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From: "xmh970252187 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/101828] New: The invocation of a virtual function for an
 object that is not usable in constant expression is accepeted by GCC
Date: Mon, 09 Aug 2021 09:53:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101828

            Bug ID: 101828
           Summary: The invocation of a virtual function for an object
                    that is not usable in constant expression is accepeted
                    by GCC
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xmh970252187 at gmail dot com
  Target Milestone: ---

struct A{
    constexpr int virtual fun() const{
        return 0;
    }
};
A obj;
constexpr A const& rf =3D obj;
constexpr int v =3D rf.fun();

According to [expr.const#5.5]

> An expression E is a core constant expression unless the evaluation of E,=
 following the rules of the abstract machine ([intro.execution]), would eva=
luate one of the following:=20

>> an invocation of a virtual function for an object unless
>>> the object is usable in constant expressions or
>>> its lifetime began within the evaluation of E;

Since `obj` is not an object that is usable in constant expression as per
[expr.const#4] and its lifetime has begun before the declaration of `v`, he=
nce
its full-expression of the initialization cannot be a constant expression. =
It
should be ill-formed. https://godbolt.org/z/8hKWs3WEf=