Correctly handle conversions in null coalescing

[333fred]

We weren't correctly using the results of the left conversion for cases when the rhs of a null-coalescing assignment determines the output type and was a user-defined conversion. I've also added tests for #29955, which I don't believe needs to be addressed as I couldn't find any scenario that was broken due to it.

Fixes #51975. Fixes #29955.

[333fred]

@dotnet/roslyn-compiler @RikkiGibson for review.

[RikkiGibson]

I don't yet completely understand the nullable changes. Maybe we can go over them.

[RikkiGibson]

nit: it feels like it could be good to change the implementation to something like:

var isUserDefined = Kind.IsUserDefinedConversion();
Debug.Assert(!isUserDefined || (Method is not null && MethodSymbol is not null))
return isUserDefined;

#Resolved

[333fred]

I don't believe this is necessary. When construction occurs, these conditions are validated. #Closed

[jcouv]

I was a bit thrown off by the use of MemberNotNullWhen here too because this method doesn't initialize or check Method or MethodSymbol.
Adding the assertion would make sense to me, but I'm not sure it would hold true.
Seems like we can create user-defined conversions that return null Method:

        internal Conversion(UserDefinedConversionResult conversionResult, bool isImplicit)
        {
            _kind = conversionResult.Kind == UserDefinedConversionResultKind.NoApplicableOperators
                ? ConversionKind.NoConversion
                : isImplicit ? ConversionKind.ImplicitUserDefined : ConversionKind.ExplicitUserDefined;

            _uncommonData = new UncommonData(
                isExtensionMethod: false,
                isArrayIndex: false,
                conversionResult: conversionResult,
                conversionMethod: null, // this is used for `Method`
                nestedConversions: default);
        }

---

In reply to: 599078854 [](ancestors = 599078854)

[333fred]

@dotnet/roslyn-compiler for a second review please.

[333fred]

@dotnet/roslyn-compiler for a second review please.

[333fred]

@dotnet/roslyn-compiler for a second review please.

[333fred]

@dotnet/roslyn-compiler for a second review please.

[jcouv]

Done with review pass (iteration 2)

[333fred]

@jcouv addressed feedback.

[jcouv]

LGTM Thanks (iteration 3)