That's BIP30, he linked BIP34:
https://github.com/bitcoin/bitcoin/blob/master/src/validation.cpp#L3004

On Tue, Apr 4, 2017 at 11:32 AM, Tom Zander via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Can you tell me where it is enforced?
>
> The only place I found was here;
> https://github.com/bitcoin/bitcoin/blob/master/src/validation.cpp#L1793
>
> which doesn’t enforce it, all that code does is check that the txid is
> unknown or fully spent.
> And since the below idea from Russel would change the txid, it would seem
> no
> full client would reject this.
>
> Maybe its in a BIP, but I can’t find it in the code.
>
> On Tuesday, 4 April 2017 16:59:12 CEST James Hilliard wrote:
> > It is a consensus rule
> > https://github.com/bitcoin/bips/blob/master/bip-0034.mediawiki
> >
> > On Tue, Apr 4, 2017 at 6:47 AM, Tom Zander via bitcoin-dev
> >
> > <bitcoin-dev@lists.linuxfoundation.org> wrote:
> > > On Sunday, 2 April 2017 22:39:13 CEST Russell O'Connor via bitcoin-dev
> > >
> > > wrote:
> > >>  Someone told me a while back that it would be more natural if we move
> > >>  the
> > >>
> > >> nHeight from the coinbase script to the coinbase locktime.  Have you
> > >> considered doing this?
> > >
> > > That change would not be a consensus change and thus free to make any
> > > day.
>
>
> --
> Tom Zander
> Blog: https://zander.github.io
> Vlog: https://vimeo.com/channels/tomscryptochannel
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