Expose plain arguments

[LuisHsu]

Sometimes the argument may need to forward to other user-defined interface, and the argument type will lead to syntax error.
Expose argument name only for such purpose.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.85%. Comparing base (82f6b87) to head (a137f16).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   89.78%   89.85%   +0.07%     
==========================================
  Files           5        5              
  Lines         421      424       +3     
==========================================
+ Hits          378      381       +3     
  Misses         43       43              

Flag	Coverage Δ
unittests	89.85% <100.00%> (+0.07%)	⬆️

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