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.

Inizia riproducendo l’errore 2526 con gli esempi di guard per i literal di oggetto in TypeScript Playground, quindi traccia il percorso del controllo dei tipi per una firma di metodo esplicita this: SomeType e la relativa condizione di guard. Il lavoro è completato quando la diagnostica viene rimossa per questo caso, entrambi gli esempi superano il controllo dei tipi come mostrato e il comportamento esistente per l’uso non valido di this rimane invariato.