SaveChanges circular dependency in unique unfiltered index

[roji]

#28065 raised a 7.0 SaveChanges circular dependency issue with unique indexes; two repros were posted there - one with a filtered index, one without. The fix for that issue (#28923) seems to have fixed only the filtered index case, but the unfiltered one still fails on 7.0.

Repro

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;

var db = new TestDbContext();
await db.Database.EnsureDeletedAsync();
await db.Database.EnsureCreatedAsync();

var t1 = new TestEntity();

db.Tests.Add(t1);

await db.SaveChangesAsync();

var dependent = new TestDependentEntity()
{
    TestEntityId = t1.Id,
    UniqueOn = null,
    ToChange = 1,
};

var dependent2 = new TestDependentEntity()
{
    TestEntityId = t1.Id,
    UniqueOn = null,
    ToChange = 2,
};

db.TestDependents.Add(dependent);
db.TestDependents.Add(dependent2);

await db.SaveChangesAsync();

dependent.ToChange = null;
dependent2.ToChange = null;

await db.SaveChangesAsync();

public class TestDbContext : DbContext
{
    public DbSet<TestEntity> Tests { get; set; }
    public DbSet<TestDependentEntity> TestDependents { get; set; }
    
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlite(@"Data Source=foo.sqlite")
            .LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<TestDependentEntity>().HasIndex(e => new { e.TestEntityId, e.UniqueOn })
            .IsUnique();
    }
}

public class TestEntity
{
    public int Id { get; set; }
}

public class TestDependentEntity
{
    public int Id { get; set; }
    public int TestEntityId { get; set; }

    public bool? UniqueOn { get; set; }
    public int? ToChange { get; set; }
}

/cc @AndriySvyryd

[AndriySvyryd]

A workaround for this is to define a filter:

modelBuilder.Entity<TestDependentEntity>().HasIndex(e => new { e.TestEntityId, e.UniqueOn })
            .IsUnique().HasFilter("true");

This doesn't repro on SQL Server because the provider defines a filter whenever the index contains a nullable column.