reported upstream as
https://unicode-org.atlassian.net/projects/ICU/issues/ICU-20236

--- icu/source/test/intltest/convtest.cpp.orig
+++ icu/source/test/intltest/convtest.cpp
@@ -744,7 +744,7 @@
     assertEquals("overflow", U_BUFFER_OVERFLOW_ERROR, errorCode.reset());
     length = static_cast<int32_t>(target - result);
     assertEquals("number of bytes written", 2, length);
-    assertEquals("next byte not clobbered", 5, result[2]);
+    assertEquals("next byte not clobbered", 5, (int)result[2]);
 
     // Convert the rest and flush.
     ucnv_convertEx(cnv2.getAlias(), cnv1.getAlias(),
@@ -777,7 +777,7 @@
     assertEquals("text2 overflow", U_BUFFER_OVERFLOW_ERROR, errorCode.reset());
     length = static_cast<int32_t>(target - result);
     assertEquals("text2 number of bytes written", 3, length);
-    assertEquals("text2 next byte not clobbered", 5, result[3]);
+    assertEquals("text2 next byte not clobbered", 5, (int)result[3]);
 
     // Convert the rest and flush.
     ucnv_convertEx(cnv2.getAlias(), cnv1.getAlias(),