diff --git a/icu4c/source/common/locid.cpp b/icu4c/source/common/locid.cpp
index f1b25f4..a6a5182 100644
--- a/icu4c/source/common/locid.cpp
+++ b/icu4c/source/common/locid.cpp
@@ -740,46 +740,10 @@
         return;
     }
 
-    // The maximized locale ID string is often longer, but there is no good
-    // heuristic to estimate just how much longer. Leave that to CharString.
     CharString maximizedLocaleID;
-    int32_t maximizedLocaleIDCapacity = static_cast<int32_t>(uprv_strlen(fullName));
-
-    char* buffer;
-    int32_t reslen;
-
-    for (;;) {
-        buffer = maximizedLocaleID.getAppendBuffer(
-                /*minCapacity=*/maximizedLocaleIDCapacity,
-                /*desiredCapacityHint=*/maximizedLocaleIDCapacity,
-                maximizedLocaleIDCapacity,
-                status);
-
-        if (U_FAILURE(status)) {
-            return;
-        }
-
-        reslen = uloc_addLikelySubtags(
-                fullName,
-                buffer,
-                maximizedLocaleIDCapacity,
-                &status);
-
-        if (status != U_BUFFER_OVERFLOW_ERROR) {
-            break;
-        }
-
-        maximizedLocaleIDCapacity = reslen;
-        status = U_ZERO_ERROR;
-    }
-
-    if (U_FAILURE(status)) {
-        return;
-    }
-
-    maximizedLocaleID.append(buffer, reslen, status);
-    if (status == U_STRING_NOT_TERMINATED_WARNING) {
-        status = U_ZERO_ERROR;  // Terminators provided by CharString.
+    {
+        CharStringByteSink sink(&maximizedLocaleID);
+        ulocimp_addLikelySubtags(fullName, sink, &status);
     }
 
     if (U_FAILURE(status)) {
@@ -798,50 +762,10 @@
         return;
     }
 
-    // Except for a few edge cases (like the empty string, that is minimized to
-    // "en__POSIX"), minimized locale ID strings will be either the same length
-    // or shorter than their input.
     CharString minimizedLocaleID;
-    int32_t minimizedLocaleIDCapacity = static_cast<int32_t>(uprv_strlen(fullName));
-
-    char* buffer;
-    int32_t reslen;
-
-    for (;;) {
-        buffer = minimizedLocaleID.getAppendBuffer(
-                /*minCapacity=*/minimizedLocaleIDCapacity,
-                /*desiredCapacityHint=*/minimizedLocaleIDCapacity,
-                minimizedLocaleIDCapacity,
-                status);
-
-        if (U_FAILURE(status)) {
-            return;
-        }
-
-        reslen = uloc_minimizeSubtags(
-                fullName,
-                buffer,
-                minimizedLocaleIDCapacity,
-                &status);
-
-        if (status != U_BUFFER_OVERFLOW_ERROR) {
-            break;
-        }
-
-        // Because of the internal minimal buffer size of CharString, I can't
-        // think of any input data for which this could possibly ever happen.
-        // Maybe it would be better replaced with an assertion instead?
-        minimizedLocaleIDCapacity = reslen;
-        status = U_ZERO_ERROR;
-    }
-
-    if (U_FAILURE(status)) {
-        return;
-    }
-
-    minimizedLocaleID.append(buffer, reslen, status);
-    if (status == U_STRING_NOT_TERMINATED_WARNING) {
-        status = U_ZERO_ERROR;  // Terminators provided by CharString.
+    {
+        CharStringByteSink sink(&minimizedLocaleID);
+        ulocimp_minimizeSubtags(fullName, sink, &status);
     }
 
     if (U_FAILURE(status)) {
diff --git a/icu4c/source/common/loclikely.cpp b/icu4c/source/common/loclikely.cpp
index 8ceb352..d4df914 100644
--- a/icu4c/source/common/loclikely.cpp
+++ b/icu4c/source/common/loclikely.cpp
@@ -19,6 +19,7 @@
 *   that then do not depend on resource bundle code and likely-subtags data.
 */
 
+#include "unicode/bytestream.h"
 #include "unicode/utypes.h"
 #include "unicode/locid.h"
 #include "unicode/putil.h"
@@ -26,6 +27,8 @@
 #include "unicode/uloc.h"
 #include "unicode/ures.h"
 #include "unicode/uscript.h"
+#include "bytesinkutil.h"
+#include "charstr.h"
 #include "cmemory.h"
 #include "cstring.h"
 #include "ulocimp.h"
@@ -145,12 +148,10 @@
  * @param trailing Any trailing data to append to the new tag.
  * @param trailingLength The length of the trailing data.
  * @param alternateTags A string containing any alternate tags.
- * @param tag The output buffer.
- * @param tagCapacity The capacity of the output buffer.
+ * @param sink The output sink receiving the tag string.
  * @param err A pointer to a UErrorCode for error reporting.
- * @return The length of the tag string, which may be greater than tagCapacity, or -1 on error.
  **/
-static int32_t U_CALLCONV
+static void U_CALLCONV
 createTagStringWithAlternates(
     const char* lang,
     int32_t langLength,
@@ -161,16 +162,13 @@
     const char* trailing,
     int32_t trailingLength,
     const char* alternateTags,
-    char* tag,
-    int32_t tagCapacity,
+    icu::ByteSink& sink,
     UErrorCode* err) {
 
