Partial properties: merging declarations and emit of simple cases

[RikkiGibson]

Test plan: #73090

[RikkiGibson]

I did consider introducing some new members on a common base type, or introducing an IPartialMemberSymbol interface or something similar. But ultimately I think extension methods with type tests is an appropriate level of abstraction for this feature.

I am meaning to add a debug assertion that these methods are only used with original definition symbols. I'll try and add a check for that in future PR.

[jcouv]

Done with review pass (iteration 1)

[RikkiGibson]

I have addressed the feedback, please take another look at your convenience @jcouv.

[RikkiGibson]

I have addressed the next round of feedback, please take another look at your convenience @jcouv.

[jcouv]

LGTM Thanks (iteration 3)

[333fred]

Leaving my feedback on the impl now. Still need to review tests.

[333fred]

Done review pass.

[RikkiGibson]

@333fred @jcouv I believe I have addressed all the comments. Please take another look at your convenience.

[jcouv]

nit: could we link to a tracking issue? Is the problem what's described in #46464?

[jcouv]

Alternatively, take the switch out of the if?

    Symbol implementation = sym switch
    {
        MethodSymbol method => method.PartialImplementationPart,
        SourcePropertySymbol property => property.PartialImplementationPart,
        _ => throw ExceptionUtilities.UnexpectedValue(sym)
    };

    if (implementation is { } && InSpan(implementation.GetFirstLocation(), this.syntaxTree, memberSpan))
    {
        result = implementation;
        return true;
    }

[RikkiGibson]

I am not sure, I feel like it is more likely to have the same cause as #73251

[RikkiGibson]

I would be willing to do either including a link or extracting the switch in a follow-up PR. I almost feel a little defiant toward keeping the current factoring since I want the formatter to be fixed :)

[jcouv]

I'd like it fixed too :-) But all the related issues are currently marked as Backlog :-/
It would help to add links and screenshots to get a fix prioritized

[RikkiGibson]

Done

[jcouv]

LGTM Thanks (iteration 6) with a minor comment to consider