F# 10.0.400 breaks Aether optics — confirmed upstream compiler bug (dotnet/fsharp#20253)

[7sharp9]

Follow-up to #447.

Root cause confirmed

Not our code, not Aether-specific. Upstream F# compiler regression, filed independently the same day: dotnet/fsharp#20253, "SDK 10.0.400 regression: cross-assembly inline SRTP call throws in Release." Identical repro: Aether 8.3.1, Dynamic invocation of op_HatEquals is not supported at Aether.Optic.set. Broken on 10.0.400, works on 10.0.303. Open, Backlog, unassigned, no fix PR yet.

What we additionally verified locally

• Not limited to Optic.set/^=. Optic.get/^. and lens composition >-> hit the same failure mode (repro'd in GenCORE.Lib/MinMax.fs and GenCORE.Lib/Patient.fs under 10.0.400).
• Eta-expanding call sites does not fix it, the compiler still emits an internal curried closure that fails.
• Bypassing Aether's dispatch and calling the Lens/Prism tuple's functions directly (fst/snd) does avoid it, verified against 10.0.400.

Current mitigation

PR #449 pins global.json to 10.0.302/latestPatch. Sufficient until upstream fixes #20253.

Optional follow-up (not urgent while the pin holds)

Replace Aether with a small hand-rolled, non-generic Lens/Prism module (get/set/compose as plain functions) across the affected files: MinMax.fs, GenCORE.Lib/Patient.fs, GenORDER.Lib/Patient.fs, ZForm.Lib/DoseRule.fs, PatientCategory.fs. To remove the dependency and this whole class of SRTP fragility. Track dotnet/fsharp#20253 before considering unpinning the SDK.

[halcwb]

I think we should even consider if and how we should use Aether. This is one of those places where I thought, this looks cool, but didn't really think over the broader consequences and consistent usage. This also was before the F# nested record fields accessors: https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/copy-and-update-record-expressions.

[7sharp9]

We can patch this pretty easy for now by replacing:

        let getMin = Optic.get min_
        let setMin = Optic.set min_

With

        let setMin v mm = (snd min_) v mm
        let getMin mm = (fst min_) mm

Or going further, a tiny optics lib that doesn't use fragile SRTP.

Or just factor out optic style and just use nested field accessors.

[halcwb]

We can patch this pretty easy for now by replacing:

        let getMin = Optic.get min_
        let setMin = Optic.set min_

With

        let setMin v mm = (snd min_) v mm
        let getMin mm = (fst min_) mm

Or going further, a tiny optics lib that doesn't use fragile SRTP.

Or just factor out optic style and just use nested field accessors.

I agree, this could be a typical case of YAGNI.

That said. The ZForm lib makes quite extensive use of optics. But this is a typical case where AI could do a lot of the heavy lifting.

[7sharp9]

This is one of those Haskelisms that creep in where it looks like a good idea then you later just go with the simple option. Sometimes boilerplate is good. The awful use of SRTP etc lead me to build Myriad (Mryiad lenses) so no one has to put themselves through composing those monstrosities. I think people choose yo use optic's libs rather than gen them with Myriad though, sometime people like to shoot themselves through the foot for fun for a while though :-p .

[halcwb]

This is one of those Haskelisms that creep in where it looks like a good idea then you later just go with the simple option. Sometimes boilerplate is good. The awful use of SRTP etc lead me to build Myriad (Mryiad lenses) so no one has to put themselves through composing those monstrosities. I think people choose yo use optic's libs rather than gen them with Myriad though, sometime people like to shoot themselves through the foot for fun for a while though :-p .

I think replacing this with Myriad lenses would be great! Again using AI this should be easy. Do you want to hand this over to me or would you rather want this resolved by yourself?

[7sharp9]

@halcwb I mean I'm flattered that you might want to use Myriad, but it is an additional moving part, it could automate other parts of boiler plate in GenPRES too though.