Assignment fails for types with conditional lookup for a union of a generic

[smoogly]

TypeScript Version: Version 3.6.0-dev.20190810

Search Terms: union conditional lookup

Code

type Target<T> = T extends null ? null : T;
type Target2<T> = { "one": null, "two": T }[T extends null ? "one" : "two"];
type Target3<T> = [null, T][T extends null ? 0 : 1];

function tst<T extends string>() {
    // These two pass as expected:
    const case0: Target2<T | null> = 1 as any as Target2<T | null>;
    const case1: { prop: Target<T | null> } = 1 as any as { prop: Target<T | null> };
    const case2: { prop: Target2<T> } = 1 as any as { prop: Target2<T> };
    const case3: { prop: Target3<T> } = 1 as any as { prop: Target3<T> };

    // These two fail as expected:
    const case4: { prop: Target2<T> } = 1 as any as { prop: Target2<T | null> };
    const case5: { prop: Target3<T> } = 1 as any as { prop: Target3<T | null> };

    // These two are expected to pass, but fail:
    const case6: { prop: Target2<T | null> } = 1 as any as { prop: Target2<T | null> };
    const case7: { prop: Target3<T | null> } = 1 as any as { prop: Target3<T | null> };
}

Expected behavior:
Expected cases 6 and 7 to pass, as both sides of assignment supposed to be structurally identical.

Actual behavior:
Cases 6 and 7 fail. Example output:

error TS2719: Type '{ prop: { "one": null; "two": T | null; }["one" | (T extends null ? "one" : "two")]; }' is not assignable to type '{ prop: { "one": null; "two": T | null; }["one" | (T extends null ? "one" : "two")]; }'. Two different types with this name exist, but they are unrelated.
  Types of property 'prop' are incompatible.
    Type '{ "one": null; "two": T | null; }[T extends null ? "one" : "two"] | null' is not assignable to type 'null & { "one": null; "two": T | null; }[T extends null ? "one" : "two"]'.
      Type '{ "one": null; "two": T | null; }[T extends null ? "one" : "two"]' is not assignable to type 'null & { "one": null; "two": T | null; }[T extends null ? "one" : "two"]'.
        Type 'T | null' is not assignable to type 'null & { "one": null; "two": T | null; }[T extends null ? "one" : "two"]'.
          Type 'T' is not assignable to type 'null & { "one": null; "two": T | null; }[T extends null ? "one" : "two"]'.
            Type 'string' is not assignable to type 'null & { "one": null; "two": T | null; }[T extends null ? "one" : "two"]'.
              Type 'string' is not assignable to type 'null'.
                Type 'T' is not assignable to type 'null'.
                  Type '{ "one": null; "two": T | null; }[T extends null ? "one" : "two"]' is not assignable to type 'null'.
                    Type 'T | null' is not assignable to type 'null'.
                      Type 'T' is not assignable to type 'null'.
                        Type 'string' is not assignable to type 'null'.

Playground Link

Related Issues: #30152 looks similar

[AnyhowStep]

Feels a little bit like this #32442

Except yours does not use function params.

[jack-williams]

This is a consequence of #30769 and fundamentally the same issue as discussed in #31833, however the trigger is slightly different.

The instantiation of the distributive conditional type T extends null ? "one" : "two" is "one" | (T extends null ? "one" : "two") as the conditional distributes over the union. The union key causes the index access type in the target type to simplify to an intersection (despite the property never being assigned), as per #30769.

[typescript-bot]

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