[Help-bash] Assigning builtins behavior

[Quentin L'Hours]

Hi,

I've always thought builtins could not have a custom parsing, however it 
seems that export/readonly/declare/local (all the builtins creating an 
assignment) are able to disable field splitting when an assignment takes 
place, why is that? I know a simple assignment disables field splitting, 
but after all that's because the grammar states it should, but doesn't 
POSIX requires all simple commands to have the same splitting behavior?

bash$ foo=$(echo 1 bar=2)
bash$ echo "$foo"
1 b=2

This is logic, simple assignment doesn't triggers field splitting, 
that's the rule.

bash$ set a=$foo
bash$ echo "$1"
a=1

This is logic too, field splitting happens as expected

bash$ readonly a=$foo
bash$ echo "$a"
1 b=2

Why? I know simple assignment doesn't split but I would have thought 
this should still follow the simple commands arguments expansion and 
thus should do 2 assignments. I mean builtins shouldn't be able to have 
their custom parsing, if they do then they should be classified as 
keywords (like [[ ]]). What should be the POSIX way of doing things? Is 
this a bash extension?

As a side note ksh has the same behavior, but dash follows what I 
thought would be the "normal" behavior:

dash$ foo=$(echo 1 b=2)
dash$ readonly a=$foo
dash$ echo "$a"
1
dash$ echo "$b"
2

Thanks,

--
Quentin