`X=1,2;X` does not error even though 2 is unused

[jkeiser]

X=1,2;X will assign X=1 and then create a stream (),2, so the output of this is 1. X=(1,2) work as expected.

This should either assign both 1 and 2 to X, or forbid the construction altogether. X = 1,2, X = 1 , 2, and X = 1, 2 should do the same thing as X=1,2.

A few possible solutions:

• The program could output (1,2)
• It could be an error to assign inside a comma at the same precedence level (X=1,2 or X = 1, 2).
• It could be an error to leave a value unused anywhere.
• The program could return the stream 2,1 because there are 2 statements that return values. In this case, () cannot be a value. This might be a wonderful idea, but we still need something more targeted to tell the user that X=1,2 isn't doing what it looks like it should.