Select followed by Any / Count(predicate) / Max / Sum returns false or 0: value-type default is taken as the push-down result #181

Version: BLite 5.2.1 (main at 45dbc8c), .NET 10.0.12, macOS 27 arm64 (the code path is platform-independent)

What happens

A LINQ query that ends in a Select followed by a terminal returning a value type (bool, int, long, decimal, ...) silently returns that type's default. Select(p => p.Age).Any() returns false and Select(p => p.Age).Max() returns 0 on a collection full of matching documents. No exception is thrown.

The provider tries a SELECT push-down before the general path (BTreeQueryProvider.cs:257-267):

var pushed = TryPushDownSelect<TResult>(model.SelectClause, model.WhereClause);
if (pushed is not null) return pushed;

TryPushDownSelect only applies to sequence results. For anything else it returns default to mean "not applicable" (BTreeQueryProvider.cs:599-605):

if (!resultType.IsGenericType) return default;

For a reference-type result such as string, default is null, so First() falls through and works. For a value-type result, TResult? on an unconstrained generic is just TResult. So default is false or 0, which is not null, and the caller returns it as the query result.

Count() is unaffected only because an earlier fast path handles it. Where(...).Select(...).Any() has the same problem.

Repro

Runs as-is in tests/BLite.Tests against the existing TestDbContext (PeopleV2).

[Fact]
public async Task Select_Then_Scalar_Terminal()
{
    await _db.PeopleV2.InsertBulkAsync(
        Enumerable.Range(0, 300).Select(i => new PersonV2 { Name = $"p{i}", Age = i % 50 }).ToList());
    var q = _db.PeopleV2.AsQueryable();

    Assert.True(q.Select(p => p.Age).Any());
    Assert.True(q.Select(p => p.Age).Any(a => a == 7));
    Assert.Equal(6, q.Select(p => p.Age).Count(a => a == 7));
    Assert.Equal(49, q.Select(p => p.Age).Max());
    Assert.Equal(7350, q.Select(p => p.Age).Sum());
    Assert.True(q.Where(p => p.Age == 7).Select(p => p.Name).Any());
}

Output

Select(p => p.Age).Any()                         False
Select(p => p.Age).Any(a => a == 7)              False
Select(p => p.Age).Count(a => a == 7)            0
Select(p => p.Age).Max()                         0
Select(p => p.Age).Sum()                         0
Where(p => p.Age == 7).Select(p => p.Name).Any() False

Select(p => p.Age).Count()                       300   (correct)
Select(p => p.Name).First()                      p..   (correct)
Select(p => p.Age).ToList().Count                300   (correct)

Expected: True, True, 6, 49, 7350, True.

Possible fix (for discussion)

Only try the push-down when TResult is a sequence, or have TryPushDownSelect report success with a bool and an out value instead of using default as a sentinel. Any other early-return helper that uses default to mean "not applicable" would have the same problem for value-type results; I haven't checked them all.

Happy to send a PR for this if you would like one.