acorn-loose throws SyntaxError on some numeric separators with ecmaVersion 2021

[denis-sokolov]

Unexpectedly thrown SyntaxError from acorn-loose parse when using invalid numeric separators with ecmaVersion 2021 and above:

> looseParse("0_", { ecmaVersion: 2021 })
Uncaught:
[SyntaxError: Numeric separator is not allowed in legacy octal numeric literals (1:1)
] {
  pos: 1,
  loc: Position { line: 1, column: 1 },
  raisedAt: 1
}

Does not throw with ecmaVersion 2020 and below:

> looseParse("0_", { ecmaVersion: 2020 })

Node {
  type: 'Program',
  start: 0,
  end: 2,
  body: [ Node { type: 'EmptyStatement', start: 2, end: 2 } ],
  sourceType: 'script'
}

Note that the same happens with input 1_, but the error is different (“Numeric separator is not allowed at the last of digits”).

Errors are marked as recoverable in the source code:

acorn/acorn/src/tokenize.js

Lines 466 to 468 in 82233bf

	if (isLegacyOctalNumericLiteral) this.raiseRecoverable(this.pos, "Numeric separator is not allowed in legacy octal numeric literals")
	if (lastCode === 95) this.raiseRecoverable(this.pos, "Numeric separator must be exactly one underscore")
	if (i === 0) this.raiseRecoverable(this.pos, "Numeric separator is not allowed at the first of digits")

[marijnh]

Attached patch should help here.