I don't see why this is a problem for reusing precompiled headers in the way the parent post suggested. If the compiler records the value of every #define when

I don't see why this is a problem for reusing precompiled headers in the way the parent post suggested. If the compiler records the value of every #define when it precompiles b.h, if b.h is #included again with the same #defines set, the compiler can reuse the same precompiled version of b.h. Indeed, if the compiler-buildsystem detects that b.h has been #included with a different set of #defines, it might be a good idea to issue a warning because that could be an indication that the codebase is about to footgun itself.