Why is this allowed

[tsbockman]

On Wednesday, 1 July 2020 at 20:05:51 UTC, tsbockman wrote:
> If you want the compiler to stop you from accidentally keeping 
> references to stack variables past the end of their scope, you 
> need to annotate your functions @safe and compile with 
> -preview=dip1000: https://run.dlang.io/is/3VdDaN
>
> Furthermore, the problem your example shows has nothing to do 
> with implicit static to dynamic array conversion, as without 
> @safe the same error can easily be committed with non-array 
> types: https://run.dlang.io/is/nBjibd

Hmm. Those run.dlang.io short links seem to allow editing of the 
code, so I'd better paste it here for permanence:

// Compile with -preview=dip1000
struct Database {
     int[] data;
     void set(int[] _data) @safe {
         data = _data;
     }
}
void myFunc(ref Database db) @safe {
     int[3] x;
     db.set(x);	// This is a compile-time error, as it should be.
}

Database theDB;
void main() {
     myFunc(theDB);
}

// This version shows that the problem is not using @safe and 
dip1000,
// not anything to do with arrays:
struct Database {
     int* data;
     void set(ref int _data) {
         data = &_data;
     }
}
void myFunc(ref Database db) {
     int x;
     db.set(x);	// oops
}

Database theDB;
void main() {
     myFunc(theDB);
}