gh-144490: Fix mimalloc debug build

[yoney]

Fix mimalloc C++ compilation error in debug builds.

The _mi_assert_fail declaration uses mi_decl_throw, which expands to __THROW (for __GNUC__ ). In C, __THROW is __attribute__((__nothrow__)) and works anywhere. In C++, __THROW is noexcept(true), which must come after the parameter list.

Before:

  // C:   __attribute__((__nothrow__)) void _mi_assert_fail(...);
  // C++: noexcept(true) void _mi_assert_fail(...);  <- Error, noexcept(...) must be after the params

After:

  // C:   void _mi_assert_fail(...) __attribute__((__nothrow__));
  // C++: void _mi_assert_fail(...) noexcept(true);

This fix also allows removing the Py_DEBUG guard in test_cppext/extension.cpp, so internal C API headers are now tested in debug builds too.

It’s possible this issue was introduced in #122587, though I haven’t verified it

Test:

$ ./configure --with-pydebug --disable-gil
$ make
$ ./python -m test test.test_cppext -u cpu=1

cc: @vstinner @colesbury @DinoV

• Issue: Enhancement request: make pycore_cell.h compile cleanly under C++ #144490

[kumaraditya303]

Does this issue exists in mimalloc upstream as well?

[yoney]

Does this issue exists in mimalloc upstream as well?

It looks like upstream fixed this earlier. The change is semantically the same: they just moved mi_attr_noexcept to the end. However, the naming differs, our code uses mi_decl_throw, while upstream uses mi_attr_noexcept.

microsoft/mimalloc@715acc0