Error/warn on switch case fallthough if there is more than one newline?

[cbuttner]

I ran into a case recently where I had commented out the body of a switch case which made it fall through and messing up the program logic, and it took me a quite while to realize.

switch (a) {
  case FOO:
    // commented out code... 
    // now it falls through unexpectedly
  case BAR:
    ...
}

Would it perhaps make sense to emit an error or warning if there is more than one newline between the end of case FOO: and case BAR:?

[cbuttner]

Correction: More than one newline and no statements.

[lerno]

Yes, I think it's reasonable, in fact this problem has actually entered my mind several times and then I consequently forgot about.

There is also this example:

switch (x)
{
    case 0:
        $if SOME_CONDITION:
            foo();
        $endif;
    case 1:
        ...
}

Here even the intent is unclear: did the programmer intend fallthrough if SOME_CONDITION is false – it has no statements afterwards – or what is the intention?

I can somewhat lean towards treating the $if as a ; - that is a NOP statement but maybe this type of statement should require being ended either by break or nextcase?

[lerno]

This is the downside of allowing this special case of fallthrough by the way.

[alexveden]

Totally agree with proposal, faced it many times when commented something inside case, and getting implied fallthrough.

[lerno]

So what should the exact behaviour be? One thing I could do is to require explicit break or nextcase as soon as it isn't completely empty.

[BWindey]

@lerno What's the reason behind implementing this as an error rather than a warning?

Imagine the case :

switch (myEnum) {
  // Some comment explaining why it belongs to case 2
  case ENUM1:
  // Some comment about case2
  case ENUM2:
    do_something()
  default:
    say_hi()
}

That now gives an error while I wanted that comment.
I feel like this should be just a warning?