Expanding command with optional arguments: edef

Question

I have defined a LaTeX command with an optional argument, e.g., newcommand*{commA}[1][opt]{A#1}, and I cannot get it to be part of an expanded command defintion with edef, e.g., edefexpcom{commA}.
I have tried different combinations to find out what is going on: first, if I define the command commA without optional arguments, the code compiles smoothly. Second, if I leave commA definition with optional arguments, but I define expcom with def, the code compiles smoothly, too. Thus, It is only the combination of optional argument definition and edef that it is faulty.
How can I pass a command with optional arguments to a edef definition?
See example of buggy code
documentclass{article}

begin{document}

%newcommand*{commA}{A} %This definition gives no error
newcommand*{commA}[1][opt]{A#1} %This definition raises error "! Argument of reserved@a has an extra }."

edefexpcomm{commA}
%defexpcomm{commA} %The definition with def gives no error
defnoexpcomm{commA}

noindent
This is verb|expcomm|: expcomm
This is verb|noexpcomm|; noexpcomm

renewcommand*{commA}{B}

noindent
This is verb|expcomm|: expcomm
This is verb|noexpcomm|; noexpcomm

end{document}

The output I would expect with the commA definition with optional argument is following:

When I use the commA definition without optional argument I get following, as expected:

egreg · Accepted Answer

You can get an expandable command with optional arguments provided

the optional arguments are followed by a mandatory one;
you use NewExpandableDocumentCommand. (For LaTeX release before 2020-10-01 include the xparse package)

So with
NewExpandableDocumentCommand{commA}{O{opt}m}{A#1}

you can call
edefexpcommA{commA{}}
edefexpcommB{commA[new]{}}

Note the dummy mandatory argument.
Full test document:
documentclass{article}

NewExpandableDocumentCommand{commA}{O{opt}m}{A#1}

begin{document}

edefexpcommA{commA{}}
edefexpcommB{commA[new]{}}

texttt{meaningexpcommA}

texttt{meaningexpcommB}

end{document}

I believe this is a typical XY problem, though.

Ulrich Diez · Answer

In case it doesn't matter if commA is not replaced by its replacement-text during the edef you can use protected@edef for having expanding commA suppressed while having the tokens forming its optional argument expanded if provided.
documentclass{article}

begin{document}

newcommand*{commA}[1][opt]{A#1} %This definition raises error "! Argument of reserved@a has an extra }."

csname protected@edefendcsnameexpcomm{commA}

noindent This is verb|expcomm|: texttt{meaningexpcomm}

newcommandfoobar{expansion of stringfoobar}
csname protected@edefendcsnameexpcommOptionalArgExpanded{commA[foobar]}

noindent This is verb|expcommOptionalArgExpanded|: texttt{meaningexpcommOptionalArgExpanded}

end{document}

But in your case it seems to matter as in your question you have commA redefined somewhere in the middle of your example...

Expanding command with optional arguments: edef

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