Casting between structs of the same size is treated as a bit cast?

[Dave P.]

I am confused on how casting structs works. According to point 9 
of https://dlang.org/spec/expression.html#CastExpression:

> Casting a value v to a struct S, when value is not a struct of 
> the same type, is equivalent to:
>
> ```d
> S(v)
> ```

However, the following program compiles and the resulting 
execution indicates the bits are just being reinterpreted.

```d
// casttest.d
import std;

struct Foo {
     int i;
     float x;
}

struct Bar {
     float i;
     int x;
}

void main(){
     Foo foo = Foo(10, 1.035);
     writeln(foo);
     auto b = cast(Bar)foo;
     writeln(b);
     auto c = cast(Bar)b;
     writeln(c);
     // auto d = Bar(foo); // casttest.d(21): Error: cannot 
implicitly convert expression `foo` of type `Foo` to `float`
     // auto e = Bar(b); // casttest.d(22): Error: cannot 
implicitly convert expression `b` of type `Bar` to `float`
}
```

Execution results in:

```
Foo(10, 1.035)
Bar(1.4013e-44, 1065646817)
Bar(1.4013e-44, 1065646817)
```

Additionally, the commented out lines indicate that what the spec 
claims the cast is equivalent to wouldn’t even work.

Is this a bug in my understanding? Bug in the spec? Bug in the 
compiler?