Mark signatures as stable since 5.020

[Corion]

with 5.036

[Leont]

I'm pretty sure we had a discussion about this before and there were reasons not to do this, but I don't remember the details. @rjbs ?

[ilmari]

Shouldn't it be stable as of 5.28? The ordering of attributes vs. signature changed in 5.22 and back again in 5.28.

[Corion]

Ah - I missed that. I'm happy to move it to 5.28 as well.

[rjbs]

From IRC where someone reminded me of this question:

I think the issue was that: "stable" is for when a feature in version X is experimental, but in version X+n it is not and is identical.

In version v5.2x, signatures stopped changing, except they do not warn on use of @_ in a subroutine with signatures. In current perl they do. I think this was the discussion/argument at the time.

Say you use v5.34 and had "use stable 'signatures'". You could use @_ and get no warning. Then up upgrade to some future version, thinking your code was not using something experimental: but it was, it was using @_-in-signatured-sub.

@_-in-signature is not deprecated, in which case we might say it could be removed with warning, and those warnings could come in vX. It is experimental, which mean it can change any time. The user would not be using a stable feature, and would not be getting warnings about it.

So I think I stand by my reasoning.
This may be an argument to deprecate @_-in-signatured-sub ASAP.

[Corion]

OK - so I'll postpone this PR until after @_-in-signatured-sub is deprecated.