Raise Roslyn and NetAnalyzers package versions

[gfraiteur]

Core/PostSharp.Analyzers/PostSharp.Analyzers.csproj pins Microsoft.CodeAnalysis to 4.6.0.
Analyzers built against an earlier Roslyn continue to load, but they do not recognize the C# 15
syntax kinds: union declarations, the closed modifier, labeled jump statements, with(...)
elements and unsafe(...) expressions.

• Confirm the analyzers do not report incorrect diagnostics on C# 15 source, and decide whether to
  raise the version.
• Microsoft.CodeAnalysis.NetAnalyzers is at 9.0.0 across the tree and needs raising for the new
  rule set.

Analysis: Documentation/DotNet11-CSharp15-Impact.md in postsharp/PostSharp.

— Claude for Gael

[gfraiteur]

Concrete Roslyn versions, read from a developer machine, for the version decision.

Host	Roslyn	Default C#	C# 15 available
Visual Studio Professional 2026, 18.9.1	5.9.0	14.0	only under -langversion:preview
.NET 10 software development kit, 10.0.401	5.9.0	14.0	only under -langversion:preview
.NET 11 software development kit, release candidate 1	5.11.0	15.0	yes, as a numbered version

The Roslyn minor version tracks the monthly Visual Studio train: 18.9 carries 5.9. Roslyn 5.11 is
therefore paired with Visual Studio 18.11, which is not the current stable release. The current
stable release is 18.10, from 8 September 2026. Roslyn 5.11 is available today only inside the .NET
11 software development kit, which is what dotnet build uses.

Two points for this issue.

PostSharp.Analyzers pins Microsoft.CodeAnalysis to 4.6.0, while the host in Visual Studio 2026 is
5.9 and in the .NET 11 kit is 5.11. The gap is a major version, not a minor one, so the question is
whether an analyzer compiled against 4.6.0 still loads and behaves correctly in a Roslyn 5.x host,
before the question of whether to raise the reference.

Roslyn 5.9 already compiles union and closed under -langversion:preview, and emits the same
metadata as 5.11: a sealed struct implementing IUnion with [UnionAttribute], and an abstract
class with [IsClosedTypeAttribute]. C# 15 metadata shapes are therefore reachable from Visual
Studio 2026 18.9 today, not only from the .NET 11 kit. This does not change the conclusion that the
C# 15 features have no impact on PostSharp, but it does mean the shapes can appear before a customer
moves to the .NET 11 tooling.

— Claude for Gael