Restriction for segwit OP_IF argument as a policy has got a few concept ACK. I would like to have more people to ACK or NACK, especially the real users of OP_IF. I think Lightning network would use that at lot.

Pull request: https://github.com/bitcoin/bitcoin/pull/8526

more related discussion could be found at https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2016-August/013036.html

It does have impact if your script uses the combination of "OP_SIZE OP_IF" or "OP_DEPTH OP_IF". With this policy/softfork, you need to use  "OP_SIZE OP_0NOTEQUAL OP_IF" or "OP_DEPTH OP_0NOTEQUAL OP_IF", or reconstruct your scripts.

> 
>     On August 16, 2016 at 1:53 PM Johnson Lau via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> 
>     -----BEGIN PGP SIGNED MESSAGE-----
>     Hash: SHA512
> 
>     A new BIP is prepared to deal with OP_IF and OP_NOTIF malleability in P2WSH:
>     https://github.com/jl2012/bips/blob/minimalif/bip-minimalif.mediawiki
>     https://github.com/bitcoin/bitcoin/pull/8526
> 
>     BIP: x
>     Title: Dealing with OP_IF and OP_NOTIF malleability in P2WSH
>     Author: Johnson Lau <jl2012@xbt.hk>
>     Status: Draft
>     Type: Standards Track
>     Created: 2016-08-17
> 
>     Abstract
> 
>     This document specifies proposed changes to the Bitcoin script validity rules in order to make transaction malleability related to OP_IF and OP_NOTIF impossible in pay-to-witness-script-hash (P2WSH) scripts.
> 
>     Motivation
> 
>     OP_IF and OP_NOTIF are flow control codes in the Bitcoin script system. The programme flow is decided by whether the top stake value is True or False. However, this behaviour opens a source of malleability as a third party may replace a True (False) stack item with any other True (False) value without invalidating the transaction.
> 
>     The proposed rules apply only to pay-to-witness-script-hash (P2WSH) scripts described in BIP141, which has not been activated on the Bitcoin mainnet as of writing. To ensure OP_IF and OP_NOTIF transactions created before the introduction of this BIP will still be accepted by the network, the new rules are not applied to non-segregated witness scripts.
> 
>     Specification
> 
>     In P2WSH, the argument for OP_IF and OP_NOTIF MUST be exactly an empty vector or 0x01, or the script evaluation fails immediately.
> 
>     This is deployed using BIP9 after segregated witness (BIP141) is activated. Details TBD.
> 
>     Compatibility
> 
>     This is a softfork on top of BIP141. The rules are enforced as a relay policy by the reference client since the first release of BIP141 (v0.13.1). To avoid risks of fund loss, users MUST NOT create P2WSH scripts that are incompatible with this BIP. An OP_0NOTEQUAL may be used before OP_IF or OP_NOTIF to imitate the original behaviour (which may also re-enable the malleability vector depending on the exact script).
> 
>     Implementation
> 
>     https://github.com/bitcoin/bitcoin/pull/8526
> 
>     Copyright
> 
>     This work is placed in the public domain.
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