Generality creep

[Kagamin]

On Thursday, 21 March 2019 at 16:11:04 UTC, Joseph Rushton 
Wakeling wrote:
> @property bool empty (T) (auto ref scope T a)
>     if(is(ReturnType!((T t) => t.length) : size_t))
> {
>     return !a.length;
> }

Here comes generality creep :)

struct A
{
     int a;
     @disable this(this);
     bool opCast(){ return !!a; }
}
struct S
{
     A a;
     ref A p() { return a; }
}

T property(T)(auto ref T a);
import std.traits;

void f()
{
     S s;
     assert(!s.p); //negation works
     static assert(is(typeof(property(s.p))==A));
     static assert(is(ReturnType!(()=>s.p):A));
}

The ReturnType assert doesn't pass. Any idea why? Well, maybe it 
shouldn't.