relax the check of no new variables on left side of :=

[gopherbot]

by vincent.risi:

This is not a bug but if the := already allows for some of the variable to 
be old, why not allow for them all to be old?

for instance I would like to have 

  a, b := Called(x)
  ...
  a, b := Called(a*b)

as it does not really affect the usability if this was relaxed.

[rsc]

Comment 1:

On the other hand, code that uses := intends to declare
a new variable.  If there are no new variables to declare, that
seems worth mentioning.

Labels changed: added languagechange.

Status changed to Thinking.

[gopherbot]

Comment 2 by soniakeys:

One could argue in favor of strengthening the check and requiring all variables to be
new.  The way it stands, you can't tell by looking at short declaration of multiple
variables which, if any of the variables are existing.  You have to read back through
the code and hunt for previous declarations of each variable.  If I add a short
declaration to some code, I might like the compiler to complain if I am stepping on
an existing variable.

[griesemer]

Comment 3:

This issue should be considered together with issue #377.

[rsc]

Comment 4:

Status changed to Duplicate.

Merged into issue #377.