http://d.puremagic.com/issues/show_bug.cgi?id=7835

           Summary: Ignored break inside static foreach
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2012-04-05 17:40:12 PDT ---
This D2 program compiles with no warnings or errors with DMD 2.059beta:


import core.stdc.stdio: printf;
template TypeTuple(TList...) {
    alias TList TypeTuple;
}
void main() {
    char c = 'b';
    switch (c) {
        case 'a': printf("1 a\n"); break;
        foreach (o; TypeTuple!('b', 'c')) {
            case o: printf("2 %c\n", c); break;
        }
        default: printf("default");
    }
}



But it runs in a wrong way, as you see:

...>dmd -run test.d
2 b
default

...>dmd -w -run test.d
test.d(12): Error: switch case fallthrough - use 'goto default;' if intended


So the break inside the static foreach is ignored.

(This idiom of using a static foreach inside a switch is handy to generate
switch cases.)



Note: adding a second break, like this, doesn't improve the situation:

import core.stdc.stdio: printf;
template TypeTuple(TList...) {
    alias TList TypeTuple;
}
void main() {
    char c = 'b';
    switch (c) {
        case 'a': printf("1 a\n"); break;
        foreach (o; TypeTuple!('b', 'c')) {
            case o: printf("2 %c\n", c); break; break;
        }
        default: printf("default");
    }
}


test.d(10): Warning: statement is not reachable
test.d(10): Warning: statement is not reachable
test.d(12): Error: switch case fallthrough - use 'goto default;' if intended

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Dmitry Olshansky <dmitry.olsh@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dmitry.olsh@gmail.com


--- Comment #1 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2012-04-06 07:15:55 PDT ---
(In reply to comment #0)
> This D2 program compiles with no warnings or errors with DMD 2.059beta:
> 
> 
> import core.stdc.stdio: printf;
> template TypeTuple(TList...) {
>     alias TList TypeTuple;
> }
> void main() {
>     char c = 'b';

L_MySwitch:
>     switch (c) {
>         case 'a': printf("1 a\n"); break;
>         foreach (o; TypeTuple!('b', 'c')) {
>             case o: printf("2 %c\n", c); break L_MySwitch;
>         }
>         default: printf("default");
>     }
> }
> 
> 

For these cases I recommend to use labeled breaks so that it's more clear for humans and compiler alike.


> 
> But it runs in a wrong way, as you see:
> 
> ...>dmd -run test.d
> 2 b
> default
> 
> ...>dmd -w -run test.d
> test.d(12): Error: switch case fallthrough - use 'goto default;' if intended
> 
> 
> So the break inside the static foreach is ignored.
> 
> (This idiom of using a static foreach inside a switch is handy to generate
> switch cases.)
> 
> 
> 
> Note: adding a second break, like this, doesn't improve the situation:
> 
> import core.stdc.stdio: printf;
> template TypeTuple(TList...) {
>     alias TList TypeTuple;
> }
> void main() {
>     char c = 'b';
>     switch (c) {
>         case 'a': printf("1 a\n"); break;
>         foreach (o; TypeTuple!('b', 'c')) {
>             case o: printf("2 %c\n", c); break; break;
>         }
>         default: printf("default");
>     }
> }

It can't help because the second break is by definition unreachable.

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timon.gehr@gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |timon.gehr@gmx.ch
         Resolution|                            |INVALID


--- Comment #2 from timon.gehr@gmx.ch 2012-04-06 07:38:55 PDT ---
Not a bug. break applies to the innermost statement that can be broken out from. This includes foreach.

(I use this idiom often. Use labeled break to break from the switch.)

Please reopen as enhancement if you think the code should be illegal.

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--- Comment #3 from bearophile_hugs@eml.cc 2012-04-06 10:06:56 PDT ---
(In reply to comment #2)
> Not a bug. break applies to the innermost statement that can be broken out from. This includes foreach.
> 
> (I use this idiom often. Use labeled break to break from the switch.)

You are right, thank you.
(My error was to think that "static foreach" doesn't support break.)

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|wrong-code                  |diagnostic
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |
            Summary|Ignored break inside static |switch case fallthrough
                   |foreach                     |error despite a break
                   |                            |inside static foreach


--- Comment #4 from bearophile_hugs@eml.cc 2012-04-06 10:18:05 PDT ---
Reopened, because you have missed the error message in my bug report.

Using a labeled break:


import core.stdc.stdio: printf;
template TypeTuple(TList...) {
    alias TList TypeTuple;
}
void main() {
    char c = 'b';
    MySwitch: switch (c) {
        case 'a': printf("1 a\n"); break;
        foreach (o; TypeTuple!('b', 'c')) {
            case o: printf("2 %c\n", c); break MySwitch;
        }
        default: printf("default");
    }
}


DMD 2.059 beta gives (compiling with -w):

test.d(12): Error: switch case fallthrough - use 'goto default;' if intended

I have also changed the issue title to better reflect the problem, now the Keywords is 'diagnostic' because it's giving a warning where there is nothing to warn against.

