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The construction VA_OPT(,) is:

• valid in C++20
• valid as an extension to the GNU C preprocessor (unknown versions)

The construction ##VA_ARGS is:

• valid as an extension to the GNU C preprocessor (since C99)
• valid as an extension to the Clang C preprocessor
• undocumented but valid as an extension to the MSVC C preprocessor

References:

• https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
• https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview?view=msvc-160