1 files changed, 11 insertions, 0 deletions
diff --git a/perl.h b/perl.h
index 98c6265fd0..be2bf4fbc8 100644
--- a/perl.h
+++ b/perl.h
@@ -23,6 +23,17 @@
 #define VOIDUSED 1
 #include "config.h"
 
+#if defined(USE_ITHREADS) && defined(USE_5005THREADS)
+#  include "error: USE_ITHREADS and USE_5005THREADS are incompatible"
+#endif
+
+/* XXX This next guard can disappear if the sources are revised
+   to use USE_5005THREADS throughout. -- A.D  1/6/2000
+*/
+#if defined(USE_ITHREADS) && defined(USE_THREADS)
+#  include "error: USE_ITHREADS and USE_THREADS are incompatible"
+#endif
+
 /* See L<perlguts/"The Perl API"> for detailed notes on
  * PERL_IMPLICIT_CONTEXT and PERL_IMPLICIT_SYS */