openingd: don't ask for scid_alias in channel type if remote doesn't tell us they support it.

This is actually totally fair, and LND, which does not support it without an option, rejects.

Thanks to Vincenzo and roasbeef for debugging this for me!

Fixes: #7221
Changelog-Fixed: Protocol: we can now open unannounced channels with LND nodes again.
Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

openingd/openingd.c

@@ -342,10 +342,11 @@ static u8 *funder_channel_start(struct state *state, u8 channel_flags,
 		 * But:
 		 * 1. We didn't accept this in CLN prior to v23.05.
 		 * 2. LND won't accept that without OPT_ANCHORS_ZERO_FEE_HTLC_TX.
-		 *
-		 * So we keep it off for now, until anchors merge.
+		 * 3. LND <= 18 won't accept OPT_SCID_ALIAS unless it advertizes it,
+		 *    which it does not by default.
 		 */
-		if (channel_type_has(state->channel_type, OPT_ANCHORS_ZERO_FEE_HTLC_TX)) {
+		if (channel_type_has(state->channel_type, OPT_ANCHORS_ZERO_FEE_HTLC_TX)
+		    && feature_offered(state->their_features, OPT_SCID_ALIAS)) {
 			if (!(channel_flags & CHANNEL_FLAGS_ANNOUNCE_CHANNEL))
 				channel_type_set_scid_alias(state->channel_type);
 		}