[Bug c++/64842] New: Implicitly defined constructor isn't constexpr

[Bug c++/64842] New: Implicitly defined constructor isn't constexpr

[simendsjo at simendsjo dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64842

            Bug ID: 64842
           Summary: Implicitly defined constructor isn't constexpr
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: simendsjo at simendsjo dot me

The following short example is a (somewhat reduced) example from
Bjarne Stroustroups book Programming Principles and Practice using
C++ 2nd edition first print, section 8.5.9 on page 290.

I'm able to reproduce the behavior on shorter examples, but I'm
afraid to do so as I don't know enough C++ to know if I'm hiding
the actual error.

After mailing Stroustrup about the issue, he suggested I added
a bug report.

He noted that I should copy the following text in the bugreport:

    12.8 Copying and moving class objects [class.copy]

    A copy/move constructor that is defaulted and not defined as
    deleted is implicitly defined if it is odrused (3.2) or when
    it is explicitly defaulted after its first declaration.
    [ Note: The copy/move constructor is implicitly defined even
    if the implementation elided its odr-use (3.2, 12.2). —end
    note ] If the implicitlydefined constructor would satisfy
    the requirements of a constexpr constructor (7.1.5), the
    implicitly-defined constructor is constexpr.

----

The code in question that fails to compile (a bit reduced from the
book example):

    struct Point { double x, y; };

    constexpr double xscale = 10;
    constexpr double yscale = 0.8;

    constexpr Point scale(Point p) { return 
{xscale*p.x,yscale*p.y};}

    int main() {
        Point p2 {10,10};
        constexpr Point p6 = scale(p2); // Error 'p2 not usable 
in constant expression'
    }

----

The compiler arguments used and the error it produces:

    $ g++ -std=c++11 -c -o test.o test.cpp

    test.cpp: In function ‘int main()’:
    test.cpp:10:34: error: the value of ‘p2’ is not usable in a 
constant expression
         constexpr Point p6 = scale(p2); // Error 'p2 not usable 
in constant expression'
                                      ^
    test.cpp:9:11: note: ‘p2’ was not declared ‘constexpr’
         Point p2 {10,10};
               ^
----

Versions used:
    $ gcc --version
    gcc (GCC) 4.9.2 20141224 (prerelease)
    Copyright (C) 2014 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.

    $ g++ --version
    g++ (GCC) 4.9.2 20141224 (prerelease)
    Copyright (C) 2014 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.

    $ uname -a
    Linux simendsjo-desktop 3.16.6-1-ck-bamboo #1 SMP PREEMPT Mon 
Jan 5 20:15:19 CET 2015 x86_64 GNU/Linux

Let me know if you need more information in order to reproduce
this.
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd837

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> ---
There is also a gold bug:

https://sourceware.org/bugzilla/show_bug.cgi?id\x17896

[Bug c++/64842] Implicitly defined constructor isn't constexpr

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64842

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |ville.voutilainen at gmail dot com
         Resolution|---                         |INVALID

--- Comment #1 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
The constructors for Point are constexpr, but since p2 is not, passing
it as an argument for scale() means that the invocation of scale() will not
yield a constant expression. If you change the declaration of p2 to
be
constexpr Point p2 {10,10};
the code will work. Clang agrees with gcc.

[Bug c++/64842] Implicitly defined constructor isn't constexpr

[simendsjo at simendsjo dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64842

--- Comment #2 from simendsjo at simendsjo dot me ---
(In reply to Ville Voutilainen from comment #1)
> The constructors for Point are constexpr, but since p2 is not, passing
> it as an argument for scale() means that the invocation of scale() will not
> yield a constant expression. If you change the declaration of p2 to
> be
> constexpr Point p2 {10,10};
> the code will work. Clang agrees with gcc.

Then there are a couple of possibilities:
1. The spec is wrong
2. Bjarne Stroustrup misinterprets the spec
3. gcc and clang has the same bug

You're saying that 2. holds. In that case, send a mail to Stroustrup so he can
correct his book "Programming: Principles and Practice using C++".