if alias this is used on a member value (whether simple, or another struct)
copying an instantiated struct no longer results in all struct members
being copied (only the aliased value is copied).

The version is Version D 2.048 (version text copied from changelog)

The problem is best illustrated by a minimal example:

----- self contained d sample starts -----
import std.stdio, std.math;

void main()
{
    auto p = Test(0.5, 0.5);
    assert(0.5 == p.a); // ok
    assert(0.5 == p.b); // ok
    Test q;
    //q = Test(0.5, 0.5);
    //
    // the above line can substitute
    // the one below and the same
    // problem occurs
    q = p;
    if (isNaN(q.b))
        printf("problem replicated\n");
    assert(0.5 == q.a); // ok
    assert(0.5 == q.b); // this fails
}

struct Test
{
    double a;
    double b;
    alias a this;
}
----- end of file -----