Maseches Eruvin, Chapter 9, Mishnah 3. Continuing from the previous mishnah, which dealt with a courtyard that lost the wall separating it from the public domain, the mishnah goes on to discuss a courtyard that was breached - except that here the breach was made on two of its sides.
"Chatzer shenifretzah lireshus harabim mishtei ruchoseha": - a courtyard that was breached into the public domain on two of its sides.
The Gemara explains that this refers to a breach made at the corner, at the angle where the two walls meet, so that nothing remains standing at the corner itself. Normally a breach is not considered a disqualifying breach unless it is ten amos wide, but here that measurement is not required. The reason: a breach at a corner is not considered a doorway, since it is not the way of the world to make doorways at corners, and therefore it has the status of a breach and not of a doorway.
The mishnah goes on to list additional cases:
"Vechein bayis shenifratz mishtei ruchosav" - not only a courtyard, but even a house that was breached on two sides at the corner.
"Vechein mavoi shenitlu koirosav ulchayav" - an alleyway requires something to permit carrying within it, either a korah (a beam laid across its entrance) or a lechi (a post at its side), serving as a marker at the beginning of the alleyway. Here the korah and the lechi were removed on Shabbos itself.
The dispute between Rabbi Yehudah and Rabbi Yosi:
In each of these cases, which occurred on Shabbos itself, the Tannaim disagree:
Rabbi Yehudah: "Mutarin le'osah Shabbos ve'asurin le'asid lavo" - carrying is permitted on that Shabbos, since Shabbos entered under permitted conditions: the partition was standing, as were the korah or the lechi. But for the coming Shabbos it will be forbidden, since that Shabbos will enter without the benefit of the partition, the korah and the lechi.
Rabbi Yosi: "Im mutarin le'osah Shabbos - mutarin le'asid lavo, ve'im asurin le'asid lavo - asurin le'osah Shabbos" - one may not distinguish between one Shabbos and another: if carrying is permitted on that Shabbos, it should be permitted for the coming Shabbos as well; and if it is forbidden in the future, it should be forbidden even on that very Shabbos on which the partition was breached or the korah and lechi were removed. In practice his intent is that just as the matter will be forbidden afterward, so too it is forbidden on this Shabbos itself.
In summary: in this mishnah we learned that a courtyard or a house breached on two sides at the corner - the breach is not considered a doorway and no measurement of ten amos is required, since it is not the way of the world to make a doorway at a corner; and the same applies to an alleyway whose korah and lechi were removed. According to Rabbi Yehudah carrying is permitted on that Shabbos and forbidden in the future, since Shabbos entered under permitted conditions, while according to Rabbi Yosi one may not distinguish between one Shabbos and another, and in practice the matter is forbidden even on that very Shabbos.