Boolean properties are treated differently in conditional types (functions/methods)

[bluebarcode]

Bug Report

Comparing conditional types containing boolean properties behave dififferent to other property types. But this only occures when the comparison takes place in a function/method.

🔎 Search Terms

conditional types, boolean, method, function

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ

⏯ Playground Link

Playground link with relevant code

💻 Code

type FailsType = {
 foo: boolean;
};
type WorksType = { 
 foo: string
};
class TestClass<Param> {
 testFunc: <T extends Partial<Param>>(
 param: T
 ) => {testProp?:Param extends Partial<T> ? 'yes' : 'no'} = (param)=>({});
}
type inlineTest = FailsType extends Partial<FailsType> ? 'yes' : 'no';
const inlineTestValue:inlineTest = 'yes';

// Here i use the Works Type
const testClassWorks = new TestClass<WorksType>();
const resultWorks = testClassWorks.testFunc({ foo: 'works' });
resultWorks.testProp='yes';

// Here i use the Fails Type
const testClassFails = new TestClass<FailsType>();
const resultFails = testClassFails.testFunc({ foo: false });
resultFails.testProp='yes';

🙁 Actual behavior

The type of resultFails.testProp is not 'yes'

🙂 Expected behavior

The type of resultFails.testProp should be 'yes'

[bluebarcode]

const testClassFails = new TestClass<FailType>();
const failParam = { foo: false }; // Step between!
const resultFails = testClassFails.testFunc(failParam);
resultFails.testProp='yes';

Interesting enough this works! the "typeof" used in the function seems to be slightly different...

[whzx5byb]

It is similar to #48363, which was fixed by #48380. But it seems the PR doesn't fix the issue when a boolean literal is used as a property.

[RyanCavanaugh]

boolean is an alias for true | false, and when a primitive literal is contextually typed by a union containing that literal's corresponding type, then it retains its actual literal type instead of widening to its parent primitive. So the comparison here is { foo: boolean } extends { foo?: false } ? ... which is correctly false

[bluebarcode]

Thank you for the explanation!

This is not intuitive tho. But i assume there is good reason to be as it is? Especially since when i assign the object literal to a variable before it does indeed work.

[typescript-bot]

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