Error message garbage when parameter expansion used inside (()) and variable unset

[PRussell]

Section 6.5 Shell Arithmetic says,

"Within an expression, shell variables may also be referenced by name without
using the parameter expansion syntax. A shell variable that is null or unset
evaluates to 0 when referenced by name without using the parameter expansion
syntax." - http://www.gnu.org/software/bash/manual/bash.html#Shell-Arithmetic

The above tells us what happens to an unset variable if not using parameter
expansion.

But if a shell variable uses parameter expansion and is null or unset, what
does it evaluate to inside (()) syntax?  

It evaluates to 0 inside [[]], but gives an error inside (()). The error 
message is different between bash 4.3.11 and 4.4.19. The later contains 
garbage. 

See test 3 and 4 below.


### Test 1:
# No parameter expansion. var2 not set. Should evaluate to 0 thus when compared
# to 0, 1A and 1B should echo yes. They do.

echo;echo 1A
  ( set -x;var=0;var1=var; [[ var1 -eq var2 ]] && echo yes || echo no )
echo 1B
  ( set -x;var=0;var1=var; (( var1 == var2 )) && echo yes || echo no )

### Test 2:
# No parameter expansion. var2 not set. Should evaluate to 0 thus when compared
# to 5, 2A and 2B should echo no. They do
                                                                                
                                       
echo;echo 2A                                                                    
                                       
  ( set -x;var=5;var1=var; [[ var1 -eq var2 ]] && echo yes || echo no )         
                                       
echo 2B                                                                         
                                       
  ( set -x;var=5;var1=var; (( var1 == var2 )) && echo yes || echo no )          
                                       
                                                                                
                                       
### Test 3:                                                                     
                                         
# Parameter expansion. var2 not set.                                            
                                       
                                                                                
                                       
echo;echo 3A                                                                    
                                       
  ( set -x;var=0;var1=var; [[ var1 -eq $var2 ]] && echo yes || echo no )        
                                       
echo 3B                                                                         
                                       
  ( set -x;var=0;var1=var; (( var1 == $var2 )) && echo yes || echo no )         
                                       
        
### Test 4:
echo;echo 4A                                                                    
                                       
  ( set -x;var=5;var1=var; [[ var1 -eq $var2 ]] && echo yes || echo no )        
                                       
echo 4B                                                                         
                                       
  ( set -x;var=5;var1=var; (( var1 == $var2 )) && echo yes || echo no )         
                                       
                                                                                
                                       
                                                                                
                                       
It appears that 3A and 4A evaluate to 0 because of the arithmetic context. 
3A echo's yes; 4A echo's no.

The problem is what is happening with 3B and 4B.  I tested on bash 4.3.11 and
bash 4.4.19 and got a slightly different error message.

Bash version 4.3.11:

./tt: line 18: var1: var1 ==  : syntax error: operand expected (error token is 
"==  ")

Bash version 4.4.19 (???? was garabage):

./tt: line 18: ????: var1 ==  : syntax error: operand expected (error token is 
"==  ")

Peggy Russell