NFD » ndn-cxx

I'm not getting any undefined behavior warning in this snippet:

// g++ -std=c++11 -o x x.cpp $(pkg-config --cflags --libs libndn-cxx) -fsanitize=undefined

#include <cstdint>

enum class A : uint8_t
{
  A0 = 0,
  A1 = 1,
  A2 = 2
};

int
main()
{
  static_cast<A>(3);
  static_cast<A>(259);
  static_cast<A>(65539);
}

Is the problem with "narrowing conversion", or with uint8_t value with no corresponding named constant?

Junxiao Shi wrote:

I'm not getting any undefined behavior warning in this snippet:

That's irrelevant. You can't assume that UBSan will find (or even knows about) every instance of undefined behavior in a program. Moreover, as I said in the description, technically the behavior is undefined since C++17 only; in C++11 there's no UB, but the resulting value is "unspecified" (which is equally useless and dangerous for all practical purposes).

Is the problem with "narrowing conversion", or with uint8_t value with no corresponding named constant?

Neither. It's already explained in the issue description, but I'll also cite the C++ standard (current draft):

[expr.static.cast]/10
A value of integral or enumeration type can be explicitly converted to a complete enumeration type. The value
is unchanged if the original value is within the range of the enumeration values (10.2). Otherwise, the behavior
is undefined.

Where "the range of the enumeration values" is defined as follows:

[dcl.enum]/8
For an enumeration whose underlying type is fixed, the values of the enumeration are the values of the
underlying type.
[...]
It is possible to define an enumeration that has values not defined by any of its enumerators.