Normative: Treat IterationStatements uniformly in Annex B.3.5.

[rkirsling]

Resolves #1392.

[bakkot]

It is a Syntax Error if any element of the BoundNames of |CatchParameter| also occurs in the VarDeclaredNames of |Block| unless |CatchParameter| is CatchParameter : BindingIdentifier and that element is only bound by a |VariableStatement|, or the |VariableDeclarationList|, |ForBinding|, or |BindingIdentifier| of an |IterationStatement|.

Are there any other cases? I'm not sure the bolded part of this rule is ever unsatisfied.

[rkirsling]

I think you're right, since function declarations add to a block's LexicallyDeclaredNames and not to its VarDeclaredNames.

I'm a bit confused about what this implies for the second instance though. Technically, what's written there is not exhaustive—if it read HoistableDeclaration instead of FunctionDeclaration, then we could delete the entire sentence, but as it stands it would not allow, e.g., GeneratorDeclaration.

However, it appears that not a single engine actually distinguishes among HoistableDeclarations here!

// SyntaxError in V8, SM, XS; permitted in JSC, CH
try {} catch (e) { function e() {} }
try {} catch (e) { function* e() {} }
try {} catch (e) { async function e() {} }
try {} catch (e) { async function* e() {} }

// permitted in V8, SM, CH; SyntaxError in JSC, XS
try { throw 1; } catch (e) { eval('function e() {}'); }
try { throw 1; } catch (e) { eval('function* e() {}'); }
try { throw 1; } catch (e) { eval('async function e() {}'); }
try { throw 1; } catch (e) { eval('async function* e() {}'); }

// (Side note: CH doesn't actually have async generators right now.)

So perhaps web reality suggests that we could do away with that whole sentence after all? 🤔

[syg]

Your example gives rise to another piece of esoterica: currently Annex B.3.3 interaction with B.3.5 means the following, which may or may not be surprising to you depending on how long you've stared at the abyss:

function foo() { try { throw 1; } catch (e) { { function e() {} } } print(e); }
foo() // prints "function e() {}"

but

function bar() { try { throw 1; } catch (e) { function e() {} } print(e); } // SyntaxError

[rkirsling]

@syg Thankfully though, that too applies equally to the four HoistableDeclaration types. 😄

[rkirsling]

Made @bakkot's suggested simplification as well as the web reality simplification from #1393 (comment).

[rkirsling]

Implementation status, just for the record:

• JSC: https://trac.webkit.org/r240686
• V8: https://crbug.com/v8/8759
• SM: https://bugzilla.mozilla.org/show_bug.cgi?id=1523720

[mathiasbynens]

@rkirsling Are you gonna patch Test262 as well?

[rkirsling]

@mathiasbynens tc39/test262#2023 is just awaiting the green button. 😄

[rkirsling]

Tests merged, two implementations landed. 🎊
(Thanks @mathiasbynens!)

[mathiasbynens]

…and now it landed in SpiderMonkey as well!