[Bug c++/64372] New: Spurious warning with throw in ternary operator returning const reference

[Bug c++/64372] New: Spurious warning with throw in ternary operator returning const reference

[tavianator at gmail dot com]

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

            Bug ID: 64372
           Summary: Spurious warning with throw in ternary operator
                    returning const reference
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tavianator at gmail dot com

The following program produces a spurious warning:

$ cat ternary-warning.cpp
const char&
foo(const char* ptr)
{
  return ptr ? *ptr : throw ptr;
}
$ g++ -Wall -c ternary-warning.cpp
ternary-warning.cpp: In function ‘const char& foo(const char*)’:
ternary-warning.cpp:4:29: warning: returning reference to temporary
[-Wreturn-local-addr]
   return ptr ? *ptr : throw ptr;
                             ^
>From gcc-bugs-return-471518-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Sun Dec 21 18:47:05 2014
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Subject: [Bug target/62631] gcc.dg/tree-ssa/ivopts-lt-2.c FAILs
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idb631

--- Comment #11 from John David Anglin <danglin at gcc dot gnu.org> ---
Created attachment 34308
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id4308&actioníit
hppa ivopts dump

[Bug c++/64372] Spurious warning with throw in ternary operator returning const reference

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |glisse at gcc dot gnu.org,
                   |                            |manu at gcc dot gnu.org
      Known to fail|                            |4.9.2, 5.0

--- Comment #1 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
I think *p creates a temporary that only lasts until the end of the function,
so the code is undefined. Probably even:

const char&
foz(const char* p)
{
  return *p;
}

is undefined. However, G++ and Clang currently handles this as returning p:

;; Function const char& foz(const char*) (null)
;; enabled by -tree-original


return <retval> = (const char &) NON_LVALUE_EXPR <p>;


;; Function const char& foo(const char*, const char*) (null)
;; enabled by -tree-original


return <retval> = (const char &) (p != 0B ? NON_LVALUE_EXPR <p> :
NON_LVALUE_EXPR <q>);


;; Function const char& fzz(const char*) (null)
;; enabled by -tree-original


<<cleanup_point return <retval> = (const char &) &TARGET_EXPR <D.2338, p != 0B
? (char) *p : <<< Unknown tree: throw_expr
  <<cleanup_point TARGET_EXPR <D.2335, __cxa_allocate_exception (8)>;, *(const
char * *) D.2335 = p;>>;, __cxa_throw (D.2335, (void *) &_ZTIPKc, 0B); >>>>;,
0>>;


Note that in the testcase that throws, the gimple indicates that the reference
is set to null even when not throwing, which leads to a segmentation fault.

const char& fzz(const char*) (const char * p)
{
  const charD.10 & D.2348;
  const charD.10 D.2338;
  charD.10 iftmp.1D.2349;
  voidD.45 * D.2335;

  [test.cc:18:24] if (pD.2330 != 0B) goto <D.2350>; else goto <D.2351>;
  <D.2350>:
  [test.cc:18:24] iftmp.1D.2349 = [test.cc:18:24] *pD.2330;
  goto <D.2352>;
  <D.2351>:
  [test.cc:18:24] # USE = anything 
  # CLB = anything 
  D.2335 = __cxa_allocate_exceptionD.2334 (8);
  [test.cc:18:24] try
    {
      [test.cc:18:24] [test.cc:18:24] MEM[(const charD.10 * *)D.2335] =
pD.2330;
    }
  catch
    {
      [test.cc:18:24] # USE = anything 
      # CLB = anything 
      __cxa_free_exceptionD.2336 (D.2335);
    }
  [test.cc:18:24] # USE = anything 
  # CLB = anything 
  __cxa_throwD.2333 (D.2335, &_ZTIPKcD.2337, 0B);
  <D.2352>:
  [test.cc:18:24] D.2338 = iftmp.1D.2349;
  [test.cc:18:24] try
    {
      [test.cc:18:24] D.2348 = 0;
      [test.cc:18:24] return D.2348;
    }
  finally
    {
      [test.cc:18:24] D.2338 = {CLOBBER};
    }
}
>From gcc-bugs-return-471527-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Mon Dec 22 02:26:43 2014
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ida864

Eric Gallager <egall at gwmail dot gwu.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egall at gwmail dot gwu.edu

--- Comment #2 from Eric Gallager <egall at gwmail dot gwu.edu> ---
this(In reply to Chengnian Sun from comment #0)
> Currently, GCC supports -Wswitch and -Wswitch-enum.

