https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118383
Bug ID: 118383
Summary: Constexpr default copy-assign operator of derived
class with virtual base seems should not be allowed
before C++23
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: accepts-invalid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rush102333 at gmail dot com
Target Milestone: ---
Consider the following code:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
struct base { };
struct derived : virtual base {
constexpr derived& operator=(derived const&) = default;
};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
https://godbolt.org/z/evns8zEqq
It appears that the default 'operator=' here inside a class with a virtual base
class is not permitted to be declared as constexpr function until
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2448r2.html#onwards-to-constexpr-classes,
which targets for C++23. That means the code above should not be accepted
before that.
https://timsong-cpp.github.io/cppwp/n4861/dcl.fct.def.default#3.sentence-1
(C++20):
An explicitly-defaulted function that is not defined as deleted may be declared
constexpr or consteval only if it is constexpr-compatible ([special]).
See discussion on LLVM bug tracker:
https://github.com/llvm/llvm-project/issues/97266
