Specification of continue behavior inside a switch statement makes no sense

[stereotype441]

The Dart language spec says:

Let s_c be a continue statement. If s_c is of the form continue L;, then let s_E be the innermost labeled do, for or while statement or case clause with label L enclosing s_c.

But for the typical usage of continue within a switch statement, the target of the continue statement typically doesn't enclose s_c. For example:

switch (x) {
  case 0:
    ...
    continue foo; // s_c
  foo:
  case 1: // s_E (does not enclose s_c)
    ...
    break;
}

If we took the spec literally, the target of a continue statement inside a switch would only be able to point to the originating case clause, which effectively allows turning a case clause into a loop. I'm pretty sure that wasn't the intent.

I think the language should be adjusted to say something like this:

Let s_c be a continue statement. If s_c is of the form continue L;, then let s_E be the innermost labeled do, for or while statement enclosing s_c, or the innermost or case clause with label L from a switch statement enclosing s_c.

...which allows continue expressions within switch statements to jump from one case clause to another.

I've verified that analyzer, dart2js, and the VM all behave consistently with the proposed spec change.

[kmillikin]

This seems like a duplicate of #23628. It has been fixed in the spec in the repository.

[stereotype441]

I don't believe this bug has been fixed (nor has #23628). The spec text in the repository still says:

Let s_c be a continue statement. If s_c is of the form continue L;, then let s_E be the innermost labeled do, for or while statement or case clause with label L enclosing s_c.

(see

sdk/docs/language/dartLangSpec.tex

Line 6590 in 28084d1

Let $s_c$ be a \CONTINUE{} statement. If $s_c$ is of the form \code{\CONTINUE{} $L$;}, then let $s_E$ be the innermost labeled \DO{} (\ref{do}), \FOR{} (\ref{for}) or \WHILE{} (\ref{while}) statement or case clause with label $L$ enclosing $s_c$. If $s_c$ is of the form \code{\CONTINUE{};} then let $s_E$ be the innermost \DO{} (\ref{do}), \FOR{} (\ref{for}) or \WHILE{} (\ref{while}) statement enclosing $s_c$. It is a compile-time error if no such statement or case clause $s_E$ exists within the innermost function in which $s_c$ occurs.

)

[eernstg]

I looked into the spec too, and I agree that it still does not mention how continue works in a switch.

[kmillikin]

You are right. I was misled because the duplicate had been closed.

[lrhn]

The runtime semantics are correct in the current specification, and doesn't rely on the "enclosing case clause", but the static warning still uses it. I'll reword that as well.

CL: https://codereview.chromium.org/2961603002/