struct vs built-in type

[Jack Applegame via Digitalmars-d-learn]

Code:

import std.stdio;
struct Bar {
        int payload;
        alias payload this;
}
struct Foo {
        private {
                Bar m_bar;
                int m_baz;
        }
        @property {
                Bar bar() { return m_bar; }
                void bar(Bar v) { m_bar = v; }
                int baz() { return m_baz; }
                void baz(int v) { m_baz = v; }
        }
}

void main() {
        Foo foo;
        foo.bar += 4; // Compiles. Why?
        foo.baz += 4; // Error: foo.baz() is not an lvalue
}

foo.baz() is not lvalue. It's true.
But foo.bar() is lvalue. Why? I expected the same error. This 
line has no effect, but the compiler does not issue even a 
warning.

DMD64 D Compiler v2.066.1. CentOS 7