From 31eac6fd0088710c193efebb632f1c97ef3045a6 Mon Sep 17 00:00:00 2001
From: Matthew Bauer <mjbauer95@gmail.com>
Date: Sun, 24 Jun 2018 23:15:09 -0400
Subject: [PATCH] impure.nix: fix handling of localSystem
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If we passed a localSystem in, we don’t want the current system to
override it. Now we check for localSystem first to avoid getting
"mixed" localSystem values from commands like this:

  nix-build --arg localSystem '{config="x86_64-unknown-linux-musl";}' -A hello

Which would eventually evaluate localSystem to this:

{
  config = "x86_64-unknown-linux-musl";
  system = "x86_64-darwin";
}

& Nix would not be able to run it correctly.

Fixes #41599

/cc @Ericson2314
---
 pkgs/top-level/impure.nix | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pkgs/top-level/impure.nix b/pkgs/top-level/impure.nix
index df462665dd1..dafa351c4e4 100644
--- a/pkgs/top-level/impure.nix
+++ b/pkgs/top-level/impure.nix
@@ -83,5 +83,6 @@ import ./. (builtins.removeAttrs args [ "system" "platform" ] // {
   inherit config overlays crossSystem;
   # Fallback: Assume we are building packages on the current (build, in GNU
   # Autotools parlance) system.
-  localSystem = { system = builtins.currentSystem; } // localSystem;
+  localSystem = (if args ? localSystem then {}
+                 else { system = builtins.currentSystem; }) // localSystem;
 })