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From: "fchelnokov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/110642] New: Undefined behavior in same constant expression
 is found not always
Date: Wed, 12 Jul 2023 13:52:28 +0000
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X-Bugzilla-Version: 13.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110642

            Bug ID: 110642
           Summary: Undefined behavior in same constant expression is
                    found not always
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This lambda

auto f =3D [](int i) {
    const auto y =3D i;
    return [&] () {
        return y;
    };
};

returns another lambda that makes undefined behavior: it accesses a stack
variable (y) after the end of its life.

And it is successfully caught during constant expression evaluation by GCC =
and
other compilers, if verified like this:

constexpr auto g =3D f(3)();
static_assert( f(3)() =3D=3D 3 );
Online demo: https://gcc.godbolt.org/z/3Y4cGMG15

But if one changes the order of lines:

static_assert( f(3)() =3D=3D 3 );
constexpr auto g =3D f(3)();

then GCC stops detecting undefined behavior. Online demo:
https://gcc.godbolt.org/z/dojhKasPs

Could you please review why this happens?=