Keep nullable outer-key null check in compound APPLY→JOIN predicates

[Copilot]

Fixes #35706

A collection navigation reached through an optional (nullable) key returned wrong results when combined with an additional filter: entities with a NULL key matched each other instead of yielding an empty collection.

context.People
    .Select(subject => new
    {
        subject.Name,
        Coworkers = subject.Employer!.Employees!
            .Where(employee => employee != subject) // extra filter makes the join predicate compound
            .Select(coworker => new { coworker.Name })
            .ToList(),
    });
// People with no Employer incorrectly listed all other employer-less people as coworkers.

Root cause: SelectExpression.RemoveRedundantNullChecks always stripped the outer-key IS NOT NULL check. For a single-comparison join that's safe (SqlNullabilityProcessor.ProcessJoinPredicate keeps SQL null semantics), but for a compound predicate the processor expands the equality with C# null semantics — adding OR (both sides NULL) — which then matches rows where both keys are NULL.

Changes

• SelectExpression.RemoveRedundantNullChecks: retain the null check on a nullable outer key when the join predicate is compound (more than one key comparison). Single-comparison joins are unchanged. A small CountKeyComparisons helper distinguishes the two cases.
• Baselines: updated affected composite-key navigation SQL in the GearsOfWar suites (SqlServer regular/TPC/TPT/Temporal and SQLite), which now emit outerKey IS NOT NULL AND ... instead of (outerKey = innerKey OR (both NULL)) AND ....
• Regression test: Filtered_collection_through_optional_navigation_does_not_match_on_null_keys (specification base + SqlServer/SQLite SQL assertions).

Notes for review

• The change widens to all compound collection-navigation joins over nullable keys, hence the broad-but-mechanical GearsOfWar baseline churn; results are unchanged for that data since the inner keys are PKs.
• SqlServer functional tests were not run locally (no server); SqlServer baselines mirror the verified SQLite output exactly.

[Copilot]

Pull request overview

Fixes incorrect results for collection navigations reached via an optional key when the correlated APPLY is converted to a JOIN with a compound predicate (e.g., additional filters), by ensuring outer nullable-key null checks aren’t stripped in cases where SqlNullabilityProcessor applies C# null-semantics expansion.

Changes:

• Updated SelectExpression.RemoveRedundantNullChecks to retain nullable outer-key null checks when the extracted join predicate is compound.
• Added a regression test covering filtered collections through an optional navigation, with provider-specific SQL assertions (SqlServer/Sqlite).
• Updated affected SQL baselines across GearsOfWar suites (SqlServer + Sqlite) to reflect the new join predicate shape.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/EFCore.Relational/Query/SqlExpressions/SelectExpression.cs	Keeps nullable outer-key null checks for compound extracted join predicates to prevent incorrect null matching.
test/EFCore.Specification.Tests/Query/AdHocNavigationsQueryTestBase.cs	Adds regression test for filtered collection through optional navigation not matching on null keys.
test/EFCore.SqlServer.FunctionalTests/Query/AdHocNavigationsQuerySqlServerTest.cs	Adds SQL assertion baseline for the new regression test (SqlServer).
test/EFCore.Sqlite.FunctionalTests/Query/AdHocNavigationsQuerySqliteTest.cs	Adds SQL assertion baseline for the new regression test (Sqlite).
test/EFCore.SqlServer.FunctionalTests/Query/GearsOfWarQuerySqlServerTest.cs	Updates SQL baselines to include retained outer-key null checks / simplified predicates.
test/EFCore.SqlServer.FunctionalTests/Query/TemporalGearsOfWarQuerySqlServerTest.cs	Updates temporal SQL baselines to include retained outer-key null checks / simplified predicates.
test/EFCore.SqlServer.FunctionalTests/Query/Inheritance/TPTGearsOfWarQuerySqlServerTest.cs	Updates TPT SQL baselines for the adjusted join predicates.
test/EFCore.SqlServer.FunctionalTests/Query/Inheritance/TPCGearsOfWarQuerySqlServerTest.cs	Updates TPC SQL baselines for the adjusted join predicates.
test/EFCore.Sqlite.FunctionalTests/Query/GearsOfWarQuerySqliteTest.cs	Updates Sqlite SQL baselines for the adjusted join predicates.

[Copilot]

Pull request overview

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.