[Bug c++/95701] New: undefined enum conversion accepted in constant expression

[leni536 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95701

            Bug ID: 95701
           Summary: undefined enum conversion accepted in constant
                    expression
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leni536 at gmail dot com
  Target Milestone: ---

gcc accepts the following code:

enum E {
    one = 1
};

constexpr E foo() {
    return static_cast<E>(0xffff);
}

void bar() {
    constexpr auto e = foo();
}

Relevant parts of the standard:

https://timsong-cpp.github.io/cppwp/n4659/expr.const#2.6

https://timsong-cpp.github.io/cppwp/n4659/expr.static.cast#10
> The value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]). Otherwise, the behavior is undefined.

https://timsong-cpp.github.io/cppwp/n4659/dcl.enum#8
Following the wording here the "range of the enumeration values" is [0, 1].