Typescript: 'async' modifier cannot be used in an ambient context.ts

[francesc79]

TypeScript Version: 3.8.2

Search Terms: allowJs declaration 'async' modifier cannot be used in an ambient context.ts

Code
see code

when run tsc with allowJs and declaration flags enabled the code below
```TestJS.js
export class TestJS {
callAsync() {
return Promise.resolve(1);
}

async test() {
    await this.callAsync();
}

}

produce the declaretion:
```TestJS.d.ts
export class TestJS {
    callAsync(): Promise<number>;
    async test(): Promise<void>; --> error
}

but when a try to use this class I receive 'async' modifier cannot be used in an ambient context.ts(1040)

Expected behavior:
the TestJS.d.ts shouldn't contain async

Actual behavior:
TestJS.d.ts conteins async

Playground Link: code

Related Issues:

[pocesar]

was going to post about this right now, the behavior from JS / TS is definitely off

[DanielRosenwasser]

Tests should probably include a test for generators too.

[francesc79]

with typescript 3.7.5 work well

[DanielRosenwasser]

Seems like this is fixed in the nightlies. @weswigham can you verify, track down the fix, and cherry-pick it?

[a-tarasyuk]

Seems like this is fixed in the nightlies.

This issue related to the method makeSerializePropertySymbol. In the 3.8.*_(need to find the commit where it was done)_ was added the way to get the modifier flags const modifierFlags = getDeclarationModifierFlagsFromSymbol(p); of a symbol and use these flags to create nodes. The 3.7.* handles only static modifier const staticFlag = isStatic ? ModifierFlags.Static : 0; - that's why 3.7.* doesn't emit async modifier.

cc @DanielRosenwasser @weswigham @francesc79

[weswigham]

@a-tarasyuk it regressed in https://github.com/microsoft/TypeScript/pull/35731 probably, since prior to that we weren't pulling on "original" modifier flags from anywhere.

Seems like this is fixed in the nightlies

Looks broken in the nightlies to me