The standard mentions f(a,(t=3,t+2),c); which would be an assignment-expression followed by an expression for the 2nd operator according to my understanding.
But the grammar lists it juxtaposed:
expression:
assignment-expression
expression, assignment-expression
Working Draft, Standard for Programming Language C ++ Revision N4140 (November 2014)
Is someone so nice as to explain to me please what it is that I'm missing here?
When you see
expression: assignment-expression expression, assignment-expression
It mean that there are 2 possibilities for expression. One possibility that it is just assignment-expression that is defined somewhere earlier. Or it is recursively represented as expression, assignment-expression
So after extending it you receive that expression is comma separated list of one or more assignment-expression tokens.
In the sample you're mentioned second parameter is expression (t=3,t+2) which consists of 2 comma-separated assignment-expressions - and since it appears "In contexts where comma is given a special meaning" it has to "appear only in parentheses".
To find out why assignment-expression could take a form of t+2 you have to go back from its definitions and choose first choice always
assignment-expression
-> conditional-expression
--> logical-or-expression
---> logical-and-expression
----> inclusive-or-expression
-----> exclusive-or-expression
------> and-expression
-------> equality-expression
--------> relational-expression
---------> shift-expression
----------> additive-expression - this is what you see