diff -ur icu.org/source/common/unicode/utf16.h icu/source/common/unicode/utf16.h
--- icu.org/source/common/unicode/utf16.h	2019-10-03 13:16:41.000000000 +0200
+++ icu/source/common/unicode/utf16.h	2019-10-28 18:03:07.967208272 +0100
@@ -397,6 +397,7 @@
         (s)[(i)++]=(uint16_t)(((c)&0x3ff)|0xdc00); \
     } else /* c>0x10ffff or not enough space */ { \
         (isError)=TRUE; \
+        (void)(isError); \
     } \
 } UPRV_BLOCK_MACRO_END