Microsoft/TypeScript

Parser does not reject `await a as any ** 1`

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#41,755 opened on Dec 1, 2020

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 (8 comments) (0 reactions) (0 assignees)TypeScript (48,455 stars) (6,726 forks)batch import
BugDomain: API: TransformsEffort: ModerateHelp Wanted

Description

TypeScript Version: 4.2.0-dev.20201201

Search Terms: exponentiation, await

Code

await a as any ** 1

Expected behavior: Parser should throw

An unary expression with the 'await' operator is not allowed in the left-hand side of an exponentiation expression. Consider enclosing the expression in parentheses.

Actual behavior: Does not throw

Playground Link: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201201#code/G4QwTgBCELwQCmA9gWwJYGcCmA6MWMkAbYLACgEYBKAbgCgQMBPAOwGMIyrYA+KAdxBoALlCgYoLJhABUMiBSA

Related Issues: https://github.com/microsoft/TypeScript/issues/40916, since in November meeting we have reached consensus that await x ** 1 is illegal and the parser already throws on await a! ** 1, I think it should either throw for await a as any ** 1, or add parenthesis in the output for await a if as any serves as an implicit parenthesis.

Contributor guide

Tech stack
typescript
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
TypeScript parser basicsExponentiation operator precedence
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
The issue is that the parser currently does not reject `await a as any ** 1`. The fix likely involves adding a check in the parser (src/compiler/parser.ts) similar to the existing check for `await a! ** 1`. Review the related issue #40916 and the discussion in the comments. The expected behavior is to throw an error about unary expression with await not allowed on left hand side of exponentiation.