Using struct members bypasses the compiler check for escaping reference to local variables.

This code should not compile:
--------------------
struct S { int a; }

ref int test()
{
    S s;
    s.a = 42;
    return s.a;
}
void main()
{
    auto p = &test(); // reference to the local variable
    assert(test() == 42); // assertion fails with -O
}
--------------------

Moreover, test() in the above example returnes a corrupted (or uninitialized) value when optimization is on. This causes unittests for std.functional.toDelegate() to segfault.