Index signature initialization

[ghost]

TypeScript Version: 3.5.3

Code

export class User {
  id: number = 0

  [key: string]: any
}

Expected behavior:

Should work like

export class User {
  id: number = 0; // <- semicolon

  [key: string]: any
}

export class User {
  id: number // no initialization

  [key: string]: any
}

Actual behavior:

error TS1005: ']' expected.

Playground Link:
https://www.typescriptlang.org/play/#code/KYDwDg9gTgLgBAYwDYEMDOa4FU3CnAbwCg44BLAEwC44A7AVwFsAjPOAXjgAYiS4BtANbAAnjTQwoZWgHMAujRS0RRAL5A

CC prettier/prettier#6727 , prettier/prettier#6728

[IllusionMH]

Looks like this is same problem as with class fields proposal tc39/proposal-class-fields#7.
ASI won't insert ; if next line starts with [ and therefore your code is parsed as indexed access id: number = 0[key which leads to an error, because : won't be part of valid expression for indexed access.

See proposal source

Not sure if TS can easily avoid this requirement even if [ is part of the type and not computed property.

[RyanCavanaugh]

ASI hazard is hazardous. We don't want to do a complex lookahead here on [ to figure out if it's an index signature or element access expression (I'm not even 100% convinced it's never ambiguous).

Line-leading [ should always have an associated ; in desemicoloned style