> people use a zero value to indicate that a value is missing

Go supports returning multiple results. Maybe I don't read enough Go code to understand what's actually popular, but AFAIU the idiomatic way to accomplish this in Go is something like:

  if v, exists := lookup(key); exists {
    do(v)
  }
 

or

  if r, err != foo(); err != nil {
    do(r)
  }

The compiler, unfortunately, can't enforce proper conditionals. OTOH, unlike C compilers Go won't make any dangerous assumptions about dereferencing nil pointers, which is something.

I'm not sure Generics can help here. What you really want is compiler enforcement of comprehensive condition checks. For example, via pattern matching switch statements. That's easiest to do with optional types as the unwrapping operation creates a simple point for static constraint checking, but in principle the same thing could be accomplished with annotations on multi-value return types that describe the association.