It's past time to update this macro to the specified value for C++20.
libcpp/ChangeLog:
* init.c (cpp_init_builtins): Update __cplusplus for C++20.
{
if (CPP_OPTION (pfile, lang) == CLK_CXX20
|| CPP_OPTION (pfile, lang) == CLK_GNUCXX20)
- _cpp_define_builtin (pfile, "__cplusplus 201709L");
+ _cpp_define_builtin (pfile, "__cplusplus 202002L");
else if (CPP_OPTION (pfile, lang) == CLK_CXX17
|| CPP_OPTION (pfile, lang) == CLK_GNUCXX17)
_cpp_define_builtin (pfile, "__cplusplus 201703L");