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--- Comment #5 from bearophile_hugs@eml.cc 2012-04-06 11:32:39 PDT ---
This compiles with no warnings and it seems to work correctly, but I don't fully understand it:


import core.stdc.stdio: printf;
template TypeTuple(TList...) {
    alias TList TypeTuple;
}
void main() {
    char c = 'b';
    MySwitch: switch (c) {
        case 'a': printf("1 a\n"); break;
        foreach (o; TypeTuple!('b', 'c')) {
            case o: printf("2 %c\n", c); break;
        }
        break;
        default: printf("default");
    }
}


Is it correct? if the break inside here is meant to be the foreach break: { case o: printf("2 %c\n", c); break; }

Then why a single break is enough after:

        foreach (o; TypeTuple!('b', 'c')) {
            case o: printf("2 %c\n", c); break;
        }
        break;

despite the foreach synthesizes more than one switch case?

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--- Comment #6 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2012-04-06 11:37:01 PDT ---
(In reply to comment #5)
> This compiles with no warnings and it seems to work correctly, but I don't fully understand it:
> 
> 
> import core.stdc.stdio: printf;
> template TypeTuple(TList...) {
>     alias TList TypeTuple;
> }
> void main() {
>     char c = 'b';
>     MySwitch: switch (c) {
>         case 'a': printf("1 a\n"); break;
>         foreach (o; TypeTuple!('b', 'c')) {
>             case o: printf("2 %c\n", c); break;
>         }
>         break;
>         default: printf("default");
>     }
> }
> 
> 
> Is it correct? if the break inside here is meant to be the foreach break:

Yes.

> { case o: printf("2 %c\n", c); break; }
No it's {case 0: printf("2 %c\n", c); }

the break did his job already, it can't work twice.


> 
> Then why a single break is enough after:
> 
>         foreach (o; TypeTuple!('b', 'c')) {
>             case o: printf("2 %c\n", c); break;
>         }

Then the code below gives you one break after that statement.
>         break;
> 
> despite the foreach synthesizes more than one switch case?

foreach synthesizes exactly one statement here.

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--- Comment #7 from timon.gehr@gmx.ch 2012-04-06 11:41:49 PDT ---
(In reply to comment #5)
> This compiles with no warnings and it seems to work correctly, but I don't fully understand it:
> 
> 
> import core.stdc.stdio: printf;
> template TypeTuple(TList...) {
>     alias TList TypeTuple;
> }
> void main() {
>     char c = 'b';
>     MySwitch: switch (c) {
>         case 'a': printf("1 a\n"); break;
>         foreach (o; TypeTuple!('b', 'c')) {
>             case o: printf("2 %c\n", c); break;
>         }
>         break;
>         default: printf("default");
>     }
> }
> 
> 
> Is it correct? if the break inside here is meant to be the foreach break: { case o: printf("2 %c\n", c); break; }
> 
> Then why a single break is enough after:
> 
>         foreach (o; TypeTuple!('b', 'c')) {
>             case o: printf("2 %c\n", c); break;
>         }
>         break;
> 
> despite the foreach synthesizes more than one switch case?

Your code is expanded to:

void main() {
    char c = 'b';
    switch (c) {
        case 'a': printf("1 a\n"); break;
        {case 'b': printf("2 %c\n", c); goto break_foreach;}
        {case 'c': printf("2 %c\n", c); goto break_foreach;}
        break_foreach: break;
        default: printf("default");
    }
}

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|                            |INVALID


--- Comment #8 from bearophile_hugs@eml.cc 2012-04-06 13:12:01 PDT ---
(In reply to comment #7)

> Your code is expanded to:
> 
> void main() {
>     char c = 'b';
>     switch (c) {
>         case 'a': printf("1 a\n"); break;
>         {case 'b': printf("2 %c\n", c); goto break_foreach;}
>         {case 'c': printf("2 %c\n", c); goto break_foreach;}
>         break_foreach: break;
>         default: printf("default");
>     }
> }

Thank you again Timon :-) So there is no bug here.

This was not easy to understand for me. (Maybe D newbies will enjoy to read an example of this in some D tips&trickls somewhere, or maybe it was just a conceptualization problem of mine.)

Issue closed again, as invalid.

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Brad Roberts <braddr@puremagic.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
                 CC|                            |braddr@puremagic.com
         Resolution|INVALID                     |


--- Comment #9 from Brad Roberts <braddr@puremagic.com> 2012-04-06 15:11:51 PDT ---
I think that the expansion of the static foreach is wrong.  It explains the behavior, but doesn't excuse it.

I think the bug report is valid.

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