std.typecons.Proxy requires a nothrow destructor and toHash?

[Saurabh Das via Digitalmars-d-learn]

struct A
{
    import std.stdio;
    File f;
}
struct B
{
    A a;

    import std.typecons;
    mixin Proxy!a;
}

rdmd proxytest.d:
/Library/D/dmd/src/phobos/std/typecons.d(5055): Error: 
'proxytest.A.~this' is not nothrow
/Library/D/dmd/src/phobos/std/typecons.d(5050): Error: function 
'proxytest.B.Proxy!(a).toHash' is nothrow yet may throw
Failed: ["dmd", "-v", "-o-", "proxytest.d", "-I."]

Why doesn't this work? Is it a requirement that a proxied struct 
must have a nothrow destructor and toHash?