     if (U_FAILURE(*err)) {
         goto error;
     }
-    else if (tag == NULL ||
-             tagCapacity <= 0 ||
-             langLength >= ULOC_LANG_CAPACITY ||
+    else if (langLength >= ULOC_LANG_CAPACITY ||
              scriptLength >= ULOC_SCRIPT_CAPACITY ||
              regionLength >= ULOC_COUNTRY_CAPACITY) {
         goto error;
@@ -184,7 +182,6 @@
          **/
         char tagBuffer[ULOC_FULLNAME_CAPACITY];
         int32_t tagLength = 0;
-        int32_t capacityRemaining = tagCapacity;
         UBool regionAppended = FALSE;
 
         if (langLength > 0) {
@@ -311,55 +308,28 @@
             }
         }
 
-        {
-            const int32_t toCopy =
-                tagLength >= tagCapacity ? tagCapacity : tagLength;
-
-            /**
-             * Copy the partial tag from our internal buffer to the supplied
-             * target.
-             **/
-            uprv_memcpy(
-                tag,
-                tagBuffer,
-                toCopy);
-
-            capacityRemaining -= toCopy;
-        }
+        /**
+         * Copy the partial tag from our internal buffer to the supplied
+         * target.
+         **/
+        sink.Append(tagBuffer, tagLength);
 
         if (trailingLength > 0) {
-            if (*trailing != '@' && capacityRemaining > 0) {
-                tag[tagLength++] = '_';
-                --capacityRemaining;
-                if (capacityRemaining > 0 && !regionAppended) {
+            if (*trailing != '@') {
+                sink.Append("_", 1);
+                if (!regionAppended) {
                     /* extra separator is required */
-                    tag[tagLength++] = '_';
-                    --capacityRemaining;
+                    sink.Append("_", 1);
                 }
             }
 
-            if (capacityRemaining > 0) {
-                /*
-                 * Copy the trailing data into the supplied buffer.  Use uprv_memmove, since we
-                 * don't know if the user-supplied buffers overlap.
-                 */
-                const int32_t toCopy =
-                    trailingLength >= capacityRemaining ? capacityRemaining : trailingLength;
-
-                uprv_memmove(
-                    &tag[tagLength],
-                    trailing,
-                    toCopy);
-            }
+            /*
+             * Copy the trailing data into the supplied buffer.
+             */
+            sink.Append(trailing, trailingLength);
         }
 
-        tagLength += trailingLength;
-
-        return u_terminateChars(
-                    tag,
-                    tagCapacity,
-                    tagLength,
-                    err);
+        return;
     }
 
 error:
@@ -373,8 +343,6 @@
         U_SUCCESS(*err)) {
         *err = U_ILLEGAL_ARGUMENT_ERROR;
     }
-
-    return -1;
 }
 
 /**
@@ -398,12 +366,10 @@
  * @param regionLength The length of the region tag.
  * @param trailing Any trailing data to append to the new tag.
  * @param trailingLength The length of the trailing data.
- * @param tag The output buffer.
- * @param tagCapacity The capacity of the output buffer.
+ * @param sink The output sink receiving the tag string.
  * @param err A pointer to a UErrorCode for error reporting.
- * @return The length of the tag string, which may be greater than tagCapacity.
  **/
-static int32_t U_CALLCONV
+static void U_CALLCONV
 createTagString(
     const char* lang,
     int32_t langLength,
@@ -413,11 +379,10 @@
     int32_t regionLength,
     const char* trailing,
     int32_t trailingLength,
-    char* tag,
-    int32_t tagCapacity,
+    icu::ByteSink& sink,
     UErrorCode* err)
 {
-    return createTagStringWithAlternates(
+    createTagStringWithAlternates(
                 lang,
                 langLength,
                 script,
@@ -427,8 +392,7 @@
                 trailing,
                 trailingLength,
                 NULL,
-                tag,
-                tagCapacity,
+                sink,
                 err);
 }
 
@@ -576,7 +540,7 @@
     goto exit;
 }
 
-static int32_t U_CALLCONV
+static UBool U_CALLCONV
 createLikelySubtagsString(
     const char* lang,
     int32_t langLength,
@@ -586,17 +550,14 @@
     int32_t regionLength,
     const char* variants,
     int32_t variantsLength,
-    char* tag,
-    int32_t tagCapacity,
-    UErrorCode* err)
-{
+    icu::ByteSink& sink,
+    UErrorCode* err) {
     /**
      * ULOC_FULLNAME_CAPACITY will provide enough capacity
      * that we can build a string that contains the language,
      * script and region code without worrying about overrunning
      * the user-supplied buffer.
      **/
-    char tagBuffer[ULOC_FULLNAME_CAPACITY];
     char likelySubtagsBuffer[ULOC_FULLNAME_CAPACITY];
 
     if(U_FAILURE(*err)) {
@@ -610,25 +571,28 @@
 
         const char* likelySubtags = NULL;
 
