Proposal: `unstable` and `volatile` control-flow analysis modifiers

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### 🔍 Search Terms

CFA narrowing optimistic pessimistic unstable volatile

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

Add two erasable declaration modifiers that let authors opt out of TypeScript's optimistic control-flow assumptions:

- `unstable`: narrowing is discarded after opaque execution, such as a function call, `await`, or `yield`.
- `volatile`: every read is a fresh observation, so narrowing cannot be reused by a later read.

`volatile` is the stronger mode and subsumes `unstable`.

These modifiers affect only control-flow analysis. They do not change types, participate in assignability, or emit JavaScript.

### 📃 Motivating Example

#9998 established that neither globally optimistic nor globally pessimistic CFA works well. TypeScript chose optimism, but some programs, especially parsers and state machines, need pessimism for selected declarations.

```ts
unstable let token: SyntaxKind;

if (token === SyntaxKind.ExportKeyword) {
nextToken();

// The previous narrowing was discarded.
if (token === SyntaxKind.DefaultKeyword) {
// ...
}
}
```

#57725 shows the complementary problem: TypeScript treats repeated property reads as stable even when a getter computes a new value each time.

```ts
const model = {
volatile get value(): string | undefined {
return Math.random() > 0.5 ? "Hello" : undefined;
}
};

if (model.value) {
model.value.toLowerCase();
// Error: the second read may produce undefined.
}
```

The value can be explicitly stabilized:

```ts
const value = model.value;

if (value) {
value.toLowerCase();
}
```

### 💻 Use Cases

1. What do you want to use this for? To explicitly control control-flow analysis for declarations whose values are not stable. This is particularly useful for parser state, state machines, live views over mutable state, and computed getters whose values may change between observations.

2. What shortcomings exist with current approaches? Today, the only way to influence CFA is indirectly through API shape (property vs. function) or by restructuring code. This relies on convention rather than explicit semantics and does not provide an escape hatch when the default heuristics are incorrect.

3. What workarounds are you using in the meantime? For `unstable`, I avoid relying on narrowing across function calls, or wrap mutable state behind trivial getters to defeat CFA. For `volatile`, the only practical workaround is to redesign the API by exposing a method instead of a getter, even when a property is the more natural abstraction.

### Declaration forms

Both modifiers should be accepted on the same forms:

```ts
unstable let current: string | undefined;
volatile let externalValue: string | undefined;

interface State {
unstable token: SyntaxKind;
volatile readonly temperature: number | undefined;
}

class Model {
unstable value: string | undefined;

volatile get currentTime(): number {
return Date.now();
}
}
```

They should also be supported in object types, interfaces, parameters, ambient declarations, and declaration files.

The modifiers are flow metadata only:

```ts
type Stable = { value: string | undefined };
type Volatile = { volatile value: string | undefined };
```

`Stable` and `Volatile` remain structurally assignable in both directions.

### Defaults and scope

Existing behavior remains the default:

- variables and properties are treated as stable;
- calls are treated as fresh.

This proposal only adds opt-outs when those defaults are incorrect. It does not add callable purity annotations, narrow repeated function calls, infer effects, or describe setter postconditions.

The naming is inspired by PostgreSQL's explicit [`IMMUTABLE`](https://www.postgresql.org/docs/current/sql-createfunction.html)[,](https://www.postgresql.org/docs/current/sql-createfunction.html)[`STABLE`](https://www.postgresql.org/docs/current/sql-createfunction.html)[, and](https://www.postgresql.org/docs/current/sql-createfunction.html)[`VOLATILE`](https://www.postgresql.org/docs/current/sql-createfunction.html) optimizer contracts, but the proposed semantics are specific to TypeScript CFA.

Empieza leyendo esta propuesta y los issues referenciados #9998 y #57725 para comprender los problemas de análisis de flujo de control que aborda. Define e implementa las formas de declaración `unstable` y `volatile` en los contextos indicados, preservando la asignabilidad estructural y la salida de JavaScript; se considera terminado cuando el comportamiento de narrowing propuesto sea compatible sin cambiar la emisión en tiempo de ejecución.

Escrito por el modelo de indexación a partir del texto del issue.