struct Foo {
    uint num;

    @property ref uint front() {
        return num;
    }

    ref uint opIndex(size_t index) {
        return num;
    }
}

void main() {
    Foo foo;
    foo.front++;       // Works
    foo.front += 1;    // error 1
    foo.front = 1;     // error 2
    foo[0] = 1;        // error 3
}

If something returns by reference then it should be usable for assignment, though if the class/struct contains an explicit setter property or opIndexAssign, this should override assignment via ref return.  Here are the error messages produced.

test9.d(16): Error: 'foo.front' is not a scalar, it is a @property ref uint()
test9.d(16): Error: incompatible types for ((foo.front) += (1)): '@property ref uint()' and 'int'
test9.d(17): Error: function test9.Foo.front () is not callable using argument types (int)
test9.d(17): Error: expected 0 arguments, not 1 for non-variadic function type @property ref uint()
test9.d(18): Error: operator [] assignment overload with opIndex(i, value) illegal, use opIndexAssign(value, i)

I'm aware that other bugs have reported bits and pieces of this problem, but I think this is a clearer bug report on the more general issue.  Also, marking as major because it's a significant impedement to library development in std.range.