[Bug c/7652] -Wswitch-break : Warn if a switch case falls through

["michael.chapman at cortus dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=7652

--- Comment #33 from Michael Chapman <michael.chapman at cortus dot com> ---
(In reply to Florian Weimer from comment #30)
> (In reply to Manuel López-Ibáñez from comment #29)
> 
> > I like the previous suggestion of using "goto LABEL;". In fact, the warning
> > message could explicitly say "use %<goto %D;%> to silence this warning".
> 
> Does this mean that you propose a GCC extension which allows to write this?
> 
>      goto 5;
>    case 5:
> 
> I'm not sure if the extension is worth it, and it creates another source of
> errors/unclarities if another switch branch is inserted before "case 5:". 
> It looks like fall-through, but it isn't one because the case labels aren't
> aligned.

Why an extension? What is wrong with:-

        goto case_5;
      case 5: case_5:
         ....
>From gcc-bugs-return-450284-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Apr 30 17:27:48 2014
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From: "ppluzhnikov at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/61020] New: [4.9/4.10 Regression] typeid(typeid(X)) produces 'ud2'
Date: Wed, 30 Apr 2014 17:27:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?ida020

            Bug ID: 61020
           Summary: [4.9/4.10 Regression] typeid(typeid(X)) produces 'ud2'
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppluzhnikov at google dot com

The test case:

#include <typeinfo>

struct Base {
  virtual ~Base() { }
};

struct Derived : public Base {
};

int compare(const Base& base)
{
  return typeid(base) == typeid(typeid(Derived));
}

int main()
{
  Base base;
  Derived derived;

  if (compare(base)) return 1;
  if (compare(derived)) return 2;
  return 0;
}


Using trunk @ r209848
g++ -g t.cc && ./a.out && echo OK
OK

g++ -g t.cc -O2 && ./a.out
Segmentation fault (core dumped)

(gdb) disas main
Dump of assembler code for function main():
   0x00000000004004c0 <+0>:     mov    0x8,%rax
   0x00000000004004c8 <+8>:     ud2
End of assembler dump.


It appears that GCC believes the test to invoke undefined behavior.
However, I don't see anything in the standard to support this.

P.S. Same result in C++98 and C++11
P.P.S. In the original code, the double application of typeid() was a bug.