• VSCode Version: 1.27.2
• OS Version: Windows 10 Version 1803

Steps to Reproduce:

1. Install the MemFS demo file system provider extension, follow the instructions to set up a MemFS folder
2. Add a new TypeScript file to the MemFS folder, write some example TypeScript code

I expected that auto-complete, signature help, and hover information would be available in the TypeScript file, but it was not. Basic syntax-highliting and some snippets were available, however.

Does this issue occur when all extensions are disabled?: N/A

I thought I had seen a bug for this but I can't find it anymore. Currently extensions/typescript-language-features/src/typescriptServiceClient.ts has explicit checks in the vscode.Uri to block non-file schemes (normalizedPath and getWorkspaceRootForResource). I'm not sure what else needs to be fixed if those checks are removed.