-        createTagString(
-            lang,
-            langLength,
-            script,
-            scriptLength,
-            region,
-            regionLength,
-            NULL,
-            0,
-            tagBuffer,
-            sizeof(tagBuffer),
-            err);
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
+            createTagString(
+                lang,
+                langLength,
+                script,
+                scriptLength,
+                region,
+                regionLength,
+                NULL,
+                0,
+                sink,
+                err);
+        }
         if(U_FAILURE(*err)) {
             goto error;
         }
 
         likelySubtags =
             findLikelySubtags(
-                tagBuffer,
+                tagBuffer.data(),
                 likelySubtagsBuffer,
                 sizeof(likelySubtagsBuffer),
                 err);
@@ -640,7 +604,7 @@
             /* Always use the language tag from the
                maximal string, since it may be more
                specific than the one provided. */
-            return createTagStringWithAlternates(
+            createTagStringWithAlternates(
                         NULL,
                         0,
                         NULL,
@@ -650,9 +614,9 @@
                         variants,
                         variantsLength,
                         likelySubtags,
-                        tag,
-                        tagCapacity,
+                        sink,
                         err);
+            return TRUE;
         }
     }
 
@@ -663,25 +627,28 @@
 
         const char* likelySubtags = NULL;
 
-        createTagString(
-            lang,
-            langLength,
-            script,
-            scriptLength,
-            NULL,
-            0,
-            NULL,
-            0,
-            tagBuffer,
-            sizeof(tagBuffer),
-            err);
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
+            createTagString(
+                lang,
+                langLength,
+                script,
+                scriptLength,
+                NULL,
+                0,
+                NULL,
+                0,
+                sink,
+                err);
+        }
         if(U_FAILURE(*err)) {
             goto error;
         }
 
         likelySubtags =
             findLikelySubtags(
-                tagBuffer,
+                tagBuffer.data(),
                 likelySubtagsBuffer,
                 sizeof(likelySubtagsBuffer),
                 err);
@@ -693,7 +660,7 @@
             /* Always use the language tag from the
                maximal string, since it may be more
                specific than the one provided. */
-            return createTagStringWithAlternates(
+            createTagStringWithAlternates(
                         NULL,
                         0,
                         NULL,
@@ -703,9 +670,9 @@
                         variants,
                         variantsLength,
                         likelySubtags,
-                        tag,
-                        tagCapacity,
+                        sink,
                         err);
+            return TRUE;
         }
     }
 
@@ -716,25 +683,28 @@
 
         const char* likelySubtags = NULL;
 
-        createTagString(
-            lang,
-            langLength,
-            NULL,
-            0,
-            region,
-            regionLength,
-            NULL,
-            0,
-            tagBuffer,
-            sizeof(tagBuffer),
-            err);
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
+            createTagString(
+                lang,
+                langLength,
+                NULL,
+                0,
+                region,
+                regionLength,
+                NULL,
+                0,
+                sink,
+                err);
+        }
         if(U_FAILURE(*err)) {
             goto error;
         }
 
         likelySubtags =
             findLikelySubtags(
-                tagBuffer,
+                tagBuffer.data(),
                 likelySubtagsBuffer,
                 sizeof(likelySubtagsBuffer),
                 err);
@@ -746,7 +716,7 @@
             /* Always use the language tag from the
                maximal string, since it may be more
                specific than the one provided. */
-            return createTagStringWithAlternates(
+            createTagStringWithAlternates(
                         NULL,
                         0,
                         script,
@@ -756,9 +726,9 @@
                         variants,
                         variantsLength,
                         likelySubtags,
-                        tag,
-                        tagCapacity,
+                        sink,
                         err);
+            return TRUE;
         }
     }
 
@@ -768,25 +738,28 @@
     {
         const char* likelySubtags = NULL;
 
-        createTagString(
-            lang,
-            langLength,
-            NULL,
-            0,
-            NULL,
-            0,
-            NULL,
-            0,
-            tagBuffer,
-            sizeof(tagBuffer),
-            err);
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
+            createTagString(
+                lang,
+                langLength,
+                NULL,
+                0,
+                NULL,
+                0,
+                NULL,
+                0,
+                sink,
+                err);
+        }
         if(U_FAILURE(*err)) {
             goto error;
         }
 
         likelySubtags =
             findLikelySubtags(
-                tagBuffer,
+                tagBuffer.data(),
                 likelySubtagsBuffer,
                 sizeof(likelySubtagsBuffer),
                 err);
@@ -798,7 +771,7 @@
             /* Always use the language tag from the
                maximal string, since it may be more
                specific than the one provided. */
-            return createTagStringWithAlternates(
+            createTagStringWithAlternates(
                         NULL,
                         0,
                         script,
@@ -808,17 +781,13 @@
                         variants,
                         variantsLength,
                         likelySubtags,
-                        tag,
-                        tagCapacity,
+                        sink,
                         err);
+            return TRUE;
         }
     }
 
