Behaviour of an empty conditional expression is undefined?

[Qingsheng Huang]

Here shows the behavious of empty conditional expressions:

------------------------------------------------------------------------
infinite:~# [[ ]]
-bash: syntax error near unexpected token `]]'
infinite:~# [[ ]]
infinite:~# [[ "$A" ]]
-bash: syntax error near unexpected token `"$A"'
infinite:~# [[ "$A" ]]
infinite:~# [[ "$A" ]]
infinite:~# [[ ]]
infinite:~# [[ "$A" ]]
-bash: syntax error near unexpected token `"$A"'
infinite:~# echo $?
2
infinite:~# [[ "$A" ]]
infinite:~# echo $?
1
infinite:~#
infinite:~# bash --version
bash --version
GNU bash, version 5.2.15(1)-release (x86_64-pc-linux-gnu)
------------------------------------------------------------------------

The variable $A has never been defined.
Double brackets surrounding 1) a few spaces or 2) an empty variable even
quoted behave illogically.  I've also try different lengths of spaces, and
also an empty variable without quotes, the same illogical.

I am curious if I am the first guy encounter such a situation.
My observation:
1) An empty conditional expression will first triger an syntax error, return
   with code 2;
2) Immediate another empty conditional expression won't triger such an error,
   return with code 1;
3) Subsequent a conditional expression with empty variables will triger the
   error again;
4) The same conditional expression won't triger the error until different
   commands are input;

I guess it has something to do with the cache of command evaluation.
It seems there is a counter recording some state.  I hope my guess may help
in debugging.

According to the manual, I think an empty condition like ``[[   ]]'' and
``[[ $EMPTY ]]'' should always triger syntax error, but a condition with
an empty string like ``[[ "" ]]'' and ``[[ "$EMPTY" ]]'' should evaluate
to false or return 1.