This is expected. The spec says

Pointers to distinct zero-size variables may or may not be equal.

You're seeing the equal case.

Note that this has been the behavior since Go1.1.

@randall77, I don't think that is true. The section of the spec you are quoting is how comparisons are done. At least in Go1.8, pointers to zero-length values are still compared based on the pointer:

var s = struct {
	t1 T
	_  int
	t2 T
}{}

m := make(map[*T]bool)
m[&s.t1] = true
m[&s.t2] = true
fmt.Println(m) // map[0xc42000e250:true 0xc42000e258:true]

It's just that since Go1.1, the new function returned the same address for zero-length values. The allocation section does not forbid this, so it seems to be okay behavior.

@dsnet, I'm not sure what you are objecting to.

The code you posted is allowed by the spec to result in a map with only one element.

There are many ways to get a pointer to a zero-sized variable. The compiler/runtime will make some of them equal and some of them not, kind of arbitrarily.