Allow this in type guards when type of this is explicitly specified in method signature

Search Terms

"A 'this' type is available only in a non-static member of a class or interface.(2526)" guard
Relevant: #37796 (but this one is different)

Suggestion

Remove error 2526 in guard condition if type of this is specified explicitly using this: SomeType.
Since both guard condition and method body work as expected, I assume only the error check needs to be removed.

Use Cases

Defining a guard without having a class (have some cases where full class would be an overkill).

Examples

({
    guard(this: { x?: 1 }): this is { x: 1 } {
        return this.x === 1;
    }
})

https://www.typescriptlang.org/play/index.html?ssl=5&ssc=3&pln=1&pc=1#code/BQbwsAUABDUOYFcCGAnAJsALgCwJYGcAuKEKADwH5iBGKAXwEpicCpXSyb6TJY+oUAU0wIUAOygt8AOjJQAvIqjUA3L1h1IjIA

Note that this is not any and works perfectly fine in both the method body and the guard:

const o = ({
    x: 1 as 1|2,
    guard(this: { x: 1 | 2 }): this is { x: 1 } {
        // body works
        return this.x === 1;
    }
});

if (o.guard()) {
    // guard works
    const check: 1 = o.x;
}

https://www.typescriptlang.org/play/index.html#code/MYewdgzgLgBCMF4YAoDeBYAUDHMAeAXDAIwwCGEJAPgEwA0WuMA5gK5kBOAJslABYBLCEVT4ipKjBowAvgEoi-ITGWjCJWTAzYmuAPR6YAIxBcAnjADuIDgGsIjXTg4BTKKw5gYSiADo8iAhIxADcjjgyWPJhmFgCAGYoIL5snDxyclrhMAYs7NxWNvbZoJCwwHwuwLbiiHD+MTJAA

Would be good to support guard in ThisType<> as well (see #37796), but can be a separate thing.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Comienza reproduciendo el error 2526 con los ejemplos de guard para literales de objeto en TypeScript Playground y, después, sigue el flujo de comprobación de tipos para una firma de método explícita this: SomeType y su condición de guard. Se considera completado cuando el diagnóstico se elimina para este caso, ambos ejemplos pasan la comprobación de tipos tal como se muestran y el comportamiento existente para el uso no válido de this permanece sin cambios.