[C# 10] Code lens doesn't work with file scoped namespaces

[vchirikov]

csharplang/file-scoped-namespaces.md

Omnisharp: 1.37.15-beta.34
omnisharp-vscode: v1.23.15

Expected behavior

Actual behavior

[JoeRobich]

The fix would be to update the CodeStructureService to handle the new BaseNamespaceDeclarationSyntax instead of NamespaceDeclarationSyntax (see https://github.com/OmniSharp/omnisharp-roslyn/blob/master/src/OmniSharp.Roslyn.CSharp/Services/Structure/CodeStructureService.cs#L88). Moving to OmniSharp-Roslyn.