[Bug c++/97234] New: Constexpr class-scope array initializer referencing previous elements

[Bug c++/97234] New: Constexpr class-scope array initializer referencing previous elements

[botond at mozilla dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97234

            Bug ID: 97234
           Summary: Constexpr class-scope array initializer referencing
                    previous elements
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: botond at mozilla dot com
  Target Milestone: ---

The following code:

struct S {
    static constexpr int rolling_sum[4]{
        0,
        rolling_sum[0] + 1,
        rolling_sum[1] + 2,
        rolling_sum[2] + 3
    };
};

produces errors when compiled with g++ 10:

test.cpp:4:9: error: ‘rolling_sum’ was not declared in this scope
    4 |         rolling_sum[0] + 1,
      |         ^~~~~~~~~~~
test.cpp:5:9: error: ‘rolling_sum’ was not declared in this scope
    5 |         rolling_sum[1] + 2,
      |         ^~~~~~~~~~~
test.cpp:6:9: error: ‘rolling_sum’ was not declared in this scope
    6 |         rolling_sum[2] + 3
      |   

The code is accepted by clang. It's also accepted by gcc if the array is
declared at namespace scope, leading me to believe that rejecting it at class
scope is likely a bug.

[Bug c++/97234] Constexpr class-scope array initializer referencing previous elements

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97234

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
ICC and MSVC reject it similar to GCC even.  Are you sure this is valid?

[Bug c++/97234] Constexpr class-scope array initializer referencing previous elements

[botond at mozilla dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97234

--- Comment #2 from Botond Ballo <botond at mozilla dot com> ---
I believe it's valid because the point of declaration of a variable is just
before its initializer
(https://timsong-cpp.github.io/cppwp/n4861/basic.scope.pdecl#1), and thus the
variable should be in scope in its initializer.

But I'm not a wording expert and it's possible I'm mistaken, or overlooking
something else that would make this invalid.

[Bug c++/97234] Constexpr class-scope array initializer referencing previous elements

[botond at mozilla dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97234

Botond Ballo <botond at mozilla dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=99059

--- Comment #3 from Botond Ballo <botond at mozilla dot com> ---
Bug 99059 is similar (involving C++17 "inline" rather than constexpr) and
Jonathan Wakely said it's a bug.

[Bug c++/97234] Constexpr class-scope array initializer referencing previous elements

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97234

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-08-17

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, I think this one is too.