-    return u_terminateChars(
-                tag,
-                tagCapacity,
-                0,
-                err);
+    return FALSE;
 
 error:
 
@@ -826,7 +795,7 @@
         *err = U_ILLEGAL_ARGUMENT_ERROR;
     }
 
-    return -1;
+    return FALSE;
 }
 
 #define CHECK_TRAILING_VARIANT_SIZE(trailing, trailingLength) \
@@ -848,12 +817,10 @@
         } \
     }
 
-static int32_t
-_uloc_addLikelySubtags(const char*    localeID,
-         char* maximizedLocaleID,
-         int32_t maximizedLocaleIDCapacity,
-         UErrorCode* err)
-{
+static void
+_uloc_addLikelySubtags(const char* localeID,
+                       icu::ByteSink& sink,
+                       UErrorCode* err) {
     char lang[ULOC_LANG_CAPACITY];
     int32_t langLength = sizeof(lang);
     char script[ULOC_SCRIPT_CAPACITY];
@@ -863,14 +830,12 @@
     const char* trailing = "";
     int32_t trailingLength = 0;
     int32_t trailingIndex = 0;
-    int32_t resultLength = 0;
+    UBool success = FALSE;
 
     if(U_FAILURE(*err)) {
         goto error;
     }
-    else if (localeID == NULL ||
-             maximizedLocaleID == NULL ||
-             maximizedLocaleIDCapacity <= 0) {
+    if (localeID == NULL) {
         goto error;
     }
 
@@ -901,7 +866,7 @@
 
     CHECK_TRAILING_VARIANT_SIZE(trailing, trailingLength);
 
-    resultLength =
+    success =
         createLikelySubtagsString(
             lang,
             langLength,
@@ -911,55 +876,32 @@
             regionLength,
             trailing,
             trailingLength,
-            maximizedLocaleID,
-            maximizedLocaleIDCapacity,
+            sink,
             err);
 
-    if (resultLength == 0) {
+    if (!success) {
         const int32_t localIDLength = (int32_t)uprv_strlen(localeID);
 
         /*
          * If we get here, we need to return localeID.
          */
-        uprv_memcpy(
-            maximizedLocaleID,
-            localeID,
-            localIDLength <= maximizedLocaleIDCapacity ? 
-                localIDLength : maximizedLocaleIDCapacity);
-
-        resultLength =
-            u_terminateChars(
-                maximizedLocaleID,
-                maximizedLocaleIDCapacity,
-                localIDLength,
-                err);
+        sink.Append(localeID, localIDLength);
     }
 
-    return resultLength;
+    return;
 
 error:
 
     if (!U_FAILURE(*err)) {
         *err = U_ILLEGAL_ARGUMENT_ERROR;
     }
-
-    return -1;
 }
 
-static int32_t
-_uloc_minimizeSubtags(const char*    localeID,
-         char* minimizedLocaleID,
-         int32_t minimizedLocaleIDCapacity,
-         UErrorCode* err)
-{
-    /**
-     * ULOC_FULLNAME_CAPACITY will provide enough capacity
-     * that we can build a string that contains the language,
-     * script and region code without worrying about overrunning
-     * the user-supplied buffer.
-     **/
-    char maximizedTagBuffer[ULOC_FULLNAME_CAPACITY];
-    int32_t maximizedTagBufferLength = sizeof(maximizedTagBuffer);
+static void
+_uloc_minimizeSubtags(const char* localeID,
+                      icu::ByteSink& sink,
+                      UErrorCode* err) {
+    icu::CharString maximizedTagBuffer;
 
     char lang[ULOC_LANG_CAPACITY];
     int32_t langLength = sizeof(lang);
@@ -974,9 +916,7 @@
     if(U_FAILURE(*err)) {
         goto error;
     }
-    else if (localeID == NULL ||
-             minimizedLocaleID == NULL ||
-             minimizedLocaleIDCapacity <= 0) {
+    else if (localeID == NULL) {
         goto error;
     }
 
@@ -1009,32 +949,32 @@
 
     CHECK_TRAILING_VARIANT_SIZE(trailing, trailingLength);
 
-    createTagString(
-        lang,
-        langLength,
-        script,
-        scriptLength,
-        region,
-        regionLength,
-        NULL,
-        0,
-        maximizedTagBuffer,
-        maximizedTagBufferLength,
-        err);
-    if(U_FAILURE(*err)) {
-        goto error;
-    }
+    {
+        icu::CharString base;
+        {
+            icu::CharStringByteSink sink(&base);
+            createTagString(
+                lang,
+                langLength,
+                script,
+                scriptLength,
+                region,
+                regionLength,
+                NULL,
+                0,
+                sink,
+                err);
+        }
 
