diff --git i/src/timezone/private.h w/src/timezone/private.h
index 0d02e620b6..2a9f22816a 100644
--- i/src/timezone/private.h
+++ w/src/timezone/private.h
@@ -184,7 +184,9 @@
  */
 #define INITIALIZE(x)	((x) = 0)
 
+#if __STDC_VERSION__ < 202311 && !defined unreachable
 #define unreachable() pg_unreachable()
+#endif
 
 /*
  * For the benefit of GNU folk...