Table Splitting Discriminator property should be nullable by default

[MaxG117]

I ran into a problem while validating Can_manipulate_entities_sharing_row_independently in TableSplittingTestBase. The test attempts to remove one of the entities ("Engine") from a shared table ("Vehicles"), which really just sets two columns ("Description" and "Engine_Discriminator") to NULL. This fails with our ADO.NET provider, because the DbParameter.Value for the discriminator column is DbNull.Value, but DbParameter.IsNullable is false.

If I'm understanding the Table Splitting feature correctly, the discriminator properties should all be nullable by default. The TypeMappedRelationalParameter.AddParameter implementation creates a new instance of the relational parameter, which inherits IsNullable from property.IsNullable. Eventually this becomes a provider-specific implementation of DbParameter with IsNullable=false.

Please confirm if this is a bug in EF Core, or perhaps in the Functional Test (the Metadata.IsNullable could be set to true for the discriminator), or if I'm mistaken in my understanding of the feature.

Further technical details

EF Core version: 3.1.5
Database provider: the one I'm writing