GoInspectionUtil.getExpressionResultCount returns -1 for GoBuiltinCallExpr.
Example golang code with builtin make call:

func f() { m := make(map[string]int,10) ... }

Method definition:
https://github.com/eugene-auduchinok/go-lang-idea-plugin/blob/7118347bf439e09c1cecc4482d0bfd75dc604ca7/src/com/goide/inspections/GoInspectionUtil.java#L31-31

Tell me if I'm wrong and this is a proper behaviour for some reason.