Can't eta-reduce with flakes: expected a function but got a thunk

[lovesegfault]

Describe the bug

I have a flake.nix file, in which I moved the outputs into their own output.nix file.

If I try to eta-reduce the outputs = args: import ./output.nix args; call to simply be outputs = import output.nix; I get the following error:

❯ nix flake check
warning: Git tree '/home/bemeurer/src/nix-config' is dirty
error: --- Error ----------------------------------------------------------------------------------------------------------------------------------------- nix
expected a function but got a thunk at /nix/store/h21gfgrkjkhwl0f1ri2jhaqsdssrzm86-source/flake.nix:29:3

Steps To Reproduce

1. mkdir flake-bug && cd flake-bug
2. echo "{ inputs = { }; outputs = import ./outputs.nix; }" > flake.nix
3. echo "{ ... }: { }"> outputs.nix
4. nix flake check
5. See error

Expected behavior

I expected the thunk to be forced and understood by nix to be a function.

nix-env --version output
nix-env (Nix) 2.4pre20201201_5a6ddb3

[andir]

Probably a duplicate of #3966 ?

[edolstra]

Right, it's not allowed to have any evaluation to get the outputs function. This is because the arguments to outputs can specify flake inputs (e.g. { nixpkgs }: ...).