https://golang.org/ref/spec#Assignments
Assignment = ExpressionList assign_op ExpressionList .
assign_op = [ add_op | mul_op ] “=” .
The text later explains that “In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression”. So (if I interpret right) when add_op or mul_op is present then both ExpressionLists must be Expressions (not lists).
This could be expressed quite easily:
Assignment = TupleAssignment | AssignmentOperation .
TupleAssignment = ExpressionList “=” ExpressionList .
AssignmentOperation = Expression ( add_op | mul_op ) “=” Expression .
I would recommend to remedy the language specification text, if possible. Otherwise one can believe that i, j += 1, 2 should be, at least syntactically, correct. But it is not correct even syntactically, as the go compiler says: syntax error: unexpected +=, expecting := or = or comma.