After reading Walter's remark "ImportC sees those macro definitions and transforms them into manifest constant declarations" in

Issue 23622 - ImportC #defines conflict with declarations

I wondered how it was implemented:

myccode.c

int getx ()
{
   return X;
#define X 1
}

imc.d

unittest {
   import myccode;
   assert (false, "Should img.c really compile?");
}

dmd -O -checkaction=context -unittest -main myccode.c -run imc.d
imc.d(3): [unittest] Should img.c really compile?
1/1 modules FAILED unittests

On Friday, 13 January 2023 at 12:50:44 UTC, kdevel wrote:

In general, no. The purpose is to build / interface with existing C code, not to develop new C code with it. ImportC also has its own extensions by borrowing D features such as __import, CTFE, and forward references. A strict C compiler would reject those.