[Issue 23780] New: Manual __ctor call can mutate immutable object in @safe code

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23780

          Issue ID: 23780
           Summary: Manual __ctor call can mutate immutable object in
                    @safe code
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: snarwin+bugzilla at gmail.com

As of DMD 2.102.2, the following program compiles successfully and causes an
assertion failure when run:

---
struct S
{
    int n;
    @safe this(int n) immutable { this.n = n; }
}

@safe void main()
{
    immutable S s = 123;
    int before = s.n;
    s.__ctor(456);
    assert(s.n == before); // fails
}
---

The cause of the assertion failure is the mutation of the immutable object `s`
by the call to `S.__ctor`.

Since constructors are allowed to mutate immutable objects for the purpose of
initialization, @safe code must not be allowed to call the constructor of an
existing object, even if that constructor is @safe.

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