Mysterious type inference changes break safe stack apps

[forki]

A recent change in type inference (probably related to SRTPs) breaks all safe stack apps that use task {}

Original report: SAFE-Stack/SAFE-BookStore#283

Repro steps

I tried to create a repro here see #4285 but that did not work. But repro is working on bookstore SAFE-Stack/SAFE-BookStore#293

Expected behavior

No further type hints needed

Actual behavior

We need a lot of new type annotations to make it compile

Known workarounds

Adding type hints

[realvictorprm]

I wasn't aware of such a bug. Sounds worse I hope we know soon what the cause is.

[dsyme]

@forki To repro I should use https://github.com/SAFE-Stack/SAFE-BookStore/tree/forki-patch-3?

[forki]

yes. sorry I don't have better repro yet.

[dsyme]

I'll take a look. I haven't yet verified the issue, but noting #4173 as a possible cause

[dsyme]

@forki Is this the same issue as giraffe-fsharp/Giraffe#222 (comment)?

[forki]

no that one was different and is solved by opening correct namespace. forki-patch-3 already has all namespaces correctly opened and would work against .net cli 2.1.3 - only the update to 2.1.300-preview1-008009 makes it break.

you should be able to repro by running "build.cmd run" on that branch

[dsyme]

@forki I've confirmed that #4173 is indeed the cause of this change in behaviour.

This is difficult, as that PR was itself a reversion of a change which @gusty reports as causing a regression. We thought that the change was innocuous but it seems the kind of inline/SRTP/object-programming code in TaskBuilder.fs has picked up a dependency on the interim behaviour.

By far the best interim solution would be to simplify TaskBuilder.fs so it doesn't depend on this kind of subtlety and works with or without the change in #4173

[gusty]

I think if this really caused a regression then trying the code in F# previous to 4.1 should also fail.
Is there a minimal repro snippet?

[forki]

I think @rspeele and @gerardtoconnor are the experts when it comes to the task { } - I'm not sure if / how to rewrite it.