cmake/Tests/CompileDefinitions/compiletest_mixed_c.c

#ifndef LINK_CXX_DEFINE
#  error Expected LINK_CXX_DEFINE
#endif
#ifndef LINK_LANGUAGE_IS_CXX
#  error Expected LINK_LANGUAGE_IS_CXX
#endif

#ifdef LINK_C_DEFINE
#  error Unexpected LINK_C_DEFINE
#endif
#ifdef LINK_LANGUAGE_IS_C
#  error Unexpected LINK_LANGUAGE_IS_C
#endif

#ifndef C_EXECUTABLE_LINK_LANGUAGE_IS_C
#  error Expected C_EXECUTABLE_LINK_LANGUAGE_IS_C define
#endif

void someFunc(void)
{
}