Using a "set's" value in an if statement doesn't make sense either, compared to

    if _, exists := something[item]; exists { ... }

As Rob said, "Clear is better than clever." https://www.youtube.com/watch?v=PAAkCSZUG1c&t=875s

I also don't want to be hunting down whoever creates a map[T]bool and doesn't leave a comment explaining it's a set ;)

Hopefully we won't have to worry about it for much longer regardless, since I imagine we'll be using proper sets (with methods) some point in the near future.

But, that's one of those "requires extensive documentation" (and ideally, wrapping in a custom type and provide methods for checking and manipulating the set(s)).

In either way, even with that type of data type underlying a set, I would (probably) provide a functional API to interact with it.

    m["x"] = foo()
    for k := range {
        // if foo returned false, this is buggy
    }

for k, v := range m { if v { } }

But it also allows you to use "if m[key] { ... }". It is literally a trade-off for what convenience you want. And in both cases, you should (probably) wrap an abstraction around the raw map.