Some code became broken in 4.7

[HanabishiRecca]

Bug Report

🔎 Search Terms

4.7

🕗 Version & Regression Information

Such code became broken in 4.7. It works as expected in 4.6.

⏯ Playground Link

4.6 example

4.7 example

💻 Code

type TypesMap = {
    [0]: { foo: 'bar'; };
    [1]: { a: 'b'; };
};

type P<T extends keyof TypesMap> = { t: T; } & TypesMap[T];

type TypeHandlers = {
    [T in keyof TypesMap]?: (p: P<T>) => void;
};

const typeHandlers: TypeHandlers = {
    [0]: (p) => console.log(p.foo),
    [1]: (p) => console.log(p.a),
};

const onSomeEvent = <T extends keyof TypesMap>(p: P<T>) =>
    typeHandlers[p.t]?.(p);

Also it works in 4.7 with optional (?) removed from TypeHandlers index.

🙁 Actual behavior

The code won't compile.

🙂 Expected behavior

The code should compile.

[fatcerberus]

type Packet<T extends keyof PacketTypes> = { op: T; } & PacketTypes[T];

op: T is not narrowed by generic for some reason.

Not sure what you mean by "narrowed by generic"? You're going to have to give more details about what's going wrong. "The code won't compile" isn't very helpful.

[HanabishiRecca]

"The code won't compile" isn't very helpful.

I know. Sorry for that. But I can't figure out what exactly is wrong. Code that worked at 4.6 won't compile at 4.7 for some unclear reasons. It should work by my logic (and worked as expected prior 4.7). Compiler just throws some obscure errors. That's why I'm asking for help here.
You can clone my repo by the link provided and run tsc to see.

[HanabishiRecca]

Ok, I figured out something.
Such example works fine for both 4.6 and 4.7 as expected:

type TypesMap = {
    [0]: { foo: 'bar'; };
    [1]: { a: 'b'; };
};

type P<T extends keyof TypesMap> = { t: T; } & TypesMap[T];

type TypeHandlers = {
    [T in keyof TypesMap]: (p: P<T>) => void;
};

const typeHandlers: TypeHandlers = {
    [0]: (p) => console.log(p.foo),
    [1]: (p) => console.log(p.a),
};

const onSomeEvent = <T extends keyof TypesMap>(p: P<T>) =>
    typeHandlers[p.t]?.(p);

But when I add optional (?) to TypeHandlers

type TypesMap = {
    [0]: { foo: 'bar'; };
    [1]: { a: 'b'; };
};

type P<T extends keyof TypesMap> = { t: T; } & TypesMap[T];


type TypeHandlers = {
    [T in keyof TypesMap]?: (p: P<T>) => void;
};

const typeHandlers: TypeHandlers = {
    [0]: (p) => console.log(p.foo),
    [1]: (p) => console.log(p.a),
};

const onSomeEvent = <T extends keyof TypesMap>(p: P<T>) =>
    typeHandlers[p.t]?.(p);

it works as expected at 4.6 (example), but broken at 4.7 (example).

I don't know why ? breaks the code in 4.7.

[jcalz]

Could be the same as #49242

[HanabishiRecca]

Could be the same as #49242

Yeah. Too bad the error is very obscure and I wasn't even able to properly explain it.

[ahejlsberg]

This is indeed a duplicate of #49242.