SourceGenerator namespace clashes

[thomhurst]

Say you have this namespace

My.Random.Namespace

And then you're using a library with the namespace

Random.Things

If you then try to reference a class in the library, the source generator will think you're referencing the Random from your own project and thus you'll get compilation errors.

The solution is to fully qualify named types, so they begin with global:: when generating source code, so there can never be any confusion. e.g. global::Random.Things

I've already put out a PR, but this is just to track the issue.

PR: #160