[Bug c++/59329] New: Using `assert(...)` is not allowed in constexpr functions

[Bug c++/59329] New: Using `assert(...)` is not allowed in constexpr functions

[vittorio.romeo at outlook dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

            Bug ID: 59329
           Summary: Using `assert(...)` is not allowed in constexpr
                    functions
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: trivial
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vittorio.romeo at outlook dot com

inline constexpr int exampleFunction(int min, int max)
{
    assert(min <= max);
    return min + max;
}

The above function fails to compile, because of the `assert(min <= max)`. 

g++ reports that a constexpr function may only be composed of a simple return
statement.

clang++ compiles the function successfully.



Is it possible to lift this restriction?

[Bug c++/59329] Using `assert(...)` is not allowed in constexpr functions

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler@googlemail.
                   |                            |com

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
At least it should be supported in c++1y mode, where several constexpr
restrictions had been lifted by the core language (but it currently isn't, not
even in the 4.9.0 trunk).
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From: "gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/59174] [avr] Suboptimal multiplication when indexing an array
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY174

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |avr
           Priority|P3                          |P4
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-12-02
                 CC|                            |gjl at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Confirmed with the trunk.

This is an issue of cost computation in the avr backend.

If you compile with -O2 -mmcu=atmega2560 -mlog=rtx_costs you will see



avr_rtx_costs[test:combine(206)]=true (speed) total6, outer=set:
(mult:HI (reg:HI 47 [ D.1460 ])
    (const_int 10 [0xa]))

...

avr_rtx_costs[test:combine(206)]=true (speed) total2, outer=set:
(plus:HI (ashift:HI (reg:HI 47 [ D.1460 ])
        (const_int 3 [0x3]))
    (ashift:HI (reg:HI 47 [ D.1460 ])
        (const_int 1 [0x1])))


thus the MULT variant is taken as more costly that the ASHIFT + PLUS version.

[Bug c++/59329] Using `assert(...)` is not allowed in constexpr functions

[lucdanton at free dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

lucdanton at free dot fr changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lucdanton at free dot fr

--- Comment #2 from lucdanton at free dot fr ---
As a workaround I thought about (ab)using the comma operator:

    return assert( cond ), expr;

The reasoning being that C guarantees that assert(…) is a void expression
(regardless of NDEBUG). This is rejected as well, although I’m not entirely
sure why. After a quick look at the preprocessed source, it appears GCC rejects
(‘body not a return statement’, again) functions of the form:

//------

void some_extern_func();

constexpr int foo(int i)
{ return (true ? static_cast<void>(0) : some_extern_func()), i; }

//-----

If the third leg of the conditional operator is e.g. throw whatever (a common
trick to ‘halt’ CTFE) everything behaves as expected.

Does this warrant opening another bug?
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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/59872] [4.9 Regression] Cannot move std::map with move-only mapped_type
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idY872

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Thu Jan 23 17:17:04 2014
New Revision: 206994

URL: http://gcc.gnu.org/viewcvs?rev 6994&root=gcc&view=rev
Log:
    PR libstdc++/59872
    * include/bits/stl_map.h (map::operator=(map&&)): Fix comment.
    * include/bits/stl_multimap.h (multimap::operator=(multimap&&)):
    Likewise.
    * include/bits/stl_multiset.h (multiset::operator=(multiset&&)):
    Likewise.
    * include/bits/stl_set.h (set::operator=(set&&)): Likewise.
    * include/bits/stl_tree.h (_Rb_tree::_M_move_data): New overloaded
    functions to perform moving or copying of elements from rvalue tree.
    (_Rb_tree::_Rb_tree(_Rb_tree&&)): Use _M_move_data.
    (_Rb_tree::_Rb_tree(_Rb_tree&&, _Node_allocator&&)): Likewise.
    * testsuite/23_containers/map/59872.cc: New.
    * testsuite/23_containers/map/56613.cc: Remove duplicate include.

Added:
    trunk/libstdc++-v3/testsuite/23_containers/map/59872.cc
Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/bits/stl_map.h
    trunk/libstdc++-v3/include/bits/stl_multimap.h
    trunk/libstdc++-v3/include/bits/stl_multiset.h
    trunk/libstdc++-v3/include/bits/stl_set.h
    trunk/libstdc++-v3/include/bits/stl_tree.h
    trunk/libstdc++-v3/testsuite/23_containers/map/56613.cc

[Bug c++/59329] Using `assert(...)` is not allowed in constexpr functions

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org
           Severity|trivial                     |enhancement

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
G++ doesn't implement the C++14 constexpr improvements yet.

[Bug c++/59329] Using `assert(...)` is not allowed in constexpr functions

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
This works in mainline. I'm adding the testcase and closing the bug.

[Bug c++/59329] Using `assert(...)` is not allowed in constexpr functions

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

--- Comment #5 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Fri Mar 27 17:21:32 2015
New Revision: 221740

URL: https://gcc.gnu.org/viewcvs?rev=221740&root=gcc&view=rev
Log:
2015-03-27  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/59329
    * g++.dg/cpp1y/constexpr-assert1.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp1y/constexpr-assert1.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/59329] Using `assert(...)` is not allowed in constexpr functions

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59329

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.0

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Done.