Merge pull request #6586 from BBasile/issue-18993

fix issue 18993 (REG ~master) - toLower is broken for UTF chars

std/uni.d

@@ -9003,16 +9003,18 @@ if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && i
 {
     import std.array : appender, array;
     import std.ascii : isASCII;
-    import std.utf : byDchar;
+    import std.utf : byDchar, codeLength;
+
+    alias C = ElementEncodingType!S;
 
     auto r = s.byDchar;
-    for (size_t i; !r.empty; ++i, r.popFront())
+    for (size_t i; !r.empty; i += r.front.codeLength!C , r.popFront())
     {
         auto cOuter = r.front;
         ushort idx = indexFn(cOuter);
         if (idx == ushort.max)
             continue;
-        auto result = appender!(ElementEncodingType!S[])();
+        auto result = appender!(C[])();
         result.put(s[0 .. i]);
         result.reserve(s.length);
         foreach (dchar c; s[i .. $].byDchar)
@@ -9062,6 +9064,11 @@ if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && i
     assert(s == "abcdefghij");
 }
 
+@safe unittest // 18993
+{
+    static assert(`몬스터/A`.toLower.length == `몬스터/a`.toLower.length);
+}
+
 
 // generic toUpper/toLower on whole range, returns range
 private auto toCaser(alias indexFn, uint maxIdx, alias tableFn, alias asciiConvert, Range)(Range str)