IES does not call functions on operator overloads, if omitting parens

[ArthaTi]

If parens are omitted on a function call performed inside of an interpolated expression passed to an operator overload — such as foo = i"$(helloWorld)" or foo ~ i"$(helloWorld)" — the function will not be called.

string helloWorld() {
    return "Hello, World!";
}

struct Foo {
    void opAssign(Ts...)(Ts) {
      
    }
    void fun(Ts...)(Ts) {
        
    }
}

void main() {
    Foo foo;
    foo.fun(i"$(helloWorld)");  // works
    foo = i"$(helloWorld)";     // fails
}

The error message is also very confusing, because it does not point to the invocation, but only to the template:

onlineapp.d(6): Error: cannot have parameter of function type `string()`