-    /**
-     * First, we need to first get the maximization
-     * from AddLikelySubtags.
-     **/
-    maximizedTagBufferLength =
-        uloc_addLikelySubtags(
-            maximizedTagBuffer,
-            maximizedTagBuffer,
-            maximizedTagBufferLength,
-            err);
+        /**
+         * First, we need to first get the maximization
+         * from AddLikelySubtags.
+         **/
+        {
+            icu::CharStringByteSink sink(&maximizedTagBuffer);
+            ulocimp_addLikelySubtags(base.data(), sink, err);
+        }
+    }
 
     if(U_FAILURE(*err)) {
         goto error;
@@ -1044,9 +984,9 @@
      * Start first with just the language.
      **/
     {
-        char tagBuffer[ULOC_FULLNAME_CAPACITY];
-
-        const int32_t tagBufferLength =
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
             createLikelySubtagsString(
                 lang,
                 langLength,
@@ -1056,19 +996,19 @@
                 0,
                 NULL,
                 0,
-                tagBuffer,
-                sizeof(tagBuffer),
+                sink,
                 err);
+        }
 
         if(U_FAILURE(*err)) {
             goto error;
         }
         else if (uprv_strnicmp(
-                    maximizedTagBuffer,
-                    tagBuffer,
-                    tagBufferLength) == 0) {
+                    maximizedTagBuffer.data(),
+                    tagBuffer.data(),
+                    tagBuffer.length()) == 0) {
 
-            return createTagString(
+            createTagString(
                         lang,
                         langLength,
                         NULL,
@@ -1077,9 +1017,9 @@
                         0,
                         trailing,
                         trailingLength,
-                        minimizedLocaleID,
-                        minimizedLocaleIDCapacity,
+                        sink,
                         err);
+            return;
         }
     }
 
@@ -1088,9 +1028,9 @@
      **/
     if (regionLength > 0) {
 
-        char tagBuffer[ULOC_FULLNAME_CAPACITY];
-
-        const int32_t tagBufferLength =
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
             createLikelySubtagsString(
                 lang,
                 langLength,
@@ -1100,19 +1040,19 @@
                 regionLength,
                 NULL,
                 0,
-                tagBuffer,
-                sizeof(tagBuffer),
+                sink,
                 err);
+        }
 
         if(U_FAILURE(*err)) {
             goto error;
         }
         else if (uprv_strnicmp(
-                    maximizedTagBuffer,
-                    tagBuffer,
-                    tagBufferLength) == 0) {
+                    maximizedTagBuffer.data(),
+                    tagBuffer.data(),
+                    tagBuffer.length()) == 0) {
 
-            return createTagString(
+            createTagString(
                         lang,
                         langLength,
                         NULL,
@@ -1121,9 +1061,9 @@
                         regionLength,
                         trailing,
                         trailingLength,
-                        minimizedLocaleID,
-                        minimizedLocaleIDCapacity,
+                        sink,
                         err);
+            return;
         }
     }
 
@@ -1133,9 +1073,9 @@
      * maximal version that we already have.
      **/
     if (scriptLength > 0 && regionLength > 0) {
-        char tagBuffer[ULOC_FULLNAME_CAPACITY];
-
-        const int32_t tagBufferLength =
+        icu::CharString tagBuffer;
+        {
+            icu::CharStringByteSink sink(&tagBuffer);
             createLikelySubtagsString(
                 lang,
                 langLength,
@@ -1145,19 +1085,19 @@
                 0,
                 NULL,
                 0,
-                tagBuffer,
-                sizeof(tagBuffer),
+                sink,
                 err);
+        }
 
         if(U_FAILURE(*err)) {
             goto error;
         }
         else if (uprv_strnicmp(
-                    maximizedTagBuffer,
-                    tagBuffer,
-                    tagBufferLength) == 0) {
+                    maximizedTagBuffer.data(),
+                    tagBuffer.data(),
+                    tagBuffer.length()) == 0) {
 
-            return createTagString(
+            createTagString(
                         lang,
                         langLength,
                         script,
@@ -1166,9 +1106,9 @@
                         0,
                         trailing,
                         trailingLength,
-                        minimizedLocaleID,
-                        minimizedLocaleIDCapacity,
+                        sink,
                         err);
+            return;
         }
     }
 
@@ -1177,18 +1117,8 @@
          * If we got here, return the locale ID parameter.
          **/
         const int32_t localeIDLength = (int32_t)uprv_strlen(localeID);
-
-        uprv_memcpy(
-            minimizedLocaleID,
-            localeID,
-            localeIDLength <= minimizedLocaleIDCapacity ? 
-                localeIDLength : minimizedLocaleIDCapacity);
-
-        return u_terminateChars(
-                    minimizedLocaleID,
-                    minimizedLocaleIDCapacity,
-                    localeIDLength,
-                    err);
+        sink.Append(localeID, localeIDLength);
+        return;
     }
 
 error:
@@ -1196,10 +1126,6 @@
     if (!U_FAILURE(*err)) {
         *err = U_ILLEGAL_ARGUMENT_ERROR;
     }
-
-    return -1;
-
-
 }
 
