Common fields of union not resolved for generic conditional with `any` branch

🔎 Search Terms

"conditional generics with any branch in union", "union with generic conditional branch"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about conditional generics used in unions but didn't find anything that seemed relevant.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.2#code/C4TwDgpgBAkgdgYQPYFsVLlAvFA3gY1XTgC4oBnYAJwEs4BzAXwG4AoV0SKZOAExuA0MAQwA2AHgDSUCAA9gEPuQrU69KAB8ocAK4oARhCoA+bFGlyFS7XsNUoAfjwYIZXQaOMoZAEoRCVLzilLQMADRQOnAA1nBIAO5wxmyc0ACqcEJwAOoCABZ+9DqiwlQ8-IIiEhbyirzKIWqaNh4mZvDIaBjNABTlAlliUqYAZLCIRBgAlGwAZlH4lZhywihgohAZWbnABRBFJVRSMrXWjQzN7nbGPdEQIGSSU3isUG9QhHCUkZkYO3sHUr9JZiMy4LzCZRbP75QrFIEYCqDarJV7vAD06KguxoyniSCo0WUkJOkEWEF4aLeUW2sP28LKiIGVQAdIQunA2Ix2KluEyQaJ-gBBOAgY6WOoNVQXLRXIymHA1Kz1Fp2RxQYSi7xQPwBILnegRKKxBJJFLgdK-HL5EUgYHI8WnFUGy62eXtCYc3r2qrC0XDKBjDqTOAzVjzOCLLInVbrTZWv1ipWSlShdSyt0mW73R7PXBUj4Yb40mG7W0+uCgnDgjVQhM20UVoaSVHvKCY7F5XFQPIk4TYi0yKhUAlQWajgAG7OIE+0SGAJ1xggu3QnTcFDaTo3GnRnBZL1rLjf5yLZIbYbYLHf0OgXu0Une7ACpeEgIOQn1B8YSC59vmARzARN1zBCFlD6E9fU3AMg09YgwzbACkCAzd1zPDkuVYIA

💻 Code

type InCommon = {common: string};

type Conditional<K extends string | number> = K extends number ? {one: number} : Record<string, unknown>;
type UnionWithRegularConditional<K extends string | number> = InCommon | (Conditional<K> & InCommon);
function exampleUnionWithRegular<K extends string | number>(key: K) {
    const unionWithRegularConditional = {} as UnionWithRegularConditional<K>;
    // this works as expected
    unionWithRegularConditional.common;
}

type ConditionalWithAny<K extends string | number> = K extends number ? any : Record<string, unknown>;
type UnionWithAnyConditional<K extends string | number> = InCommon | (ConditionalWithAny<K> & InCommon);
function exampleUnionWithAny<K extends string | number>(key: K) {
    const unionWithAnyConditional = {} as UnionWithAnyConditional<K>;
    // this has a type error for `common` not existing on `ConditionalWithAny<K> & InCommon`
    unionWithAnyConditional.common;
    
    // but then this *does* work
    const propWithAnyConditional = {} as (ConditionalWithAny<K> & InCommon);
    propWithAnyConditional.common;
}

🙁 Actual behavior

When a union type contains a conditional generic whose branch is any, the common fields of the union are not being correctly recognized.

This is illustrated above when TS is unable to recognize that common is a field on unionWithAnyConditional.

🙂 Expected behavior

The expected behavior here is that common fields of the union should be correctly resolved.

From the above example, the expectation is that unionWithAnyConditional.common would not have a "does not exist on type" error.

Additional information about the issue

No response

Start with the linked TypeScript Playground repro and confirm the difference between the regular conditional and the conditional with an any branch. Trace the compiler's handling of common properties on the resulting union, then add coverage for unionWithAnyConditional.common. Done means the expected access no longer reports that common is missing.