so we may want to fix instances of nullOr being merged before merging this.

Interesting. Is there a test I can run that covers that? By "merged" do you mean the // { description, check } update?

Feels like that's related to the wider "addCheck is broken" issues exposed by v2 check+merge.
There is (Report error for unsupported t // { check = ...; }) that tries to warn when check is shadowed on a type that supports v2 check+merge, though. Did the warning not trigger on your example?

so we may want to fix instances of nullOr being merged before merging this.

Interesting. Is there a test I can run that covers that? By "merged" do you mean the // { description, check } update?

Feels like that's related to the wider "addCheck is broken" issues exposed by v2 check+merge.
There is report error for unsupported t // { check = ...; } that tries to warn when check is shadowed on a type that supports v2 check+merge, though. Did the warning not trigger on your example?

Yeah, I agree on hot-code. But on "normal" code, I tend to favor legibility over mirco-optimisation.

Based on roberth's comment on your linked issue I'll keep your version, though:

I support removing these calls from lib and other hot paths.

oh, i should've sent the error - yeah that's the error that triggered

 error: The option `services.displayManager.defaultSession' has a type `session name' that uses
       an ad-hoc `type // { check = ...; }' override, which is incompatible with
       the v2 merge mechanism.

       Please use `lib.types.addCheck` instead of `type // { check }' to add
       custom validation. For example:

         lib.types.addCheck baseType (value: /* your check */)

       instead of:

         baseType // { check = value: /* your check */; }

       Alternatively, this message may also occur as false positive when mixing Nixpkgs
       versions, if one Nixpkgs is between 83fed2e6..58696117 (Aug 28 - Oct 28 2025)

is there any reason why this check would pass?

nix-repl> (lib.types.listOf (lib.types.enum [])).check [ "hello" ]
true

(this is a minimal example but the original type is also just an enum, and the merging doesn't throw any errors at all)

i found this issue but i don't think that's related since it seems to be submodule- and oneOf-specific?

is there any reason why this check would pass?

nix-repl> (lib.types.listOf (lib.types.enum [])).check [ "hello" ]
true

(this is a minimal example but the original type is also just an enum, and the merging doesn't throw any errors at all)

(also sorry if this isn't the place to ask noobish question like this, it seems like mostly technical discussion ;-;)
i found this issue but i don't think that's related since it seems to be submodule- and oneOf-specific?

is there any reason why this check would pass?

nix-repl> (lib.types.listOf (lib.types.enum [])).check [ "hello" ]
true

(this is a minimal example but the original type is also just an enum, and the merging doesn't throw any errors at all)

i found this issue but i don't think that's related since it seems to be submodule- and oneOf-specific?

(also sorry if this isn't the place to ask noobish question like this, it seems like mostly technical discussion ;-;)

types.enum's check is literally x: elem x values, so (lib.types.enum []).check "hello" should fail.

types.listOf's check just uses isList, without also checking all elemType.check. This laziness was introduced in https://github.com/NixOS/nixpkgs/pull/6794

types.enum's check is literally x: elem x values, so (lib.types.enum []).check "hello" should fail.

types.listOf's check just uses isList, without also checking all elemType.check. This laziness was introduced in https://github.com/NixOS/nixpkgs/pull/6794
(specifically https://github.com/NixOS/nixpkgs/commit/e4bc2592f3c5fa2f05484e7258f99ebb0507d304)

types.enum's check is literally x: elem x values, so (lib.types.enum []).check "hello" should fail.

types.listOf's check just uses isList, without also checking all elemType.check.
This laziness was introduced in https://github.com/NixOS/nixpkgs/pull/6794
(specifically https://github.com/NixOS/nixpkgs/commit/e4bc2592f3c5fa2f05484e7258f99ebb0507d304)