add dirstate._dirtypl variable Theoretically, it's possible to forget modified dirstate parents by doing: dirstate.invalidate() dirstate.setparents(p1, p2) dirstate._map The final access to _map should call _read(), which will unconditionally overwrite dirstate._pl. This doesn't actually happen right now because invalidate accidentally ends up rebuilding dirstate._map.

#!/bin/sh

mkdir t
cd t
hg init
echo a > a
hg ci -Ama -d '1000000000 0'
echo b > b
hg branch b
hg ci -Amb -d '1000000000 0'
hg co -C 0

echo fast-forward
hg merge b
hg ci -Ammerge -d '1000000000 0'

echo bogus fast-forward should fail
hg merge b

echo done