Update console & classlib project templates to opt-in to ImplicitUsings feature

[DamianEdwards]

To prepare for the changes coming in dotnet/sdk#19521 we need to update the .NET 6 project templates to include the <ImplicitUsings>enable</ImplicitUsings> property setup in their project files.

[DamianEdwards]

FYI @vlada-shubina

[vlada-shubina]

@bekir-ozturk @DavidKarlas could you please take it over whilst I'm on leave? Thanks

[DavidKarlas]

I looked into making this change... We need SDK to make switch first so our integration tests can pass...

[DamianEdwards]

@DavidKarlas why is that? The property is benign right now so I think the opposite is true isn't it? Add the property now so that when the SDK change lands the templates continue to get implicit usings applied.

[DavidKarlas]

Problem is that if user specify following parameters:

dotnet new classlib -o MyProject --framework net6.0 --langVersion 9.0

and tries to build project using dotnet build this is result:

error CS8023: Feature 'global using directive' is not available in C# 9.0. Please use language version 10.0 or greater.

[AraHaan]

I think the issue is that the templates should be made dependent on language versions set.

However I think that with .NET 6 one should not be able to downgrade to C# 9 or lower due to the issue pointed above about global usings not being in C# 9.

[DamianEdwards]

The templates that support specifying version as an option (like classlib and console) could decide to condition the addition of <ImplicitUsings>enable</ImplicitUsings> based on language version, i.e. if --langVersion is specified and is less than 10 then do not emit the property.

[DavidKarlas]

I have that change ready, problem is that integration tests are failing with error message from previous comment with current SDK, hence need to wait for SDK change... I guess I could disable those tests temporary... I will open PR

[DamianEdwards]

@DavidKarlas ah sorry I understand now. I keep forgetting the classlib and console templates are "special" in that they support defining language version and thus you'd have tests for that 😄

Do the needful to coordinate the changes.

[AraHaan]

Perhaps what should be done is default the templates to disabled but have a line that overrides it like so:

<ImplicitUsings Condition="'$(LangVersion)' == '10' OR '$(LangVersion)' == 'preview' OR '$(LangVersion)' == 'latest'">enable</ImplicitUsings>

I think with the .NET 6 SDK that technically all 3 values would mean the same thing?

Also I think with this it would make it a minimal yet also non-breaking change for tests?

[DamianEdwards]

@AraHaan as long as the conditions are evaluated by the template engine at project creation time, rather than staying as MSBuild conditions in the project file, I think it makes sense to default the inclusion of <ImplicitUsings>enabled</ImplicitUsings> based on the template's --langVersion option.