Translate ToString() over enums

[Danevandy99]

Fixes #33635 and #20604

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[Danevandy99]

@roji @cincuranet This is ready to review again.

[roji]

In addition to the below comments, one more general note...

Translations where we duplicate an expression multiple times are problematic, since the expression may be arbitrarily complex. For example, if instance here is a complex subquery, that subquery would get evaluated by the database N times (once for each CASE); this is bad enough for perf that we refrain from translating in most such cases (e.g. entity equality).

At least for now, we should do the same here, i.e. perform the CASE/WHEN translation only when instance is a column or a parameter - can you please add that check?

[roji]

Extract the converter type once to a variable instead of calling GetType() repeatedly.

Can also have a single "converter is not null && converterType.IsGenericType" rather than repeating twice.

[roji]

This looks like it should go into the relational layer rather than be a SQL Server-specific translation, as nothing here seems SQL Server-specific (and so it would work the same way on SQLite, PostgreSQL...). We have StringMethodTranslator in relational where this would fit well.

I'd leave a comment here pointing to StringMethodTranslator (and also in SqliteObjectToStringTranslator).

[Danevandy99]

The existing methods handled by StringMethodTranslator are those called on string objects or on the string class (such as string.IsNullOrEmpty). Would it be better to rename EnumHasFlagTranslator to EnumMethodTranslator and include this code there? This would align with the fact that we are translating the ToString method called on an enum, similar to how we translate the HasFlag method on an enum.

[roji]

Yep, sounds good!

[roji]

Since the enum is already mapped to a string in the database, an actual SQL cast shouldn't be necessary here. However, there's still a CLR type change here, and we currently lack the ability to "change" the CLR type without a corresponding SQL expression node.

Can you please leave a TODO comment here, referencing #33733?

[Danevandy99]

Yep! I did attempt to just return the instance without the cast at one point, but ran into issues that seemed larger than the scope of this issue.

[roji]

Can you explain why this change is needed?

[Danevandy99]

Calling ToString on a nullable value type like an enum returns an empty string if the value is null, rather than null: https://learn.microsoft.com/en-us/dotnet/api/system.nullable-1.tostring?view=net-8.0 Without that check, the in-memory query returns null if the value of the nullable enum is null, rather than an empty string like C# usually does.

[Danevandy99]

Looks like that might also allow ToString to be called on a reference type, since @object!.Type.IsNullableType() returns true for non-value types and we're only comparing the method names. The new check should probably include @object!.Type.IsNullableValueType()