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

+= operator is a compound assignment. The standard explicitly permits initializer lists on the right-hand side of assignments:

§8.5.4/1 [...] Note: List-initialization can be used

...

— on the right-hand side of an assignment (5.17)

§5.17 talks about all assignments, including compound ones:

assignment-expression:
- conditional-expression
- logical-or-expression assignment-operator initializer-clause
- throw-expression

assignment-operator: one of
= *= /= %= += -= >>= <<= &= ˆ= |=