Your second example works fine for me:
https://play.golang.org/p/ay4QlHnQFU

The first example doesn't work because you are defining a block. You can think of blocks as subroutines; the variables from one block do not leak into the scope of the second block. This is specifically documented:

A template invocation does not inherit variables from the point of its invocation.

Thanks for pointing that out. Yes, the second example should be inside a define block. I think they should be in scope at block definition, not when they're applied/run.

The scenario I had in mind:

type Names struct {
    First string
    Last []string
}

{{$first := .Names.First}}
{{define "names"}} {{range .}} {{$first}} {{.}} {{end}} {{end}}
[...]
{{template "names" .Names.Last}}

This won't work, but seems like it should, and it would be useful.

Okay, so look at this example then:

https://play.golang.org/p/sAKdMc6Sbv

In that example, if the contents of the block had been {{ $first }}, what should the code have printed?

Remember that templates can be invoked without invoking their enclosing code.

I'd expect a not-defined error, like it works now.

I think I missed your last sentence. Are you saying that a template defined in another template can be used without running the outer template?

Yes, that's what my example attempts to demonstrate.

Is the template namespace global? Is the whole thing parsed before scanning for templates to register?