diff --git a/icu4c/source/i18n/dtptngen.cpp b/icu4c/source/i18n/dtptngen.cpp
index 26ebee2..02be4f0 100644
--- a/icu4c/source/i18n/dtptngen.cpp
+++ b/icu4c/source/i18n/dtptngen.cpp
@@ -2197,6 +2197,11 @@
     copyFrom(other.skeleton);
 }
 
+DateTimeMatcher& DateTimeMatcher::operator=(const DateTimeMatcher& other) {
+    copyFrom(other.skeleton);
+    return *this;
+}
+
 
 void
 DateTimeMatcher::set(const UnicodeString& pattern, FormatParser* fp) {
diff --git a/icu4c/source/i18n/dtptngen_impl.h b/icu4c/source/i18n/dtptngen_impl.h
index 95219f0..ade9f57 100644
--- a/icu4c/source/i18n/dtptngen_impl.h
+++ b/icu4c/source/i18n/dtptngen_impl.h
@@ -238,6 +238,7 @@
     int32_t getDistance(const DateTimeMatcher& other, int32_t includeMask, DistanceInfo& distanceInfo) const;
     DateTimeMatcher();
     DateTimeMatcher(const DateTimeMatcher& other);
+    DateTimeMatcher& operator=(const DateTimeMatcher& other);
     virtual ~DateTimeMatcher();
     int32_t getFieldMask() const;
 };
diff --git a/icu4c/source/i18n/unicode/basictz.h b/icu4c/source/i18n/unicode/basictz.h
index c4d0876..fc2cb8e 100644
--- a/icu4c/source/i18n/unicode/basictz.h
+++ b/icu4c/source/i18n/unicode/basictz.h
@@ -207,6 +207,12 @@
     BasicTimeZone(const BasicTimeZone& source);
 
     /**
+     * Copy assignment.
+     * @stable ICU 3.8
+     */
+    BasicTimeZone& operator=(const BasicTimeZone&) = default;
+
+    /**
      * Gets the set of TimeZoneRule instances applicable to the specified time and after.
      * @param start     The start date used for extracting time zone rules
      * @param initial   Receives the InitialTimeZone, always not NULL
diff --git a/icu4c/source/i18n/unicode/numsys.h b/icu4c/source/i18n/unicode/numsys.h
index b2b16fa..7def703 100644
--- a/icu4c/source/i18n/unicode/numsys.h
+++ b/icu4c/source/i18n/unicode/numsys.h
@@ -74,6 +74,12 @@
     NumberingSystem(const NumberingSystem& other);
 
     /**
+     * Copy assignment.
+     * @stable ICU 4.2
+     */
+    NumberingSystem& operator=(const NumberingSystem& other) = default;
+
+    /**
      * Destructor.
      * @stable ICU 4.2
      */
diff --git a/icu4c/source/test/cintltst/ccapitst.c b/icu4c/source/test/cintltst/ccapitst.c
index 73663f1..aae16f9 100644
--- a/icu4c/source/test/cintltst/ccapitst.c
+++ b/icu4c/source/test/cintltst/ccapitst.c
@@ -3035,7 +3035,7 @@
 
     err = U_ZERO_ERROR;
     myConvName[UCNV_MAX_CONVERTER_NAME_LENGTH-1] = ',';
-    strncpy(myConvName + UCNV_MAX_CONVERTER_NAME_LENGTH, "locale=", 7);
+    memcpy(myConvName + UCNV_MAX_CONVERTER_NAME_LENGTH, "locale=", 7);
     cnv = ucnv_open(myConvName, &err);
     if (cnv || err != U_ILLEGAL_ARGUMENT_ERROR) {
         log_err("4) Didn't get U_ILLEGAL_ARGUMENT_ERROR as expected %s\n", u_errorName(err));
diff --git a/icu4c/source/test/intltest/convtest.cpp b/icu4c/source/test/intltest/convtest.cpp
index 53d8cfd..949bd3b 100644
--- a/icu4c/source/test/intltest/convtest.cpp
+++ b/icu4c/source/test/intltest/convtest.cpp
@@ -1185,9 +1185,13 @@
             cc.offsets=NULL;
         }
         else {
-            memset(resultOffsets, -1, UPRV_LENGTHOF(resultOffsets));
+            for (int32_t i = 0; i < UPRV_LENGTHOF(resultOffsets); i++) {
+                resultOffsets[i] = -1;
+            }
         }
-        memset(result, -1, UPRV_LENGTHOF(result));
+        for (int32_t i = 0; i < UPRV_LENGTHOF(result); i++) {
+            result[i] = -1;
+        }
         errorCode.reset();
         resultLength=stepToUnicode(cc, cnv.getAlias(),
                                 result, UPRV_LENGTHOF(result),
@@ -1615,8 +1619,12 @@
     ok=TRUE;
     for(i=0; i<UPRV_LENGTHOF(steps) && ok; ++i) {
         step=steps[i].step;
-        memset(resultOffsets, -1, UPRV_LENGTHOF(resultOffsets));
-        memset(result, -1, UPRV_LENGTHOF(result));
+        for (int32_t i = 0; i < UPRV_LENGTHOF(resultOffsets); i++) {
+            resultOffsets[i] = -1;
+        }
+        for (int32_t i = 0; i < UPRV_LENGTHOF(result); i++) {
+            result[i] = -1;
+        }
         errorCode=U_ZERO_ERROR;
         resultLength=stepFromUnicode(cc, cnv,
                                 result, UPRV_LENGTHOF(result),
diff --git a/icu4c/source/test/intltest/rbbitst.cpp b/icu4c/source/test/intltest/rbbitst.cpp
index 92e7ade..d5728bd 100644
--- a/icu4c/source/test/intltest/rbbitst.cpp
+++ b/icu4c/source/test/intltest/rbbitst.cpp
@@ -4285,7 +4285,7 @@
                     u_charName(c, U_EXTENDED_CHAR_NAME, cName, sizeof(cName), &status);
 