 static UBool
@@ -1230,51 +1156,83 @@
 }
 
 U_CAPI int32_t U_EXPORT2
-uloc_addLikelySubtags(const char*    localeID,
-         char* maximizedLocaleID,
-         int32_t maximizedLocaleIDCapacity,
-         UErrorCode* err)
-{
+uloc_addLikelySubtags(const char* localeID,
+                      char* maximizedLocaleID,
+                      int32_t maximizedLocaleIDCapacity,
+                      UErrorCode* status) {
+    if (U_FAILURE(*status)) {
+        return 0;
+    }
+
+    icu::CheckedArrayByteSink sink(
+            maximizedLocaleID, maximizedLocaleIDCapacity);
+
+    ulocimp_addLikelySubtags(localeID, sink, status);
+    int32_t reslen = sink.NumberOfBytesAppended();
+
+    if (U_FAILURE(*status)) {
+        return sink.Overflowed() ? reslen : -1;
+    }
+
+    if (sink.Overflowed()) {
+        *status = U_BUFFER_OVERFLOW_ERROR;
+    } else {
+        u_terminateChars(
+                maximizedLocaleID, maximizedLocaleIDCapacity, reslen, status);
+    }
+
+    return reslen;
+}
+
+U_CAPI void U_EXPORT2
+ulocimp_addLikelySubtags(const char* localeID,
+                         icu::ByteSink& sink,
+                         UErrorCode* status) {
     char localeBuffer[ULOC_FULLNAME_CAPACITY];
 
-    if (!do_canonicalize(
-        localeID,
-        localeBuffer,
-        sizeof(localeBuffer),
-        err)) {
-        return -1;
+    if (do_canonicalize(localeID, localeBuffer, sizeof localeBuffer, status)) {
+        _uloc_addLikelySubtags(localeBuffer, sink, status);
     }
-    else {
-        return _uloc_addLikelySubtags(
-                    localeBuffer,
-                    maximizedLocaleID,
-                    maximizedLocaleIDCapacity,
-                    err);
-    }    
 }
 
 U_CAPI int32_t U_EXPORT2
-uloc_minimizeSubtags(const char*    localeID,
-         char* minimizedLocaleID,
-         int32_t minimizedLocaleIDCapacity,
-         UErrorCode* err)
-{
+uloc_minimizeSubtags(const char* localeID,
+                     char* minimizedLocaleID,
+                     int32_t minimizedLocaleIDCapacity,
+                     UErrorCode* status) {
+    if (U_FAILURE(*status)) {
+        return 0;
+    }
+
+    icu::CheckedArrayByteSink sink(
+            minimizedLocaleID, minimizedLocaleIDCapacity);
+
+    ulocimp_minimizeSubtags(localeID, sink, status);
+    int32_t reslen = sink.NumberOfBytesAppended();
+
+    if (U_FAILURE(*status)) {
+        return sink.Overflowed() ? reslen : -1;
+    }
+
+    if (sink.Overflowed()) {
+        *status = U_BUFFER_OVERFLOW_ERROR;
+    } else {
+        u_terminateChars(
+                minimizedLocaleID, minimizedLocaleIDCapacity, reslen, status);
+    }
+
+    return reslen;
+}
+
+U_CAPI void U_EXPORT2
+ulocimp_minimizeSubtags(const char* localeID,
+                        icu::ByteSink& sink,
+                        UErrorCode* status) {
     char localeBuffer[ULOC_FULLNAME_CAPACITY];
 
-    if (!do_canonicalize(
-        localeID,
-        localeBuffer,
-        sizeof(localeBuffer),
-        err)) {
-        return -1;
+    if (do_canonicalize(localeID, localeBuffer, sizeof localeBuffer, status)) {
+        _uloc_minimizeSubtags(localeBuffer, sink, status);
     }
-    else {
-        return _uloc_minimizeSubtags(
-                    localeBuffer,
-                    minimizedLocaleID,
-                    minimizedLocaleIDCapacity,
-                    err);
-    }    
 }
 
 // Pairs of (language subtag, + or -) for finding out fast if common languages
@@ -1282,7 +1240,7 @@
 static const char LANG_DIR_STRING[] =
         "root-en-es-pt-zh-ja-ko-de-fr-it-ar+he+fa+ru-nl-pl-th-tr-";
 
