Why can I use initializer lists on the right-hand side of operator += but not operator+?

Question

This is a follow-up to an earlier question about why I can\'t use a brace-enclosed initializer as an argument to operator+, which was resolved by looking at this earlier que

Answer 1

It is explained in the answer to this question (which is linked from the question you linked to).

The language grammar only allows a braced list in certain grammatical contexts, not in place of an arbitrary expression. That list includes the right-hand side of assignment operators, but NOT the right-hand side of operators in general.

+= is an assignment operator, + is not.

The grammar for assignment expressions is:

  assignment-expression:
     conditional-expression
     logical-or-expression assignment-operator initializer-clause
     throw-expression
  assignment-operator: one of
      = *= *= /= %= += -= >>= <<= &= ^= |=