error: integer value 31 is outside the valid range of values [0, 1] for the enumeration type '_SYSTEM_INFORMATION_CLASS'

[phallot]

When compiling with a recent clang (17+ if I am not mistaken), the following error is raised:
error: integer value 31 is outside the valid range of values [0, 1] for the enumeration type '_SYSTEM_INFORMATION_CLASS' [-Wenum-constexpr-conversion]

This refers to:
constexpr auto SystemPerformanceTraceInformation{ static_cast<SYSTEM_INFORMATION_CLASS>(0x1f) };

And indeed SYSTEM_INFORMATION_CLASS is declared as:

typedef enum _SYSTEM_INFORMATION_CLASS {
} SYSTEM_INFORMATION_CLASS;

I don't understand what this cast is supposed to achieve, but the compiler looks right to me: casting a value to a non-existing enum value is UB in C++.
If 0x1f/31 is some kind of default that needs to be supported, then I guess it should be added as an enum value?

Thanks!

[swannman]

0x1F corresponds to SystemPerformanceTraceInformation: https://www.geoffchappell.com/studies/windows/km/ntoskrnl/inc/api/ntetw/event_trace_performance_information.htm

[phallot]

Hello @swannman, I think you closed this a bit too quickly.
Indeed the value exists, but the issue is casting to a type too small to contain it.

The size of an enum in implementation defined, and in the case of Clang, it's too small to contain 0x1f.

It would make sense to fix the size of the enum, for instance:

typedef enum _SYSTEM_INFORMATION_CLASS : uint8_t {
} SYSTEM_INFORMATION_CLASS;

To avoid the issue with the cast.

Thanks.

[swannman]

@phallot thank you for asking us to reconsider this! @kallanreed resolved this just now with #269