diff --git a/lib/Frontend/TextDiagnostic.cpp b/lib/Frontend/TextDiagnostic.cpp
index 1572d0f..2da66d3 100644
--- a/lib/Frontend/TextDiagnostic.cpp
+++ b/lib/Frontend/TextDiagnostic.cpp
@@ -336,13 +336,10 @@
   if (MaxColumns <= Columns)
     return;
 
-  // no special characters allowed in CaretLine or FixItInsertionLine
+  // No special characters are allowed in CaretLine.
   assert(CaretLine.end() ==
          std::find_if(CaretLine.begin(), CaretLine.end(),
          char_out_of_range(' ','~')));
-  assert(FixItInsertionLine.end() ==
-         std::find_if(FixItInsertionLine.begin(), FixItInsertionLine.end(),
-         char_out_of_range(' ','~')));
 
   // Find the slice that we need to display the full caret line
   // correctly.
@@ -370,8 +367,15 @@
       if (!isWhitespace(FixItInsertionLine[FixItEnd - 1]))
         break;
 
-    CaretStart = std::min(FixItStart, CaretStart);
-    CaretEnd = std::max(FixItEnd, CaretEnd);
+    // We can safely use the byte offset FixItStart as the column offset
+    // because the characters up until FixItStart are all ASCII whitespace
+    // characters.
+    unsigned FixItStartCol = FixItStart;
+    unsigned FixItEndCol
+      = llvm::sys::locale::columnWidth(FixItInsertionLine.substr(0, FixItEnd));
+
+    CaretStart = std::min(FixItStartCol, CaretStart);
+    CaretEnd = std::max(FixItEndCol, CaretEnd);
   }
 
   // CaretEnd may have been set at the middle of a character
@@ -1023,24 +1027,18 @@
         if (HintCol < PrevHintEndCol)
           HintCol = PrevHintEndCol + 1;
 
-        // FIXME: This function handles multibyte characters in the source, but
-        // not in the fixits. This assertion is intended to catch unintended
-        // use of multibyte characters in fixits. If we decide to do this, we'll
-        // have to track separate byte widths for the source and fixit lines.
-        assert((size_t)llvm::sys::locale::columnWidth(I->CodeToInsert) ==
-               I->CodeToInsert.size());
-
-        // This relies on one byte per column in our fixit hints.
         // This should NOT use HintByteOffset, because the source might have
         // Unicode characters in earlier columns.
-        unsigned LastColumnModified = HintCol + I->CodeToInsert.size();
-        if (LastColumnModified > FixItInsertionLine.size())
-          FixItInsertionLine.resize(LastColumnModified, ' ');
+        unsigned NewFixItLineSize = FixItInsertionLine.size() +
+          (HintCol - PrevHintEndCol) + I->CodeToInsert.size();
+        if (NewFixItLineSize > FixItInsertionLine.size())
+          FixItInsertionLine.resize(NewFixItLineSize, ' ');
 
         std::copy(I->CodeToInsert.begin(), I->CodeToInsert.end(),
-                  FixItInsertionLine.begin() + HintCol);
+                  FixItInsertionLine.end() - I->CodeToInsert.size());
 
-        PrevHintEndCol = LastColumnModified;
+        PrevHintEndCol =
+          HintCol + llvm::sys::locale::columnWidth(I->CodeToInsert);
       } else {
         FixItInsertionLine.clear();
         break;