[Bug c++/110463] New: Mutable subobject is usable in a constant expression

[fchelnokov at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110463

            Bug ID: 110463
           Summary: Mutable subobject is usable in a constant expression
           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: ---

The following program is invalid per
https://timsong-cpp.github.io/cppwp/n4861/expr.const#4.8 (An object or
reference is usable in constant expressions if it is ... a non-mutable
subobject)

struct U {
    mutable int x = 2;
};

int main() {
    constexpr U u{};
    u.x = 1;
    static_assert( u.x == 2 ); // must fail, but ok in GCC
}

but GCC accepts it. Online demo: https://gcc.godbolt.org/z/n8MYzhbad