Why does \newcommand on an already defined active character not yield an error?

Question

We (or at least I) usually think of an active character as just another command whose name just happens to a character rather than a control sequence. So why does \newcommand<active character> not yield an error if the <active character> was already defined?

\documentclass{article}

\begin{document}

\newcommand\newcommand{coffee} % error

\newcommand~{coffee} % no error

~

\end{document}

Answer 1

As with any "why" command about latex the answer is related to the token memory available in 1985 (2.0x) or 1993 (2e)

This century things are different

\documentclass{article}

\begin{document}



\NewDocumentCommand~{coffee} % no error

~

\end{document}

gives

! LaTeX cmd Error: Command '~' already defined.

For immediate help type H <return>.
 ...                                              
                                                  
l.8 
    
? 

Answer 2

An answer looking at the source code: First we have the macros \@ifundefined and \@ifdefinable:

Note that \@ifundefined takes in strings (technically "the name of the control sequence") as the argument, not the control sequence itself.

However \@ifdefinable takes in a control sequence as the input. So if it wants to use \@ifundefined in its implementation, it has to extract the name:

It's not too hard to see the check only works for control sequences, not active characters.

Side note, assuming nobody wants to use this command when \escapechar is -1 or 32 (space character), it would work correctly.

Side note 2, \@ifundefined, contrary to what its name implies, is supposed to^(*) check if the argument's meaning is either undefined or \relax.
Nevertheless, it first uses \ifcsname ... \endcsname to check if it's undefined first, then if it is not, construct the control sequence with the name to check if it's \relax, presumably to save an entry in the hash table when the name is not defined.
(I suspect in pre-ε-TeX days, only the csname-relax check is performed, then after that the ifcsname is added in to save an entry; but to maintain backwards compatibility it still need to check if the meaning is \relax -- although I didn't check this in the old LaTeX source code)

Maybe someone who can find want source code of older LaTeX versions can confirm that these versions use just the csname-\relax check.

^(*): Or so I think.