command variable assignment exports for function

[apprehensions]

If no command name results [from processing the command line], or if the command name is a special built-in or function, variable assignments shall affect the current execution environment. Otherwise, the variable assignments shall be exported for the execution environment of the command and shall not affect the current execution environment except as a side-effect of the expansions performed [while doing tilde expansion and other stuff to get the command line]

$ type true
true is a shell builtin
$ FOO=bar /bin/true
$ echo $FOO

$ FOO=bar true      
$ echo $FOO

$ true() builtin true
$ FOO=bar true        
$ echo $FOO
bar
$