Landing http://codereview.chromium.org/1594017 for Ryan.

test/cctest/test-strings.cc

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 
 // Check that we can traverse very deep stacks of ConsStrings using
 // StringInputBuffer.  Check that Get(int) works on very deep stacks
 // of ConsStrings.  These operations may not be very fast, but they
 // should be possible without getting errors due to too deep recursion.
 
 #include <stdlib.h>
 
 #include "v8.h"
@@ skipping 329 matching lines @@
     // Check that the contents are correct
     for (int j = 0; j < lengths[i]; j++)
       CHECK_EQ(as_utf8[j], static_cast<unsigned char>(buffer[j]));
     // Check that the rest of the buffer hasn't been touched
     for (int j = lengths[i]; j < 11; j++)
       CHECK_EQ(kNoChar, buffer[j]);
   }
 }
 
 
+TEST(StringConcatFlatten) {
+  InitializeVM();
+  v8::HandleScope handle_scope;
+
+  const char* stringA = "0123456789";
+  const char* stringB = "ABCDEFGHIJ";
+
+  v8::Local<v8::String> a = v8::String::New(stringA);
+  v8::Local<v8::String> b = v8::String::New(stringB);
+
+  v8::Local<v8::String> cons = v8::String::Concat(a, b);
+
+  i::Handle<i::String> str = v8::Utils::OpenHandle(*cons);
+  CHECK(!str->IsFlat());
+
+  cons->Flatten();
+
+  CHECK(str->IsFlat());
+
+  char buffer[21];
+  cons->WriteUtf8(buffer);
+
+  for (int i = 0; i < 10; i++) {
+    CHECK_EQ(stringA[i], buffer[i]);
+  }
+
+  for (int i = 0; i < 10; i++) {
+    CHECK_EQ(stringB[i], buffer[i + 10]);
+  }
+}
+
+
 TEST(ExternalShortStringAdd) {
   ZoneScope zone(DELETE_ON_EXIT);
 
   InitializeVM();
   v8::HandleScope handle_scope;
 
   // Make sure we cover all always-flat lengths and at least one above.
   static const int kMaxLength = 20;
   CHECK_GT(kMaxLength, i::String::kMinNonFlatLength);
 
@@ skipping 66 matching lines @@
     "      if (non_ascii[i] !="
     "          (non_ascii[j] + external_non_ascii[i - j])) return 12;"
     "    }"
     "  }"
     "  return 0;"
     "};"
     "test()";
   CHECK_EQ(0,
            v8::Script::Compile(v8::String::New(source))->Run()->Int32Value());
 }