diff -ur icu.org/source/common/unicode/utf16.h icu/source/common/unicode/utf16.h
--- icu.org/source/common/unicode/utf16.h	2020-10-28 22:21:12.000000000 +0100
+++ icu/source/common/unicode/utf16.h	2020-11-16 19:31:03.356478154 +0100
@@ -398,6 +398,7 @@
         (s)[(i)++]=(uint16_t)(((c)&0x3ff)|0xdc00); \
     } else /* c>0x10ffff or not enough space */ { \
         (isError)=true; \
+        (void)(isError); \
     } \
 } UPRV_BLOCK_MACRO_END
 
