualNixpkgs = import <nixpkgs-warned> args;
-in actualNixpkgs.lib.warn
-("Using legacy channel nixpkgs from " + <nixpkgs-warned>) actualNixpkgs
+let
+  # We declare some parameters explicitly because nix's --arg command line
+  # flag requires this.  Getting the list of parameters & constructing this
+  # function dynamically would require import-from-derivation, which is not
+  # permitted in some evaluation contexts, so we can't do that.  Instead, we
+  # duplicate nixpkgs' list of explicit parameters here, with a check in case
+  # it drifts out of sync.
+  unused = builtins.throw "These default values are not used";
+  toplevel = args@{ localSystem ? unused, system ? unused, platform ? unused
+    , crossSystem ? unused, config ? unused, overlays ? unused
+    , crossOverlays ? unused, ... }:
+    let
+
+      actualNixpkgs = import <nixpkgs-warned> args;
+      noisyNixpkgs = actualNixpkgs.lib.warn
+        ("Using legacy channel nixpkgs from " + <nixpkgs-warned>) actualNixpkgs;
+
+      local-parameters = builtins.attrNames (builtins.functionArgs toplevel);
+      remote-parameters =
+        builtins.attrNames (builtins.functionArgs (import <nixpkgs-warned>));
+      missing-parameters =
+        actualNixpkgs.lib.subtractLists local-parameters remote-parameters;
+      # Don't warn about extra parameters to avoid spurious warnings as everyone
+      # rolls past nixpkgs commit 2dde58903e0f2f490088c3b0cedadc9b479da085
+      # which removed the platform parameter.
+      # extra-parameters =
+      #   actualNixpkgs.lib.subtractLists remote-parameters local-parameters;
+    in actualNixpkgs.lib.traceIf (missing-parameters != [ ])
+    ("Note: warn-nixpkgs doesn't know about these nixpkgs parameters, warned"
+      + "so they might not work with --arg on the nix command line: "
+      + actualNixpkgs.lib.concatStringsSep " " missing-parameters) noisyNixpkgs;
+
+in toplevel
-- 
2.44.1