It also supports "-Wswitch-default", which seems to be the exact opposite of
clang's "-Wcovered-switch-default". Personally I think gcc's "-Wswitch-default"
is the better one to listen to; having a default case is a good back-up even if
it will never actually be reached. Here is a link to a post that explains it
better than I could: http://programmers.stackexchange.com/a/242896/161051

[Bug c++/64372] Spurious warning with throw in ternary operator returning const reference

[harald at gigawatt dot nl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

Harald van Dijk <harald at gigawatt dot nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |harald at gigawatt dot nl

--- Comment #2 from Harald van Dijk <harald at gigawatt dot nl> ---
(In reply to Manuel López-Ibáñez from comment #1)
> Probably even:
> 
> const char&
> foz(const char* p)
> {
>   return *p;
> }
> 
> is undefined.

No, that is fine. *p is an lvalue already, so no temporary gets created.

The original code was supposed to get a warning because ptr ? *ptr : throw ptr
was not an lvalue: for a ? b : c to be an lvalue, both b and c needed to be
lvalues.

However, the C++ standard has since changed, and if one operand is a
throw-expression, and the other is an lvalue, the result is now an lvalue. CWG
issue 1560:
  http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1560

So in -std=c++11 mode, there should be no temporary, and no warning. (I haven't
checked if current versions of GCC already implement that, but I can find no
bug reports about it.)
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From: "harald at gigawatt dot nl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/64372] Spurious warning with throw in ternary operator returning const reference
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd372

--- Comment #3 from Harald van Dijk <harald at gigawatt dot nl> ---
BTW, *(ptr ? ptr : throw ptr) would be fine, would not create any temporary, in
any of the language standards.

[Bug c++/64372] Spurious warning with throw in ternary operator returning const reference

[tavianator at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

--- Comment #4 from Tavian Barnes <tavianator at gmail dot com> ---
I meant to include -std=c++11 in the OP, it still happens with that flag set. 
Good suggestion for the replacement though.  The actual code was closer to i <
length ? a[i] : throw ... but I guess that can become (i < length ? a : throw
...)[i].

[Bug c++/64372] [DR1560] Gratuitous lvalue-to-rvalue conversion in conditional-expression with throw-expression operand

[harald at gigawatt dot nl]

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

--- Comment #7 from Harald van Dijk <harald at gigawatt dot nl> ---
(In reply to Manuel López-Ibáñez from comment #6)

Even that is probably too much right now. :) I do hope to be able to make some
bigger contributions in the future (in code too), and I really appreciate your
comment.
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Subject: [Bug c++/64385] New: odd behaviour of std::is_move_constructible< std::ostringstream >
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd385

            Bug ID: 64385
           Summary: odd behaviour of std::is_move_constructible<
                    std::ostringstream >
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alan at birtles dot org.uk

Created attachment 34320
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id4320&actioníit
file to reproduce the bug

When compiling the attached code without "A" defined the code outputs 0, 0
which is expected as std::ostringstream doesn't have a move constructor in gcc
4.9. However when "A" is defined the code outputs 1, 1 which is incorrect.
Compiled with "g++ test.cpp -std=c++11 -D A" on Ubuntu 14.10.
g++ --version:
g++ (Ubuntu 4.9.1-16ubuntu6) 4.9.1

[Bug c++/64372] [DR1560] Gratuitous lvalue-to-rvalue conversion in conditional-expression with throw-expression operand

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zhonghao at pku dot org.cn

--- Comment #17 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 86475 has been marked as a duplicate of this bug. ***

[Bug c++/64372] [DR1560] Gratuitous lvalue-to-rvalue conversion in conditional-expression with throw-expression operand

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64372

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kaballo86 at hotmail dot com

--- Comment #18 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 61736 has been marked as a duplicate of this bug. ***