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Create a new tiddler with below content
<$list filter="[function[not.defined]count[]]"/>
<$count filter="[function[not.defined]]"/>
<$count filter="[not.defined[]]"/>
All of these returns 1782 which is equal to <$count filter="[all[]]"/>!
But this is confusing and error prone!
When used in a filter, functions are “selection modifiers”. This means that they expect to have preceding filter syntax that provides a list of items as input to the function.
However, when a selection modifier occurs as the first operator in a filter run that does not begin with + or :then, there are no specified input items and it is instead preceded by an implicit all[tiddlers].
This behavior is consistent with all the other selection modifiers, such as [tag[...]], [has[...]], [addprefix[...]], and even math operators like [abs[]] (which will return 1782 zeros!).
The only issue I see is that I can’t find where this handling is documented.
-e
Hi Eric,
Thank you for your clarification!
I expect a not.defined function returns nothing so <$list filter="[all[]function[not.defined]]"/> should return nothing, but it passes through all input!
This is a bit confusing especially when you have the function but its scope is local and cannot be seen in calling tiddler.
Filter Operators starts with:
A filter operator is a predefined keyword attached to an individual step of a filter. It defines the particular action of that step.
Important: In general, each first filter step of a filter run not given any input titles receives the output of [all[tiddlers]] as its input.
While it definitely belongs there, it might also make sense in Filters and Filter Syntax.
If only there was some way to include small pieces in many places. 
This is also a pretty useful trick when dynamically creating filters. For example, selectively defining a function that is a selection modifier based on a user UI selection. If the function does not exist, everything is passed through.
Combined with using text references to specify function names and thus switching between which function is called, I have been getting a lot of mileage out of functions to replace string concatenation to create dynamic filter strings.
How’s this for reducing confusion: when the filter fails, what you get is …
unfiltered
Which is to say, a firehose of all of the tiddlers. Makes sense, no?