[Bug c++/94929] New: GCC 8 chokes on std::max in alignas

[Bug c++/94929] New: GCC 8 chokes on std::max in alignas

[soap at gentoo dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

            Bug ID: 94929
           Summary: GCC 8 chokes on std::max in alignas
           Product: gcc
           Version: 8.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: soap at gentoo dot org
  Target Milestone: ---

The following snippet

  #include <algorithm>

  struct One {};
  struct Two {};
  struct Foo { alignas(std::max(alignof(One), alignof(Two))) char value; };

compiles fine in GCC 9.3, but in GCC 8.4 fails with

test.cpp:5:65: error: requested alignment is not an integer constant
 struct Foo { alignas(std::max(alignof(One), alignof(Two))) char value; };
                                                                 ^~~~~

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
.

*** This bug has been marked as a duplicate of bug 90736 ***

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |6.4.0, 7.5.0, 8.4.0
                 CC|                            |polacek at redhat dot com
     Ever confirmed|0                           |1
           Keywords|                            |rejects-valid
             Status|RESOLVED                    |NEW
      Known to work|                            |10.0, 11.0, 5.5.0, 9.2.0
   Last reconfirmed|                            |2020-05-03
         Resolution|DUPLICATE                   |---

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Hmm, although this was fixed for gcc 9.2 by the patch for PR 90736 it seems to
be a different root cause. This compiled with GCC 5 but not 6, 7 or 8:

template<typename T>
constexpr const T& max(const T& a, const T& b) { return a < b ? b : a; }

struct One {};
struct Two {};
struct Foo { alignas(max(alignof(One), alignof(Two))) char value; };

This regressed with r230365, "Merge C++ delayed folding branch."

Marek, looks like backporting r272219 would fix this.

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |8.5

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
             Status|NEW                         |ASSIGNED
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I'm going to backport the fix to 8 if it passes the usual testing.

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Done in r8-10235-gf6965321b1c00bfb2b9c8407df56bcf38f096088.

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[soap at gentoo dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

--- Comment #5 from David Seifert <soap at gentoo dot org> ---
(In reply to Marek Polacek from comment #3)
> I'm going to backport the fix to 8 if it passes the usual testing.

Hi Marek,
could you also test the inlined code. Defining some const and then using it
alignas worked previously, the problem comes from doing all of it in alignas().

[Bug c++/94929] [8 Regression] GCC 8 chokes on std::max in alignas

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94929

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to David Seifert from comment #5)
> (In reply to Marek Polacek from comment #3)
> > I'm going to backport the fix to 8 if it passes the usual testing.
> 
> Hi Marek,
> could you also test the inlined code. Defining some const and then using it
> alignas worked previously, the problem comes from doing all of it in
> alignas().

Both the original testcase in this PR and the test in Comment 2 now compile
even with GCC 8.