Just one to point out that the assumption in the motivating example
(that started this discussion) is incorrect:

> On 5/17/23 2:16 AM, "许传奇(夜灯)" <chuanqi.xcq_at_[hidden]> wrote:
>
> > First, the motivation example:
> >
> > ```
> > // a.cpp
> > import "foo.h";
> >
> > // b.cpp
> > #define M 43
> > import "foo.h"
> > ```
> >
> > The question is, for build systems, how can they know if they should
> > use the same BMI for these 2 imports. Since **currently** the build
> > systems don't have a **good** method to know whether a macro is
> > interesting to a specific file.

The standard is pretty clear that an imported header does not "see"
any macros defined by the importer.