So there is more than one way to skin this cat. ‘nixpkgs’ as handed to you as part of a flakes ‘inputs’ is (jargon alert) partially applied, which is to say you

So there is more than one way to skin this cat. ‘nixpkgs’ as handed to you as part of a flakes ‘inputs’ is (jargon alert) partially applied, which is to say you have to put a battery in it. In this instance what you have to pass is ‘system’ because nixpkgs supports many systems (e.g. ‘linux-x86_64’ or ‘darwin-aarach64’). It sort of makes sense that it wouldn’t know what GCC you want without that information. You can do this up top by hand, e.g. P = inputs.nixpkgs { inherit system; }, but more commonly you would use a library like ‘flake-utils’ or ‘flake-utils-plus’ to issue system configuration for all supported packages to all your packages/derivations/devShells.