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From: "xry111 at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110401] New: Unhelpful "goto is not a constant expression"
 in ill-formed constexpr function template
Date: Sun, 25 Jun 2023 08:56:41 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110401

            Bug ID: 110401
           Summary: Unhelpful "goto is not a constant expression" in
                    ill-formed constexpr function template
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xry111 at gcc dot gnu.org
  Target Milestone: ---

#include <array>

template <int N>
constexpr std::array<int, N> get_sqr()
{
    std::array<int, N> ret;
    for (int i =3D 0; i < N; i++)
        ret[i] =3D i * i;
    return ret;
}

constexpr auto sqr =3D get_sqr<10000>();

GCC says "error: 'goto' is not a constant expression".  This is puzzling
because there is no "goto" in the function.

Note that if it's not a template, GCC outputs the correct message:

#include <array>

constexpr std::array<int, 10000> get_sqr()
{
    std::array<int, 10000> ret;
    for (int i =3D 0; i < 10000; i++)
        ret[i] =3D i * i;
    return ret;
}

constexpr auto sqr =3D get_sqr();

t.cc: In function 'constexpr std::array<int, 10000> get_sqr()':
t.cc:5:28: error: uninitialized variable 'ret' in 'constexpr' function
    5 |     std::array<int, 10000> ret;
      |                            ^~~=