Disallow usage of invalid keyword after export abstract statement in Typescript

[armano2]

Q	A
Fixed Issues?	Fixes #9304
Patch: Bug Fix?	👍
Major: Breaking Change?	👎
Minor: New Feature?	👎
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?	👎
License	MIT

This PR makes parser error on unexpected keywords used after export abstract and disallow usage of unknown keywords as "classes":

example of fixed cases:

export abstract interface X {}

export abstract putHereWhathever X {}

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this.parseClass does not check if its actually class, it's just doing this.next() on first token
it can be also solved by moving this check there

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/9832/

[nicolo-ribaudo]

Don't the other things inside that switch (enum, interface) have the same problem?

[armano2]

no, in other cases we are checking identifier and only abstract is multi keyword abstract class

additionally issue is present only if export keyword is in front of it

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before this change

abstract interface will throw error
export abstract interface is valid

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i found this issue by doing alignment tests between typescript-estree with babel 🐱