Core differ on SQL Server scaffolds a NULLS-ordered or expression index that fails at apply; the core README says migrations add fails #172

Found by a pre-release sweep of main @ eff80b4 (lens: docs-claims); reproduced in a scratch test. Not blocking the tag: the failure is loud. Moving the refusal to migrations add in a 5.5.x patch changes no applied answer, and it makes the packed README true.

Problem

The core package's packed README (src/EFCore.ComplexIndexes/README.md:98-100) says: "On SQL Server, and on providers without their own package such as SQLite, an index that uses [DbOrder.NullsFirst/NullsLast] makes dotnet ef migrations add fail."

On SQL Server, with the core differ, it doesn't fail there. ValidateRenderableParts returns early for any provider listed in Satellites (if (_satellite is not null) return;), and #150's ValidateNothingOnlyTheSatelliteRenders checks only options, constraints and the nullable-unique filter. So the migration scaffolds with the Npgsql sentinel column. database update then fails with an invalid column name that points at UseNpgsqlComplexIndexes(), not at the SQL Server package. The same applies to expression parts.

This happens in two cases:

• A SQL Server consumer installs only the core package, which is who reads that README.
• At runtime, UseComplexIndexes() is called in place of UseSqlServerComplexIndexes() (EnsureCreated, GenerateCreateScript).

With the SQL Server satellite's differ, it does fail at migrations add, so docs/sqlserver.md and the SQL Server README are right. The README sentence is older than #166; 934ec55 says the same.

Model

mb.Entity<NCommit>(b => b.HasComplexCompositeIndex(
    x => new { x.Created, Reviewed = DbOrder.NullsLast(x.ReviewedAt) }, indexName: "ix_commits_reviewed"));

MigrationHarness.Diff<CustomMigrationsModelDiffer>(sqlServerOptions, null, MigrationHarness.SqlServerModel<NCtx>()) scaffolds without an error, and renders:

CREATE INDEX [ix_commits_reviewed] ON [NCommit] ([Created], [ReviewedAt], [__requires_UseNpgsqlComplexIndexes__]);

Change

In the core differ, refuse NULLS-ordered and expression parts on a provider whose satellite isn't the one diffing. Do this in ValidateNothingOnlyTheSatelliteRenders, with the satellite's remedy (UseSqlServerComplexIndexes(), or reference EFCore.ComplexIndexes.SqlServer). On PostgreSQL the same refusal would replace an apply-time sentinel failure with a migrations add one. Rewording the README instead is the fallback.

Test

The model above through the core differ on SQL Server throws InvalidOperationException naming the SQL Server package. Add the same for an expression part, and for the core differ on Npgsql.