This reverts commit 9b2ab41037f45ad92ab4e850591093ffc45d3e10 and
reinstates e62dc1f6252c1dcdcc2a64e8e3b07a32412e9d89 with changes.
This fix is speculative, since I don't have access to a crashing test
case for the old code, and fixing the crash bug on Windows when C++20 is
enabled seems more important than running it down.
This reverts commit e62dc1f6252c1dcdcc2a64e8e3b07a32412e9d89.
Reverting as per discussion with the patch author.
This patch causes module import error, but there was no intended
behavior change for code that does not use Microsoft extensions.
An enum may be considered to be a complete type if it was forward
declared. It may be declared with a fixed underlying type, or, in MSVC
compatiblity mode, with no type at all.
Previously, the code was written with special handling for fixed enums.
I generalized the code to check if the underlying integer type is known,
which should be the case when targetting the MSVC C++ ABI.
Fixes PR45409
