[Issue 24034] New: Changing this in constructor allows to modify immutable members of other instance

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24034

          Issue ID: 24034
           Summary: Changing this in constructor allows to modify
                    immutable members of other instance
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: tim.dlang at t-online.de

Pull request https://github.com/dlang/dmd/pull/15389 for issue 24024 allows to
modify `this` for classes inside functions. In constructors this allows to
modify immutable members of another instance of the same class. The constructor
will now also return the wrong pointer.

import std.stdio;
class C
{
    immutable int i;
    this(int i, C other)
    {
        if (other !is null)
            this = other;
        this.i = i;
    }
}
void main()
{
    C c1 = new C(1, null);
    writeln(c1.i); // prints 1
    C c2 = new C(2, c1);
    writeln(c1.i); // prints 2
    writeln(c2.i); // prints 2
    writeln(c1 is c2); // prints true
}

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