-// Implemented here because this calls uloc_addLikelySubtags().
+// Implemented here because this calls ulocimp_addLikelySubtags().
 U_CAPI UBool U_EXPORT2
 uloc_isRightToLeft(const char *locale) {
     UErrorCode errorCode = U_ZERO_ERROR;
@@ -1309,12 +1267,15 @@
         }
         // Otherwise, find the likely script.
         errorCode = U_ZERO_ERROR;
-        char likely[ULOC_FULLNAME_CAPACITY];
-        (void)uloc_addLikelySubtags(locale, likely, UPRV_LENGTHOF(likely), &errorCode);
+        icu::CharString likely;
+        {
+            icu::CharStringByteSink sink(&likely);
+            ulocimp_addLikelySubtags(locale, sink, &errorCode);
+        }
         if (U_FAILURE(errorCode) || errorCode == U_STRING_NOT_TERMINATED_WARNING) {
             return FALSE;
         }
-        scriptLength = uloc_getScript(likely, script, UPRV_LENGTHOF(script), &errorCode);
+        scriptLength = uloc_getScript(likely.data(), script, UPRV_LENGTHOF(script), &errorCode);
         if (U_FAILURE(errorCode) || errorCode == U_STRING_NOT_TERMINATED_WARNING ||
                 scriptLength == 0) {
             return FALSE;
@@ -1365,11 +1326,14 @@
             rgLen = 0;
         } else if (rgLen == 0 && inferRegion) {
             // no unicode_region_subtag but inferRegion TRUE, try likely subtags
-            char locBuf[ULOC_FULLNAME_CAPACITY];
             rgStatus = U_ZERO_ERROR;
-            (void)uloc_addLikelySubtags(localeID, locBuf, ULOC_FULLNAME_CAPACITY, &rgStatus);
+            icu::CharString locBuf;
+            {
+                icu::CharStringByteSink sink(&locBuf);
+                ulocimp_addLikelySubtags(localeID, sink, &rgStatus);
+            }
             if (U_SUCCESS(rgStatus)) {
-                rgLen = uloc_getCountry(locBuf, rgBuf, ULOC_RG_BUFLEN, status);
+                rgLen = uloc_getCountry(locBuf.data(), rgBuf, ULOC_RG_BUFLEN, status);
                 if (U_FAILURE(*status)) {
                     rgLen = 0;
                 }
diff --git a/icu4c/source/common/ulocimp.h b/icu4c/source/common/ulocimp.h
index fd16af5..31d3c44 100644
--- a/icu4c/source/common/ulocimp.h
+++ b/icu4c/source/common/ulocimp.h
@@ -145,6 +145,74 @@
 ulocimp_getRegionForSupplementalData(const char *localeID, UBool inferRegion,
                                      char *region, int32_t regionCapacity, UErrorCode* status);
 
+/**
+ * Add the likely subtags for a provided locale ID, per the algorithm described
+ * in the following CLDR technical report:
+ *
+ *   http://www.unicode.org/reports/tr35/#Likely_Subtags
+ *
+ * If localeID is already in the maximal form, or there is no data available
+ * for maximization, it will be copied to the output buffer.  For example,
+ * "und-Zzzz" cannot be maximized, since there is no reasonable maximization.
+ *
+ * Examples:
+ *
+ * "en" maximizes to "en_Latn_US"
+ *
+ * "de" maximizes to "de_Latn_US"
+ *
+ * "sr" maximizes to "sr_Cyrl_RS"
+ *
+ * "sh" maximizes to "sr_Latn_RS" (Note this will not reverse.)
+ *
+ * "zh_Hani" maximizes to "zh_Hans_CN" (Note this will not reverse.)
+ *
+ * @param localeID The locale to maximize
+ * @param sink The output sink receiving the maximized locale
+ * @param err Error information if maximizing the locale failed.  If the length
+ * of the localeID and the null-terminator is greater than the maximum allowed size,
+ * or the localeId is not well-formed, the error code is U_ILLEGAL_ARGUMENT_ERROR.
+ * @internal ICU 64
+ */
+U_STABLE void U_EXPORT2
+ulocimp_addLikelySubtags(const char* localeID,
+                         icu::ByteSink& sink,
+                         UErrorCode* err);
+
+/**
+ * Minimize the subtags for a provided locale ID, per the algorithm described
+ * in the following CLDR technical report:
+ *
+ *   http://www.unicode.org/reports/tr35/#Likely_Subtags
+ *
+ * If localeID is already in the minimal form, or there is no data available
+ * for minimization, it will be copied to the output buffer.  Since the
+ * minimization algorithm relies on proper maximization, see the comments
+ * for ulocimp_addLikelySubtags for reasons why there might not be any data.
+ *
+ * Examples:
+ *
+ * "en_Latn_US" minimizes to "en"
+ *
+ * "de_Latn_US" minimizes to "de"
+ *
+ * "sr_Cyrl_RS" minimizes to "sr"
+ *
+ * "zh_Hant_TW" minimizes to "zh_TW" (The region is preferred to the
+ * script, and minimizing to "zh" would imply "zh_Hans_CN".)
+ *
+ * @param localeID The locale to minimize
+ * @param sink The output sink receiving the maximized locale
+ * @param err Error information if minimizing the locale failed.  If the length
+ * of the localeID and the null-terminator is greater than the maximum allowed size,
+ * or the localeId is not well-formed, the error code is U_ILLEGAL_ARGUMENT_ERROR.
+ * @internal ICU 64
+ */
+U_STABLE void U_EXPORT2
+ulocimp_minimizeSubtags(const char* localeID,
+                        icu::ByteSink& sink,
+                        UErrorCode* err);
+
 U_CAPI const char * U_EXPORT2
 locale_getKeywordsStart(const char *localeID);
 
