[Issue 18712] [Reg 2.072] bogus "switch skips declaration" error with case in mixin

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18712

Walter Bright <bugzilla at digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #4 from Walter Bright <bugzilla at digitalmars.com> ---
Here's what's happening. After a case/default statement, the parser makes the
code between one case/default and the next into a scope. So it looks like:

    int test(int n)
    {
        switch(n)
        {
            mixin("case 0:");
            int x = 1;
            return x;
            case 1:
            {
                int y = 2;
                return y;
            }
            default:
            {
                return -1;
            }
        }
    }

and, of course, now the error message makes sense. `x` is visible to the
following two scopes, whether or not the error message is generated. Any time
the case/default is not directly in the switch body (not nested via { } or a
mixin) the implicit { } scope is not generated. Oops.

Try putting { } in various combinations, and you'll see how it all comes
unglued.

I can't think of any solution that 1) works in all cases and 2) doesn't break a
lot of existing code. So we're just stuck with it. Fortunately, there is a
workaround. Recode the switch like this:

    int test(int n)
    {
        switch(n)
        {
        mixin("case 0:");
        {
                int x = 1;
                return x;
        }
        case 1:
                int y = 2;
                return y;
        default:
                return -1;
        }
    }

I'm going to close this as WONTFIX. If anyone has a brainwave on how to make it
work in all cases without breaking code, reopen with proof.

Note that the fundamental problem is a combination of:

1. allowing case/default statements to appear inside nested scopes

2. implicit generation of scopes between case/default pairs

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