Track nullable state for extension properties

[jcouv]

Fixes #81913
This was discussed with extensions working group (Aleksey and Mads)

Problem

C#'s nullable flow analysis (NullableWalker) did not track null-state for extension instance properties (the extension(Receiver) { public T P { get; set; } } feature). As a result:

• A null-test did not narrow: if (o.P != null) o.P.ToString(); still warned (the customer-reported false positive from Extensions: Confirm whether we should track nullable state for extension properties #81913).
• Assignment did not update state: after o.P = new object();, a prior o.P = null; still produced spurious CS8602 warnings.

#nullable enable
object o = new object();
o.P = null;
o.P.ToString();      // warns (correct)
o.P = new object();
o.P.ToString();      // BEFORE: warns (bug)  /  AFTER: no warning

static class E
{
    extension(object o)
    {
        public object? P { get => throw null!; set => throw null!; }
    }
}

Fix

Three changes in NullableWalker.cs:

1. IsSlotMember — for an extension instance member, key against the extension parameter's (substituted) type via TryGetInstanceExtensionParameter instead of the marker ContainingType, so the write path can create a stable member slot.
2. VisitPropertyAccess (extension branch) — add read-side slot consultation (MakeMemberSlot + GetState) mirroring the regular VisitMemberAccess path.
3. InheritDefaultState — for extension block members, use the symbol directly instead of AsMemberOfType (which asserts the member is not an extension block member and would otherwise crash).

The read path keys on the reinferred updatedProperty while the write path keys on node.PropertySymbol; these resolve to the same slot because VariableIdentifier.Equals compares with TypeCompareKind.AllIgnoreOptions (ignores nullability). Generic extensions work because the binder substitutes type arguments at the concrete access site.

Extension indexers are intentionally unaffected — regular indexers aren't slot-tracked either (their identity includes argument values).

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR updates C# nullable flow analysis (NullableWalker) to track and consult null-state for extension instance properties, aligning their behavior with regular instance properties and addressing incorrect warnings/flow behavior in extension-property scenarios.

Changes:

• Update member-slot identity and default-state inheritance logic for extension block members in NullableWalker.
• Consult tracked member-slot state on reads of extension properties (mirroring normal member access).
• Add/adjust compiler nullability regression tests for extension properties, including state reset and attribute/postcondition/pattern interactions.

Show a summary per file

File	Description
src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs	Tracks extension property members via receiver parameter type, avoids AsMemberOfType for extension members, and consults member-slot state on extension-property reads.
src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests2.cs	Adds/updates nullability tests validating extension-property state tracking and adjusts expectations in several pattern/postcondition scenarios.
src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs	Adds nullability tests around AllowNull/DisallowNull on extension property setters in object initializers and with expressions (source + metadata).

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 2