Erasing as or satisfies can expose an invalid left operand of ** #65170

🔎 Search Terms

erasableSyntaxOnly, as, satisfies, exponentiation, unary expression, await

🕗 Version & Regression Information

Reproduced with TypeScript 6.0.3 and 7.0.2. The Playground uses 6.0.3 to demonstrate the missing diagnostic.

⏯ Playground Link

TypeScript Playground

💻 Code

// @erasableSyntaxOnly: true
export function negate(x: number) {
    return -x as number ** 2;
}

export async function awaited(x: Promise<number>) {
    return await x as number ** 2;
}

🙁 Actual behavior

Both functions compile without diagnostics, including with --erasableSyntaxOnly. The native builds emit (-x) ** 2 and (await x) ** 2. Replacing the type syntax with spaces leaves -x ** 2 and await x ** 2, both of which Node.js rejects with a SyntaxError.

Replacing as number with satisfies number or using chained assertions reproduces the issue. Erasure also produces invalid JavaScript when it exposes an unparenthesized +, ~, !, typeof, void, or delete expression to the left of **.

🙂 Expected behavior

The parser already rejects related cases where erasing as or satisfies changes the grouping of a binary expression (#63527 and #63661). Reporting a parser diagnostic here would be consistent with those fixes. In these examples, erasure leaves an invalid left operand for **.

Even if these forms remain accepted by default, a diagnostic under --erasableSyntaxOnly would still catch the unsafe erasure.

Explicitly parenthesized forms such as (-x) as number ** 2 and (await x) as number ** 2 should still compile.

Additional information about the issue

• The original parser fix handles as and satisfies when erasure changes how surrounding binary operators bind. microsoft/TypeScript#63527
• The follow-up parser fix covers the right associativity of **, where erasure can change grouping even at equal precedence. microsoft/TypeScript#63661
• Ryan explained why ordinary parsing accepts await a as any ** 1 and confirmed that current emit preserves its grouping by adding parentheses. Ryan’s reply on microsoft/TypeScript#41755