eval, apparently inconsistent behavior

[antonio]

#!/bin/bash

a=( 1 2 3 )
b=( 4 5 6 )

x=a
eval b=( \${$x[@]} )
echo ${b[@]}

#output:
#1 2 3


x=b
eval $x=( \${a[@]} )

#output:
#./tst: line 15: syntax error near unexpected token `('
#./tst: line 15: `eval $x=( \${a[@]} ) '