Grammar ambiguity in for ( async of
#2034
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why allow keyword like |
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Because they’ve always been allowed, and it wouldn’t be web compatible to disallow them now. |
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Feb 10, 2021
Implements tc39/ecma262#2034 Currently the token sequence `for (async of` is ambiguous. It can be the prefix for either `(async of => {};;);` or `for (async of foo);`. This CL disallows the token sequence. Note that `for await (async of` is still allowed, since there is no C-style `for await (;;)`, and thus no ambiguity. Bug: v8:11412 Change-Id: I3fede83a69420996baa2bc8b6c1cff000535d990 Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2683221 Commit-Queue: Shu-yu Guo <syg@chromium.org> Reviewed-by: Marja Hölttä <marja@chromium.org> Cr-Commit-Position: refs/heads/master@{#72607}
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For reference, the following code prints var async;
for (async of [42]) {}
console.log(async); |
strager
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Feb 19, 2021
For a while, the code 'for (async of []);' is ambiguous in the JS
standard. It's going to be made unambiguously a SyntaxError. Make
quick-lint-js report an error for this code, treating 'async' as a
variable name, and gracefully continue.
Many other forms, such as 'for (async.prop of []);' and
'for (async of => {}; cond; update);' are unambiguous, and shouldn't
result in any errors. This commit adds tests for these similar cases.
Discussion on the ambiguity and its resolution:
tc39/ecma262#2034
strager
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Feb 20, 2021
For a while, the code 'for (async of []);' is ambiguous in the JS
standard. It's going to be made unambiguously a SyntaxError. Make
quick-lint-js report an error for this code, treating 'async' as a
variable name, and gracefully continue.
Many other forms, such as 'for (async.prop of []);' and
'for (async of => {}; cond; update);' are unambiguous, and shouldn't
result in any errors. This commit adds tests for these similar cases.
Discussion on the ambiguity and its resolution:
tc39/ecma262#2034
strager
added a commit
to quick-lint/quick-lint-js
that referenced
this issue
Feb 20, 2021
For a while, the code 'for (async of []);' is ambiguous in the JS
standard. It's going to be made unambiguously a SyntaxError. Make
quick-lint-js report an error for this code, treating 'async' as a
variable name, and gracefully continue.
Many other forms, such as 'for (async.prop of []);' and
'for (async of => {}; cond; update);' are unambiguous, and shouldn't
result in any errors. This commit adds tests for these similar cases.
Discussion on the ambiguity and its resolution:
tc39/ecma262#2034
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@strager A fix was recently merged (https://chromium-review.googlesource.com/c/v8/v8/+/2683221), and Chrome 90 / V8 9.0 will correctly disallow |
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The token sequence
for ( async ofcan begin two different parses: eitherfor (async of => {};;);orfor (async of foo);.@waldemarhorwat raised this issue at the December 2019 meeting. During that discussion we decided to resolve this by using a lookahead restriction to disallow for-of loops which start with the token
async.The text was updated successfully, but these errors were encountered: