Handle block boundaries in utf8_to_utf16

include/fmt/format-inl.h

@@ -1292,9 +1292,12 @@ FMT_FUNC internal::utf8_to_utf16::utf8_to_utf16(string_view s) {
     for (auto end = p + s.size() - block_size + 1; p < end;) p = transcode(p);
   }
   if (auto num_chars_left = s.data() + s.size() - p) {
-    char buf[4] = {};
+    char buf[2 * block_size - 1] = {};
     memcpy(buf, p, num_chars_left);
-    transcode(buf);
+    p = buf;
+    do {
+      p = transcode(p);
+    } while (p - buf < num_chars_left);
   }
   buffer_.push_back(0);
 }

test/format-test.cc

@@ -410,6 +410,7 @@ TEST(UtilTest, UTF8ToUTF16) {
                    "invalid utf8");
   EXPECT_THROW_MSG(fmt::internal::utf8_to_utf16(fmt::string_view("л", 1)),
                    std::runtime_error, "invalid utf8");
+  EXPECT_EQ(L"123456", fmt::internal::utf8_to_utf16("123456").str());
 }
 
 TEST(UtilTest, UTF8ToUTF16EmptyString) {