Property 'X' does not exist on type Y

Bug Report

🔎 Search Terms

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "does not exist on type"

⏯ Playground Link

Playground link with relevant code

💻 Code

type Listener = {
    callback(): boolean;
};

const fn = <T extends Listener>(listener: T) => { };

// this works
fn({
    context: new Map<string, number>(),
    callback() {
        const num = this.context.get('number') ?? 0;

        if (num > 0) return true;
        else return false;
    },
});

// this does not
fn({
    context: new Map<string, number>(),
    callback() {
        const num = this.context.get('number') ?? 0; // Property 'context' does not exist on type 'Listener'.(2339)
        return num > 0;
    },
});

🙁 Actual behavior

It is giving a compilation error as soon as I change the return variable.

🙂 Expected behavior

Should not give compilation errors.

Inizia con l’esempio collegato di TypeScript Playground e confronta le due chiamate a fn, concentrandoti sul contextual typing e sull’inferenza generica per il valore this del callback. L’issue è completata quando il secondo esempio viene compilato senza l’errore Property 'context' segnalato, preservando il comportamento del primo esempio.