Fix CI

Author: YingboMa

src/discretedomain.jl

@@ -228,7 +228,7 @@ Base.:-(k::ShiftIndex, i::Int) = k + (-i)
 
 Return the time-domain type (`Continuous` or `InferredDiscrete`) that `op` operates on.
 """
-function input_timedomain(s::Shift, arg)
+function input_timedomain(s::Shift, arg = nothing)
     if has_time_domain(arg)
         return get_time_domain(arg)
     end
@@ -240,26 +240,26 @@ end
 
 Return the time-domain type (`Continuous` or `InferredDiscrete`) that `op` results in.
 """
-function output_timedomain(s::Shift, arg)
+function output_timedomain(s::Shift, arg = nothing)
     if has_time_domain(t, arg)
         return get_time_domain(t, arg)
     end
     InferredDiscrete
 end
 
-input_timedomain(::Sample, _) = Continuous
-output_timedomain(s::Sample, _) = s.clock
+input_timedomain(::Sample, _ = nothing) = Continuous
+output_timedomain(s::Sample, _ = nothing) = s.clock
 
-function input_timedomain(h::Hold, arg)
+function input_timedomain(h::Hold, arg = nothing)
     if has_time_domain(arg)
         return get_time_domain(arg)
     end
     InferredDiscrete # the Hold accepts any discrete
 end
-output_timedomain(::Hold, _) = Continuous
+output_timedomain(::Hold, _ = nothing) = Continuous
 
-sampletime(_, op::Sample, _) = sampletime(op.clock)
-sampletime(_, op::ShiftIndex, _) = sampletime(op.clock)
+sampletime(op::Sample, _ = nothing) = sampletime(op.clock)
+sampletime(op::ShiftIndex, _ = nothing) = sampletime(op.clock)
 
 changes_domain(op) = isoperator(op, Union{Sample, Hold})