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digitalmars.D.bugs - [Issue 603] New: Undocumented behaviour: case and default create a scope

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=603

           Summary: Undocumented behaviour: case and default create a scope
           Product: D
           Version: 0.175
          Platform: PC
               URL: http://www.digitalmars.com/d/statement.html#SwitchStatem
                    ent
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid, spec
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: smjg iname.com


I've noticed an interesting behaviour: the code between two consecutive case or
default labels creates a scope.

----------
import std.stdio;

void main() {
    int qwert;
    switch (qwert) {
        case 42:
            int yuiop;
            int asdfg;

        default:
            int hjkl = 98;
            writefln(yuiop);
            writefln(asdfg);
            writefln(hjkl);
    }
}
----------
D:\My Documents\Programming\D\Tests\label_scope_2.d(12): Error: undefined
identifier yuiop
D:\My Documents\Programming\D\Tests\label_scope_2.d(13): Error: undefined
identifier asdfg
----------

While this is intuitively sensible (it enables cases to define their own
variables independently of each other), it doesn't follow from the logical code
structure (in which all cases are at one level immediately below the
SwitchStatement) or from anything on the relevant page of the spec.


-- 
Nov 26 2006
next sibling parent reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=603


bugzilla digitalmars.com changed:

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




------- Comment #1 from bugzilla digitalmars.com  2006-12-15 03:10 -------
What happens is the { } after the switch creates a new scope. The case and
default statements are just labels. I believe the grammar implies this
behavior, and no changes are necessary.


-- 
Dec 15 2006
parent Stewart Gordon <smjg_1998 yahoo.com> writes:
d-bugmail puremagic.com wrote:
 ------- Comment #1 from bugzilla digitalmars.com  2006-12-15 03:10 -------
 What happens is the { } after the switch creates a new scope. The case and
 default statements are just labels. I believe the grammar implies this
 behavior, and no changes are necessary.

d.puremagic.com seems to be down at the moment - so I wonder if this'll make it there.... Your marking this INVALID is nonsense. A bug is a bug, whether it's the spec or the compiler that's doing what you intended. If the case and default statements weren't themselves creating scopes, then there would be no "undefined identifier" errors. If the errors aren't coming up when you try my testcase, then DMD has an OS-version-dependent bug. Stewart.
Dec 15 2006
prev sibling next sibling parent d-bugmail puremagic.com writes:
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smjg iname.com changed:

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




------- Comment #2 from smjg iname.com  2006-12-16 20:36 -------
(Originally posted to digitalmars.D.bugs)

A bug is a bug, whether it's the spec or the compiler that's doing what you
intended.  If the case and default statements weren't themselves creating
scopes, then there would be no "undefined identifier" errors.

If the errors aren't coming up when you try my testcase, then DMD has an
OS-version-dependent bug.


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Dec 16 2006
prev sibling next sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=603





------- Comment #3 from baryluk mpi.int.pl  2007-01-07 12:36 -------
We can say that switch do implicit goto, but spec say that useing goto for
skiping initialisation is an error.


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Jan 07 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
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------- Comment #4 from smjg iname.com  2007-01-07 15:06 -------
See issue 602.  Moreover, it isn't clear whether a goto should be allowed to
skip a declaration with no explicit initializer.  While you could question the
validity of the code on this basis, you can't sensibly claim this as the reason
for the particular error message reported.


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Jan 07 2007
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------- Comment #5 from smjg iname.com  2007-01-27 07:48 -------
(In reply to comment #3)
 We can say that switch do implicit goto, but spec say that useing goto for
 skiping initialisation is an error.

With that premise, the code would be invalid even if the default section doesn't touch yuiop or asdfg. It's still skipping initialisation even if what hasn't been initialised is never used. The variables are nonetheless in scope, if you believe the spec rather than the compiler. --
Jan 27 2007
prev sibling next sibling parent d-bugmail puremagic.com writes:
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kamm-removethis incasoftware.de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kamm-
                   |                            |removethis incasoftware.de




------- Comment #6 from kamm-removethis incasoftware.de  2008-06-20 08:09
-------
*** Bug 2155 has been marked as a duplicate of this bug. ***


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Jun 20 2008
prev sibling next sibling parent d-bugmail puremagic.com writes:
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--- Comment #7 from Christian Kamm <kamm-removethis incasoftware.de> 
2009-07-07 11:50:12 PDT ---
The fact that default and case statements create a new scope is evident in the
frontend code:

statements = new Statements();
while (token.value != TOKcase &&
       token.value != TOKdefault &&
       token.value != TOKrcurly)
{
    statements->push(parseStatement(PSsemi | PScurlyscope));
}
s = new CompoundStatement(loc, statements);
s = new ScopeStatement(loc, s);

With this in mind, it would make sense to add this to the section on switch
statements:

Case and default statements create a new scope that contains all statements up
until the next case or default statement with the same parent, or the end of
the enclosing scope.

Example:

switch(i) {
  case 1:
     ...
  case 2:
    if (i) {
      case 3:
        ...
      case 4:
        ...
    }
  case 5:
}

is equivalent to

switch(i) {
  case 1:
  {  ...  }
  case 2:
  {
    if (i) {
      case 3:
      { ... }
      case 4:
      { ... }
    }
  }
  case 5:
}

I'm not marking this as 'patch' because I'm not happy with 'with the same
parent'. Suggestions? Also, can someone suggest a grammar change that would
explain this behavior? Replacing

case ExpressionList : Statement
with
case ExpressionList : ScopeStatement

isn't right as ScopeStatement is either BlockStatement or NonEmptyStatement. I
think we need a new ScopeCaseStatement here.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Sobirari Muhomori <maxmo pochta.ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|rejects-valid               |diagnostic




--- Comment #8 from Sobirari Muhomori <maxmo pochta.ru>  2009-07-08 01:45:16
PDT ---
Scoped case is a step towards switch redesign :) +1.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

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


--- Comment #9 from Walter Bright <bugzilla digitalmars.com> 2010-11-08
00:00:23 PST ---
http://www.dsource.org/projects/phobos/changeset/2129

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

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


--- Comment #10 from Walter Bright <bugzilla digitalmars.com> 2010-11-08
00:08:08 PST ---
Ahh, I'm confused. Wrong fix.

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Walter Bright <bugzilla digitalmars.com> changed:

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


--- Comment #11 from Walter Bright <bugzilla digitalmars.com> 2010-11-08
00:35:29 PST ---
http://www.dsource.org/projects/phobos/changeset/2130

This should do it.

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