Possibly-unreachable specification text

[jugglinmike]

I'm having trouble verifying step 4.a.ii.1 in section 13.12.9 ("Runtime Semantics: CaseBlockEvaluation"):

CaseBlock : { CaseClauses }

1. Let V = undefined.
2. Let A be the List of CaseClause items in CaseClauses, in source text order.
3. Let found be false.
4. Repeat for each CaseClause C in A,
   a. If found is false, then
      i. Let clauseSelector be the result of CaseSelectorEvaluation of C.
      ii. If clauseSelector is an abrupt completion, then
          1. If clauseSelector.[[value]] is empty, return
             Completion{[[type]]: clauseSelector.[[type]], [[value]]:
             undefined, [[target]]: clauseSelector.[[target]]}.

source: http://www.ecma-international.org/ecma-262/6.0/#sec-runtime-semantics-caseblockevaluation

(The algorithm for blocks containing the default case have an equivalent step.)

Because CaseSelectorEvaluation is defined as follows:

CaseClause : case Expression : StatementListopt

1. Let exprRef be the result of evaluating Expression.
2. Return GetValue(exprRef).

...the question I'm having trouble answering is: which Expression will return an empty abrupt completion?

[anba]

These steps were initially added to support do-expressions, but currently they aren't needed (and got removed in 748ec83).

[anba]

These steps were initially added to support do-expressions, but currently they aren't needed (and got removed in 748ec83).

[jugglinmike]

@anba Thank you!

[jugglinmike]

@anba Thank you!

[anba]

(BTW there is a similar issue in CreatePerIterationEnvironment: lastIterationEnvRec.GetBindingValue can never return an abrupt completion in ES2015/2016. The call to ReturnIfAbrupt was only added to handle this case: for (let x = do { continue; }; ;) {}. )

[anba]

(BTW there is a similar issue in CreatePerIterationEnvironment: lastIterationEnvRec.GetBindingValue can never return an abrupt completion in ES2015/2016. The call to ReturnIfAbrupt was only added to handle this case: for (let x = do { continue; }; ;) {}. )