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* [Bug c++/101828] New: The invocation of a virtual function for an object that is not usable in constant expression is accepeted by GCC
@ 2021-08-09 9:53 xmh970252187 at gmail dot com
2021-08-09 18:00 ` [Bug c++/101828] The invocation of a virtual function for an object that is not usable in constant expression is accepted " pinskia at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: xmh970252187 at gmail dot com @ 2021-08-09 9:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101828
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 = obj;
constexpr int v = 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 evaluate one of the following:
>> 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`, hence
its full-expression of the initialization cannot be a constant expression. It
should be ill-formed. https://godbolt.org/z/8hKWs3WEf
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug c++/101828] The invocation of a virtual function for an object that is not usable in constant expression is accepted by GCC
2021-08-09 9:53 [Bug c++/101828] New: The invocation of a virtual function for an object that is not usable in constant expression is accepeted by GCC xmh970252187 at gmail dot com
@ 2021-08-09 18:00 ` pinskia at gcc dot gnu.org
0 siblings, 0 replies; 2+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-09 18:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101828
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2021-08-09
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
With -pedantic-errors -std=c++17, we do reject it:
<source>:2:19: error: member 'fun' can be declared both 'virtual' and
'constexpr' only in '-std=c++20' or '-std=gnu++20' [-Wc++20-extensions]
2 | constexpr int virtual fun() const{
| ~~~~~~~~~ ^~~~~~~
With -pedantic-errors -std=c++20 we don't reject this even though we should,
clang gives:
Output of x86-64 clang (trunk) (Compiler #2)
<source>:8:15: error: constexpr variable 'v' must be initialized by a constant
expression
constexpr int v = rf.fun();
^ ~~~~~~~~
<source>:8:22: note: virtual function called on object 'obj' whose dynamic type
is not constant
constexpr int v = rf.fun();
^
^ permalink raw reply [flat|nested] 2+ messages in thread
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2021-08-09 9:53 [Bug c++/101828] New: The invocation of a virtual function for an object that is not usable in constant expression is accepeted by GCC xmh970252187 at gmail dot com
2021-08-09 18:00 ` [Bug c++/101828] The invocation of a virtual function for an object that is not usable in constant expression is accepted " pinskia at gcc dot gnu.org
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