Program.cs

using Microsoft.Data.Sqlite;
using Microsoft.EntityFrameworkCore;

using var connection = new SqliteConnection("DataSource=:memory:");
connection.Open();

var options = new DbContextOptionsBuilder<TestContext>()
    .UseSqlite(connection)
    .LogTo(Console.WriteLine)
    .EnableSensitiveDataLogging().Options;

try
{
    using (TestContext testContext = new(options))
    {
        await testContext.Database.EnsureCreatedAsync();
    }

    using (TestContext testContext = new(options))
    {
        testContext.TestEntities.Add(new TestEntity { Id = 1, UserId = 1, IsPrimary = false });
        testContext.TestEntities.Add(new TestEntity { Id = 2, UserId = 1, IsPrimary = true });
        await testContext.SaveChangesAsync();
    }

    using (TestContext testContext = new(options))
    {
        TestEntity test1 = await testContext.TestEntities.SingleAsync(t => t.Id == 1);
        TestEntity test2 = await testContext.TestEntities.SingleAsync(t => t.Id == 2);

        test1.IsPrimary = true;
        test2.IsPrimary = false;
        await testContext.SaveChangesAsync();
    }
}
catch (Exception ex)
{
    // Does not throw in EF Core 6. Throws in EF Core 7 and 8.
    throw;
}


public class TestContext : DbContext
{
    public DbSet<TestEntity> TestEntities { get; set; }

    public TestContext(DbContextOptions<TestContext> options) : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<TestEntity>()
            .HasIndex(e => new { e.UserId, e.IsPrimaryNormalized })
            .IsUnique();

        modelBuilder.Entity<TestEntity>()
            .Property(e => e.IsPrimaryNormalized)
            .HasComputedColumnSql($"IIF(IsPrimary, true, null)", stored: true);
    }
}

public class TestEntity
{
    public int Id { get; set; }
    public int UserId { get; set; }
    public bool IsPrimary { get; set; }
    public bool? IsPrimaryNormalized { get => IsPrimary ? true : null; set { } }
}