If BSON_EXTRA_ALIGN is 0 the BSON_ALIGNED_BEGIN macro defined here is invalid.

MSVC does not allow mathematical expressions in align:

__declspec( align( # ) ) declarator

...

# is the alignment value. Valid entries are integer powers of two from 1 to 8192 (bytes), such as 2, 4, 8, 16, 32, or 64.

https://msdn.microsoft.com/en-us/library/83ythb65.aspx

The cmake file always sets BSON_EXTRA_ALIGN to 1 so this was never noticed.
https://github.com/mongodb/libbson/blob/33851d13/CMakeLists.txt#L70-L92