[bug] Discrepencies in results between JavaScript and TypeScript

[Pamplemousse]

Trying to match and rewrite lambda functions in TypeScript, I get a simple pattern that doesn't work, but matches correctly in JavaScript.

⏯ Playground Link

Issue can't be reproduced in a single playground, as it is about inconsistencies between pattern matching in JavaScript and TypeScript.

💻 Code

# yolo.ts
const foo = (x) => bar(x);

🙁 Actual behavior

$ ast-grep run -p '($P) => $F($P)' yolo.ts
# nothing

$ ast-grep run --lang typescript -p '($P) => $F($P)' yolo.ts
# nothing

$ ast-grep run --lang javascript -p '($P) => $F($P)' yolo.ts
yolo.ts
1│const foo = (x) => bar(x);

🙂 Expected behavior

I was expecting the first pattern to match the code in TypeScript, like it does when the language is explicitly set to JavaScript.

[HerringtonDarkholme]

I can reproduce it on playground

https://ast-grep.github.io/playground.html#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

[HerringtonDarkholme]

The problem is first $P captures requried_parameter instead of identifier. But the second $P expects identifier

[HerringtonDarkholme]

https://ast-grep.github.io/playground.html#eyJtb2RlIjoiQ29uZmlnIiwibGFuZyI6InR5cGVzY3JpcHQiLCJxdWVyeSI6IiIsInJld3JpdGUiOiJsb2dnZXIubG9nKCRNQVRDSCkiLCJjb25maWciOiJydWxlOlxuICBwYXR0ZXJuOiAoJEEpID0+ICRCKCRDKVxuY29uc3RyYWludHM6XG4gIEE6XG4gICAgaGFzOlxuICAgICAgcGF0dGVybjogJEMiLCJzb3VyY2UiOiJjb25zdCBmb28gPSAoeCkgPT4gYmFyKHgpOyJ9

You can use this yaml rule as work around

[HerringtonDarkholme]

I have made pattern more permissive and it will be released in 0.21.3.