Packing of Struct Fields

[Per Nordlöw via Digitalmars-d-learn]

Why is `T.sizeof` 12 instead of 8 when `U.sizeof` is 8 in the 
following example?

struct S
{
    int i;
    bool b;
}
struct T
{
    S s;
    char c;
}

struct U
{
    int i;
    bool b;
    char c;
}

?