Type assertion does not creates a type union anymore

[drtex0]

Bug Report

Since version 5.1.X I am facing this issue with type assertion. In version 5.0.X the type resolves with an Union of the original type & the assertion type. However in 5.1.X there is only the original type left

🔎 Search Terms

🕗 Version & Regression Information

• This is a crash
• This changed between versions 5.0.1 and 5.1.3
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type assertion
• I was unable to test this on prior versions because _______

⏯ Playground Link

Playground link with relevant code

💻 Code

import { mapValues } from 'lodash';

type Dinero  = {
  mul: () => number
}

const isDinero = (obj: unknown): obj is Dinero => 
  typeof obj === "object" && obj !== null && Object.hasOwn(obj, "mul")

const transformValues = <Contract extends object>(contract : Contract)  => {
   mapValues(contract, (value, key) => {
      if(isDinero(value)) {
        //  value -> Contract[keyof Contract] in TS 5.1.X
        // value: Contract[keyof Contract] & Dinero in TS 5.0.X
        value.mul() 
        
      }

    })

}

🙁 Actual behavior

The type value is Contract[keyof Contract

🙂 Expected behavior

The type value to be at least Contract[keyof Contract] & Dinero

[fatcerberus]

1. isDinero is a type predicate, not a type assertion. Type assertion is e.g. as Foo
2. The expected type is an intersection, not a union.

[thibault-lr]

Indeed.

1. isDinero is a type predicate, not a type assertion. Type assertion is e.g. as Foo
2. The expected type is an intersection, not a union.

Indeed, I'm not allowed to update the post content however.

[RyanCavanaugh]

Adding a self-contained repro so this can be bisected...

type Dinero  = {
  mul: () => number
}
declare const isDinero: (obj: unknown) => obj is Dinero;
const transformValues = <Contract extends object>(contract : Contract)  => {
    let value: Contract[keyof Contract] = null as any;
    if(isDinero(value)) {
      value.mul() 
    }
}

[typescript-bot]

The change between origin/release-5.0 and origin/release-5.1 occurred at 84a09c7.

[RyanCavanaugh]

The originating PR #53059 is being reverted in favor of #54845, so I think it's safe to call this a duplicate

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.