Optional discriminated union's callback parameter type is not inferred

[wzono]

Bug Report

Optional discriminated union's callback parameter type is not inferred

🔎 Search Terms

#35769

🕗 Version & Regression Information

typescript@4.4.4

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.5.0-beta#code/C4TwDgpgBAglC8UDeAoK6qkgfgFxQHIYCAaNDAQ3wAoBjfJKAE3wDsBXAWwCMIAnKAF8AlAgB8UAG4B7AJZMA3CkEoUWaACEEycunX4CG0rqhUodBszZdefElHpQAzsD6zWAcyGj4EmfKUVVXVYDQBhbTgAHygtJRRJCgEzGHCAbQBdKBiAJQhaaT4mAB4XN097VLDMiUQ0k1QMJtMaMCSKTicfCUbmvpMmlUGUDKA

💻 Code

type A = {
    type?: 'A',
    a: (c: { d: number }) => void;
}

type B = {
    type: 'B',
    a: (c: { d: number, c: string }) => void;
}


type ABC = A | B ;

var a: ABC[] | Record<string, ABC[]> = [
    {
        a: (params) => {
            
        }
    }
]

🙁 Actual behavior

Parameters of method 'a' can not be inferred correctly.

🙂 Expected behavior

Parameters of method 'a 'can be inferred correctly: {d: number}

[RyanCavanaugh]

Behavior dates back to at least 3.3.3. It looks like our discriminant detection is getting a little confused by the Record part of the union.

[wzono]

@RyanCavanaugh Any progress?

[RyanCavanaugh]

@wingsico no

[Eldemarkki]

Any updates?

I'm not sure if this Playground link is the same problem as this, but if true, then still exists in 5.1.6: https://www.typescriptlang.org/play?ts=5.1.6#code/C4TwDgpgBAEghgOwCYBsICcAKd1wLYTAYDOUAvFAN4BQUdUAjgK4YgBqcKAlknMAPboAXFAAUASnIA+KACN+-NIgDctegAtEqDCNHNWIylB4jiwdFwQBzKAF9JZGQDd+PVbagAfKmrr707Jw8fIIA-CJMyBAAZpYQSKr0UJrIaMJi-iARUbEI8Q7OrgnUtqrUAMb8CGZQYDj4pBQ0SZkc3LwC6RLSUOYsADS+yVppupkFPklJldWKEAB0KPxWeiwB4kO2tB7EfFzEsRCk8KkY2LgEROjEqkA