I happened to encounter the same problem.
I don’t know how you can use methods 1 and 2 correctly, the simplest method is also required

asInstanceOf[ManagedChannelBuilder[_]]

Actually

public static ManagedChannelBuilder forTarget(String target) { return ManagedChannelProvider.provider().builderForTarget(target); }

What is returned is ManagedChannelBuilder[Any]. In fact, it shouldn't be Any here. But a

T extends ManagedChannelBuilder

Then nameResolverFactory returns this T as Any.
Then it becomes calling the build method of Any, which obviously cannot pass compilation.

I hope there is a better way~

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Update

Simplify the problem:

class A[T <: A[T]] { def retA: A[_] = ??? def retT: T = ??? } class B extends A[B] (new B).retA.retT

This is the original definition. If you change it to this
def retA: A[_ <: A[_]] = ???
it should be fine.

So, it can be like this

(new B).retA.asInstanceOf[A[_ <: A[_]]].retT.retT....

Similarly, when returningManagedChannelBuilder,ManagedChannelBuilder的时候就
.asInstanceOf[ManagedChannelBuilder[_ <: ManagedChannelBuilder[_]]].asInstanceOf[ManagedChannelBuilder[_ <: ManagedChannelBuilder[_]]]