Should arrow functions allow `yield` as identifier name in StrictFormalParameters?

[ianwjhalliday]

Currently arrow functions pass along the Yield parameter to their parameter list grammar production, making yield a reserved word in that context.

However, function expressions do not pass the Yield parameter along to their FormalParameters production.

Corner case but seems an odd asymmetry.

[michaelficarra]

Currently, 14.2.1 defines an early error that disallows _YieldExpression_s in ArrowParameters.

It is a Syntax Error if ArrowParameters Contains YieldExpression is true.

So the only difference this grammar change would make would be allowing yield used as an identifier in ArrowParameters (or conversely disallowing yield used as an identifier in FormalParameters, but that would be web incompatible). 👍 let's drop the flag from ArrowParameters.

[michaelficarra]

Currently, 14.2.1 defines an early error that disallows _YieldExpression_s in ArrowParameters.

It is a Syntax Error if ArrowParameters Contains YieldExpression is true.

So the only difference this grammar change would make would be allowing yield used as an identifier in ArrowParameters (or conversely disallowing yield used as an identifier in FormalParameters, but that would be web incompatible). 👍 let's drop the flag from ArrowParameters.

[anba]

Even when [Yield] gets removed, yield still cannot be used as the name of an arrow function default parameter in a generator function context.

function* g() {
  // Default parameters cannot be named "yield", because `yield = 0`
  // is initially parsed as an assignment expression.
  var f = (yield = 0) => {};
}

And removing [Yield] probably also complicates correctly reinterpreting CoverParenthesizedExpressionAndArrowParameterList as ArrowFormalParameters:

function* g() {
  // Lexical token sequence for `yield/a/i` is `YIELD REGEXP`.
  // When CoverParenthesizedExpressionAndArrowParameterList is reinterpreted/reparsed
  // as ArrowFormalParameters, the lexer/parser is not allowed to change this to the
  // token sequence `YIELD DIV IDENT DIV IDENT`.
  var f = (p = yield/a/i) => {};
}

[anba]

Even when [Yield] gets removed, yield still cannot be used as the name of an arrow function default parameter in a generator function context.

function* g() {
  // Default parameters cannot be named "yield", because `yield = 0`
  // is initially parsed as an assignment expression.
  var f = (yield = 0) => {};
}

And removing [Yield] probably also complicates correctly reinterpreting CoverParenthesizedExpressionAndArrowParameterList as ArrowFormalParameters:

function* g() {
  // Lexical token sequence for `yield/a/i` is `YIELD REGEXP`.
  // When CoverParenthesizedExpressionAndArrowParameterList is reinterpreted/reparsed
  // as ArrowFormalParameters, the lexer/parser is not allowed to change this to the
  // token sequence `YIELD DIV IDENT DIV IDENT`.
  var f = (p = yield/a/i) => {};
}

[ianwjhalliday]

@anba ah, there's the gotcha with arrow functions! Thanks for pointing that out. I see the need for the asymmetry now.

[ianwjhalliday]

@anba ah, there's the gotcha with arrow functions! Thanks for pointing that out. I see the need for the asymmetry now.