diff --git a/icu4c/source/common/uscript.cpp b/icu4c/source/common/uscript.cpp
index 83b5f7e..98528c1 100644
--- a/icu4c/source/common/uscript.cpp
+++ b/icu4c/source/common/uscript.cpp
@@ -18,8 +18,11 @@
 #include "unicode/uchar.h"
 #include "unicode/uscript.h"
 #include "unicode/uloc.h"
+#include "bytesinkutil.h"
+#include "charstr.h"
 #include "cmemory.h"
 #include "cstring.h"
+#include "ulocimp.h"
 
 static const UScriptCode JAPANESE[3] = { USCRIPT_KATAKANA, USCRIPT_HIRAGANA, USCRIPT_HAN };
 static const UScriptCode KOREAN[2] = { USCRIPT_HANGUL, USCRIPT_HAN };
@@ -98,7 +101,6 @@
                 int32_t capacity,
                 UErrorCode* err){
     UBool triedCode;
-    char likely[ULOC_FULLNAME_CAPACITY];
     UErrorCode internalErrorCode;
     int32_t length;
 
@@ -125,10 +127,13 @@
     if(U_FAILURE(*err) || length != 0) {
         return length;
     }
-    (void)uloc_addLikelySubtags(nameOrAbbrOrLocale,
-                                likely, UPRV_LENGTHOF(likely), &internalErrorCode);
+    icu::CharString likely;
+    {
+        icu::CharStringByteSink sink(&likely);
+        ulocimp_addLikelySubtags(nameOrAbbrOrLocale, sink, &internalErrorCode);
+    }
     if(U_SUCCESS(internalErrorCode) && internalErrorCode != U_STRING_NOT_TERMINATED_WARNING) {
-        length = getCodesFromLocale(likely, fillIn, capacity, err);
+        length = getCodesFromLocale(likely.data(), fillIn, capacity, err);
         if(U_FAILURE(*err) || length != 0) {
             return length;
         }
diff --git a/icu4c/source/i18n/calendar.cpp b/icu4c/source/i18n/calendar.cpp
index 63197bd..4f451ee 100644
--- a/icu4c/source/i18n/calendar.cpp
+++ b/icu4c/source/i18n/calendar.cpp
@@ -3796,18 +3796,16 @@
     // 2). If the locale has a script designation then we ignore it,
     //     then remove it ( i.e. "en_Latn_US" becomes "en_US" )
 
-    char minLocaleID[ULOC_FULLNAME_CAPACITY] = { 0 };
     UErrorCode myStatus = U_ZERO_ERROR;
 
-    uloc_minimizeSubtags(desiredLocale.getName(),minLocaleID,ULOC_FULLNAME_CAPACITY,&myStatus);
-    Locale min = Locale::createFromName(minLocaleID);
+    Locale min(desiredLocale);
+    min.minimizeSubtags(myStatus);
     Locale useLocale;
     if ( uprv_strlen(desiredLocale.getCountry()) == 0 ||
          (uprv_strlen(desiredLocale.getScript()) > 0 && uprv_strlen(min.getScript()) == 0) ) {
-        char maxLocaleID[ULOC_FULLNAME_CAPACITY] = { 0 };
         myStatus = U_ZERO_ERROR;
-        uloc_addLikelySubtags(desiredLocale.getName(),maxLocaleID,ULOC_FULLNAME_CAPACITY,&myStatus);
-        Locale max = Locale::createFromName(maxLocaleID);
+        Locale max(desiredLocale);
+        max.addLikelySubtags(myStatus);
         useLocale = Locale(max.getLanguage(),max.getCountry());
     } else {
         useLocale = desiredLocale;
diff --git a/icu4c/source/i18n/dtptngen.cpp b/icu4c/source/i18n/dtptngen.cpp
index e0f19f0..fcc5977 100644
--- a/icu4c/source/i18n/dtptngen.cpp
+++ b/icu4c/source/i18n/dtptngen.cpp
@@ -615,16 +615,11 @@
 
 void DateTimePatternGenerator::getAllowedHourFormats(const Locale &locale, UErrorCode &status) {
     if (U_FAILURE(status)) { return; }
-    const char *localeID = locale.getName();
-    char maxLocaleID[ULOC_FULLNAME_CAPACITY];
-    int32_t length = uloc_addLikelySubtags(localeID, maxLocaleID, ULOC_FULLNAME_CAPACITY, &status);
+    Locale maxLocale(locale);
+    maxLocale.addLikelySubtags(status);
     if (U_FAILURE(status)) {
         return;
-    } else if (length == ULOC_FULLNAME_CAPACITY) {  // no room for NUL
-        status = U_BUFFER_OVERFLOW_ERROR;
-        return;
     }
-    Locale maxLocale = Locale(maxLocaleID);
 
     const char *country = maxLocale.getCountry();
     if (*country == '\0') { country = "001"; }