Item templates should pick correct language from project

Is your feature request related to a problem? Please describe.

When using a item template inside a F# project, e.g. dotnet new avalonia.templatedcontrol I would expect that the option -lang=F# would not be required but it is. So it is using the C# version by default when the -lang=F# was omitted/forgotten.

Describe the solution you’d like.

The solution should be that the templating engine should detect that the item template is added to a project of a known language, when the item template has multiple matching identities with different lang tags the correct one should be picked.

Additional context

Somewhat related to: #5358

Also I am aware of #2684 but I don’t think that is a solution when e.g. you mostly use C# but only sometimes F#.

[YuliiaKovalova]

Hi everyone,

I have contacted the author of the https://github.com/microsoft/VSProjectSystem/blob/master/doc/overview/project_capabilities.md - the document is outdated. F# is added to project capabilities.

The problem is on the templating side - the required information isn’t retrieved during the evaluation phase [ProjectCapabilityConstraint.cs](https://github.com/dotnet/sdk/blob/3eea35e0d77f9a1bc418a364805ff89df818e371/src/Cli/dotnet/commands/dotnet-new/MSBuildEvaluation/ProjectCapabilityConstraint.cs#:~:text=static void-,AddProjectCapabilities,-(HashSet%3C )

We are working on fixing the issue.

[vlada-shubina]

After impediment is resolved, the work in template resolution still needs to be done. Picking up correct template language based on project context is not available yet, and template resolution logic should be adjusted to take constraints into account.

Relevant code managing constraints (https://github.com/dotnet/sdk/blob/11f4fee47293e8c9dae5fa902074eb26062d8f99/src/Cli/Microsoft.TemplateEngine.Cli/Commands/create/TemplateCommand.cs#L158-L168) happens only when single template is defined.

It should be moved earlier in the invocation cycle, potentially in this method: https://github.com/dotnet/sdk/blob/11f4fee47293e8c9dae5fa902074eb26062d8f99/src/Cli/Microsoft.TemplateEngine.Cli/Commands/create/InstantiateCommand.cs#L363-L385

If constraints are satisfied only for single template, it should be preferred. This should fix auto language pick-up.