expression must have a constant value / cannot use reinterpret_cast in constant-expression evaluation

[akenter-mathworks]

Hi,

I would like to report an issue but I cannot create one with the New Issue button, so I follow the suggestion in Contributing.md of posting it here in the discussions.

WIth this repro.cpp:

struct S { int a; int b; };
constexpr long off =
reinterpret_cast<const char*>(
&reinterpret_cast<const S*>(16)->b) -
reinterpret_cast<const char*>(16);

I get the following messages on Windows with edgcpfe --microsoft --microsoft_version=1944 --ms_c++20 -w repro.cpp:

"repro.cpp", line 3: error: expression must have a constant value
reinterpret_cast<const char*>(
^
"repro.cpp", line 3: note: cannot use reinterpret_cast in constant-expression evaluation
reinterpret_cast<const char*>(
^

I checked on godbolt.org that all versions of Visual accept this repro. The problem was observed with Boost header protobuf3\include\google\protobuf\implicit_weak_message.h. The reinterpret_casts come from macro PROTOBUF_FIELD_OFFSET, when neither of clang and GNUC is defined.

I investigated and I could solve it by adding these lines:

if (microsoft_mode) {
allow_reinterpret_cast = TRUE;
}

at line 34333 of interpret_dynamic_init_full (immediately after the if (!in_front_end || il_lowering_underway) at the beginning of the function.

That works with our regression tests (we have ~30,000 at my company), but I must admit I didn't verify with the EDG tests, yet.

I built the front-end with this configuration:

#define DEFAULT_USE_PREDEFINED_MACRO_FILE 0
#include "defines_win32.h"

Thanks,
Arjan Kenter