Match no longer compatible with identity

[thewilkybarkid]

What version of Effect is running?

3.13.6

What steps can reproduce the bug?

import { identity, Match, pipe } from "effect"

const withIdentity: (u: "foo" | "bar") => "foo" | "baz" = pipe(
  Match.type<"foo" | "bar">(),
  Match.when("foo", identity),
  Match.when("bar", () => "baz" as const),
  Match.exhaustive
)

const withoutIdentity: (u: "foo" | "bar") => "foo" | "baz" = pipe(
  Match.type<"foo" | "bar">(),
  Match.when("foo", (value) => value),
  Match.when("bar", () => "baz" as const),
  Match.exhaustive
)

https://effect.website/play#8b9250181318

What is the expected behavior?

Both are assignable.

What do you see instead?

withIdentity has a type error:

Type '(u: "foo" | "bar") => unknown' is not assignable to type '(u: "foo" | "bar") => "foo" | "baz"'.

Additional information

PREreview/prereview.org#2279 is failing, as some of our matchers now have an inferred return type of unknown (e.g. https://github.com/PREreview/prereview.org/blob/59804ce6fc1f444f0fa4ed579a94d01af6d1c6d1/src/Comments/Evolve.ts#L5-L17)

Could be related to #4530

[tim-smart]

That PR didn't touch the .when signature, so I think the cause must be something else 🤔

[thewilkybarkid]

I spent some time trying to debug, but I couldn't isolate the change that caused the failure.

It's working fine with 3.13.4 but starts failing with 3.13.5.

[gcanti]

@thewilkybarkid I get the same error with 3.13.4

[thewilkybarkid]

Looking at it closer, my reproduction isn't right: when using Match.tag it works in 3.13.4 but fails after. When using Match.when, it never seems to have worked...

[thewilkybarkid]

Works in 3.13.4, but not newer versions:

import { identity, Match, pipe } from "effect"

type Foo = { _tag: 'foo' }
type Bar = { _tag: 'bar' }

const withIdentity: (u: Foo | Bar) => Foo | "baz" = pipe(
  Match.type<Foo | Bar>(),
  Match.tag("foo", identity),
  Match.tag("bar", () => "baz" as const),
  Match.exhaustive
)

const withoutIdentity: (u: Foo | Bar) => Foo | "baz" = pipe(
  Match.type<Foo | Bar>(),
  Match.tag("foo", (value) => value),
  Match.tag("bar", () => "baz" as const),
  Match.exhaustive
)

https://effect.website/play#2ddf17cdcbe1