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

S(v)

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

// 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?

On Wednesday, 20 October 2021 at 04:14:37 UTC, Dave P. wrote:

S(v)

It says value v and struct S, but you go on to cast a struct into another struct.

Point 9 is exhibited by

struct Foo {
    int i;
}

unittest {
    int n = 2;
    Foo f = cast(Foo) n;
}

It looks at least like a gap in the spec. Adding "or a struct to another struct" to point six would fit the observed behavior.