https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106294
Bug ID: 106294
Summary: GCC accepts the undefined behavior operation in a
constant expression
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: xmh970252187 at gmail dot com
Target Milestone: ---
````cpp
enum A {a = 0};
constexpr A e = static_cast<A>(1024);
````
According to [expr.static.cast p10
> If the enumeration type does not have a fixed underlying type, the value is
> unchanged if the original value is within the range of the enumeration values
> ([dcl.enum]), and **otherwise, the behavior is undefined**.
[dcl.enum] p8 defines the range of the enumeration values, which says
> Otherwise, the values of the enumeration are the values representable by a
> hypothetical integer type with **minimal** width M such that all enumerators
> can be represented.
In this case, it is sufficient to represent the value `0` of the unique
enumerator if `M` is `1`. Apparently, the value `1024` cannot be representable
in the hypothetical integer type with minimal width M. So, the full-expression
of the initialization is not a constant expression since the violation of
[expr.const] p5
> an operation that would have undefined behavior as specified in [intro]
> through [cpp];
However, GCC accepts this example without any diagnosis.
