Annex B.3.5 CatchParameter legacy remedy could be narrower

[ajklein]

The early errors for Try/Catch names includes the following passage:

"It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block."

However, this is modified in B.3.5 to allow code of the form:

try {
  ...
} catch (e) {
  var e = 2;
}

to run without complaint, presumably for legacy compat reasons. The spec text says:

"It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block, unless that element is only bound by a VariableStatement or the VariableDeclarationList of a for statement, or the ForBinding of a for-in statement."

But this means that the following is also allowed:

try {
  ...
} catch ({stack, code}) {
  var stack = 1;
  var code = 2;
}

What's the motivation for allowing the destructured form, given that there will be no legacy concerns in that case? One could imagine the spec could instead read:

"It is a Syntax Error if any element of the BoundNames of CatchParameter also occurs in the VarDeclaredNames of Block, unless BoundNames contains a single element and that element is only bound by a VariableStatement or the VariableDeclarationList of a for statement, or the ForBinding of a for-in statement."

[allenwb]

Yes, this sounds like a good narrowing of the B.3.5 form of the rule.

[allenwb]

Yes, this sounds like a good narrowing of the B.3.5 form of the rule.

[anba]

The current specification also allows this:

try {
  ...
} catch (e) {
  var [e] = ...
}

Should that form also be made invalid?

[...] unless BoundNames contains a single element and [...]

"unless CatchParameter is CatchParameter : BindingIdentifier and " to handle catch ({stack}) {...} correctly.

[anba]

The current specification also allows this:

try {
  ...
} catch (e) {
  var [e] = ...
}

Should that form also be made invalid?

[...] unless BoundNames contains a single element and [...]

"unless CatchParameter is CatchParameter : BindingIdentifier and " to handle catch ({stack}) {...} correctly.

[michaelficarra]

Yes, that final change should do it.

[michaelficarra]

Yes, that final change should do it.