The spec says:

if the map is nil or does not contain such an entry, a[x] is the zero value for the element type of M

and:

The "++" and "--" statements increment or decrement their operands by the untyped constant 1.

And:

The following assignment statements are semantically equivalent:
x++ .... x += 1

And:

An assignment operation x op= y where op is a binary arithmetic operator is equivalent to x = x op (y) but evaluates x only once.

Therefore,

m["go"]++

is equivalent to:

m["go"] = m["go"] + 1

And because m["missing key"] means the zero value, that means:

m["go"] = 0 + 1

I think this is fully specified.

I concur, this is not a bug.

Interesting. My mistake. Sorry for wasting your time