                     char buffer[200];
-                    snprintf(buffer, 200,
+                    auto ret = snprintf(buffer, UPRV_LENGTHOF(buffer),
                              "%4s %3i :  %1s  %1s  %10s  %-*s  %-40s  %-40s",
                              currentLineFlag.c_str(),
                              ci,
@@ -4295,6 +4295,8 @@
                              classNameSize,
                              mk.classNameFromCodepoint(c).c_str(),
                              mk.getAppliedRule(ci).c_str(), cName);
+                    (void)ret;
+                    U_ASSERT(0 <= ret && ret < UPRV_LENGTHOF(buffer));
 
                     // Output the error
                     if (ci == i) {
diff --git a/icu4c/source/tools/pkgdata/pkgdata.cpp b/icu4c/source/tools/pkgdata/pkgdata.cpp
index ea75302..5ab682e 100644
--- a/icu4c/source/tools/pkgdata/pkgdata.cpp
+++ b/icu4c/source/tools/pkgdata/pkgdata.cpp
@@ -46,6 +46,7 @@
 #include "flagparser.h"
 #include "filetools.h"
 #include "charstr.h"
+#include "uassert.h"
 
 #if U_HAVE_POPEN
 # include <unistd.h>
@@ -1132,12 +1133,15 @@
     int32_t result = 0;
     char cmd[SMALL_BUFFER_MAX_SIZE];
 
-    sprintf(cmd, "cd %s && %s %s %s%s%s",
+    auto ret = snprintf(cmd,
+            SMALL_BUFFER_MAX_SIZE,
+            "cd %s && %s %s %s%s%s",
             targetDir,
             pkgDataFlags[INSTALL_CMD],
             libFileNames[LIB_FILE_VERSION],
-            installDir, PKGDATA_FILE_SEP_STRING, libFileNames[LIB_FILE_VERSION]
-            );
+            installDir, PKGDATA_FILE_SEP_STRING, libFileNames[LIB_FILE_VERSION]);
+    (void)ret;
+    U_ASSERT(0 <= ret && ret < SMALL_BUFFER_MAX_SIZE);
 
     result = runCommand(cmd);
 
@@ -1255,10 +1259,14 @@
                     buffer[bufferLength-1] = 0;
                 }
 
-                sprintf(cmd, "%s %s%s%s %s%s%s",
+                auto ret = snprintf(cmd,
+                        SMALL_BUFFER_MAX_SIZE,
+                        "%s %s%s%s %s%s%s",
                         pkgDataFlags[INSTALL_CMD],
                         srcDir, PKGDATA_FILE_SEP_STRING, buffer,
                         installDir, PKGDATA_FILE_SEP_STRING, buffer);
+                (void)ret;
+                U_ASSERT(0 <= ret && ret < SMALL_BUFFER_MAX_SIZE);
 
                 result = runCommand(cmd);
                 if (result != 0) {
@@ -1690,12 +1698,20 @@
                             break;
                         }
                     }
-                    sprintf(newName, "%s_%s",
+                    auto ret = snprintf(newName,
+                            SMALL_BUFFER_MAX_SIZE,
+                            "%s_%s",
                             DATA_PREFIX[n],
                             newNameTmp);
-                    sprintf(dataName, "%s_%s",
+                    (void)ret;
+                    U_ASSERT(0 <= ret && ret < SMALL_BUFFER_MAX_SIZE);
+                    ret = snprintf(dataName,
+                            SMALL_BUFFER_MAX_SIZE,
+                            "%s_%s",
                             o->shortName,
                             DATA_PREFIX[n]);
+                    (void)ret;
+                    U_ASSERT(0 <= ret && ret < SMALL_BUFFER_MAX_SIZE);
                 }
                 if (newName[0] != 0) {
                     break;