SQL Server: translate `dateOnly.ToDateTime(timeOnly)` to `DATETIMEFROMPARTS`

[IanKemp]

As title.

Transform to

EF.Functions.DateTimeFromParts(
    dateOnly.Year, dateOnly.Month, dateOnly.Day,
    timeOnly.Hour, timeOnly.Minute, timeOnly.Second, timeOnly.Millisecond)

which would then translate (using the current mapping of EF.Functions.DateTimeFromParts) to the SQL

DATETIMEFROMPARTS(
    DATEPART(year, dateOnly), DATEPART(month, dateOnly), DATEPART(day, dateOnly),
    DATEPART(hour, timeOnly), DATEPART(minute, timeOnly), DATEPART(second, timeOnly), DATEPART(millisecond, timeOnly))

[roji]

Putting in the backlog as a possible translation. One problem with this kind of translation is that it duplicates its argument (in this many times over); this is OK if the argument is a simple value (column), but if it's e.g. a complex scalar subquery, that would reevaluate the subquery many times over. At least at first, I'd not perform the translation in that case.

[mseada94]

@roji
I would like to work on this method translation.
I am new to the repo, I will try to understand the translation and then start working on the code.
I think this PR would be a good reference to understand the method translation #31180

[roji]

@mseada94 sure thing, go ahead.

[mseada94]

@roji
Should I perform the translation for SQL Server Only and skip SQLite considering the issue #18844 and #25103?

[mseada94]

I will start by adding two tests should these test be enough?

    [ConditionalTheory]
    [MemberData(nameof(IsAsyncData))]
    public virtual Task Where_DateOnly_ToDateTime_With_Constant_TimeOnly(bool async)
        => AssertQuery(
            async,
            ss => ss.Set<Mission>()
                .Where(o => o.Date.ToDateTime(new TimeOnly(21, 5, 19, 94)) == new DateTime(1990, 11, 10, 21, 5, 19, 94))
                .AsTracking());


    [ConditionalTheory]
    [MemberData(nameof(IsAsyncData))]
    public virtual Task Where_DateOnly_ToDateTime_With_Property_TimeOnly(bool async)
        => AssertQuery(
            async,
            ss => ss.Set<Mission>()
                .Where(o => o.Date.ToDateTime(o.Time) == new DateTime(1990, 11, 10, 10, 15, 50, 500))
                .AsTracking());

[roji]

@mseada94 please submit a PR rather than posting implementation questions here - I'll review the PR (though it may take some time).