TPH + HasTrigger: Potential undocumented breaking change in EFC8

[Suchiman]

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We've just tried our Code with EFC8 and ran into the following issue:

InvalidOperationException: Can't configure a trigger on entity type 'Derived', which is in a TPH hierarchy and isn't the root. Configure the trigger on the TPH root entity type 'Base' instead.

The reason this occurs is due to the BlankTriggerAddingConvention workaround that was described in the EFC7 breaking changes that we've applied https://learn.microsoft.com/en-us/ef/core/what-is-new/ef-core-7.0/breaking-changes#sql-server-tables-with-triggers-or-certain-computed-columns-now-require-special-ef-core-configuration

That workaround calls HasTrigger on every table which seems to be no longer allowed on every table in EFC8.

This change should be documented and a revised workaround provided.

Include provider and version information

EF Core version: 8.0 Preview 7
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 8.0 P7
Operating system: Windows 10
IDE: Visual Studio 2022 17.8P1

[ajcvickers]

Note from triage: make it a warning and update the guidance to indicate that only the root entity type needs to have triggers defined on it.

[roji]

Note that in EF 8.0 we have the new TableBuilder.UseSqlOutputClause (for SQL Server) and TableBuilder.UseSqlReturningClause (for SQLite) which replaces the "add a blank trigger" technique (#29916).