[Bug c++/67193] New: Overzealous -Wstack-usage warning

["tobi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67193

            Bug ID: 67193
           Summary: Overzealous -Wstack-usage warning
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tobi at gcc dot gnu.org
  Target Milestone: ---

-Wstack-usage throws a warning if it cannot prove that the stack is bounded. 
Therefore it is more noisy with optimization turned off.  That is fine, but
sometimes it's really too noisy, such as in this example (this is with current
trunk):

$ cat t.cc
#include <alloca.h>

void f(char *p) {
  char *q = (char*)alloca(1);
  *q = *p;
  return;
}
$ bin/g++ -c -Wstack-usage=898989 t.cc
t.cc: In function ‘void f(char*)’:
t.cc:3:6: warning: stack usage might be unbounded [-Wstack-usage=]
 void f(char *p) {
      ^
$ bin/g++ -O -c -Wstack-usage=898989 t.cc
$ bin/g++ --version
g++ (GCC) 6.0.0 20150812 (experimental)
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ 

Constant folding should suffice to get rid of this warning but apparently
alloca is converted to some more complicated operation before the warning is
issued.  (I know that alloca with a fixed argument is fairly pointless, but
this can happen with templates.)
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From: "vanyacpp at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/63809] Missing warning on extra template
Date: Wed, 12 Aug 2015 16:09:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc809

--- Comment #2 from Ivan Sorokin <vanyacpp at gmail dot com> ---
(In reply to Marc Glisse from comment #0)
> g++ fails to warn about the extra parameter list. clang does warn, see
> PR63750.

Why is it a warning? Isn't this code incorrect? I can not find anything in
[temp.expl.spec] that forbid a code like this, but I can not find anything that
assign a specific meaning to this code either.

At least one compiler (MSVC) rejects this code with an error.