go.go:8: multiple-value f() in single-value context
which looks like the right thing.
Also if you uncomment the case, gotype says:
go.go:10:7: cannot compare f() == f() (operator == not defined for (int, int))
gotype should reject f() in switch case at earlier stage and say something along the lines of the gc error message (the issue is not with comparison, it is with multiple-value in single-value context).