https://issues.dlang.org/show_bug.cgi?id=19621

          Issue ID: 19621
           Summary: The specification is self-contradictory on
                    immutability
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dlang.org
          Assignee: nobody@puremagic.com
          Reporter: porton@narod.ru

https://dlang.org/spec/const3.html says both of the following:

---
The second way is to cast data to immutable. When doing so, it is up to the programmer to ensure that no other mutable references to the same data exist.

char[] s = ...;
immutable(char)[] p = cast(immutable)s;     // undefined behavior
immutable(char)[] p = cast(immutable)s.dup; // ok, unique reference
---

and

---
An immutable or const type qualifier can be removed with a cast:

immutable int* p = ...;
int* q = cast(int*)p;

This does not mean, however, that one can change the data:

*q = 3; // allowed by compiler, but result is undefined behavior
---

The first says that the existence of mutable reference is enough for the undefined behavior but the second says that for wrong behavior one needs to actually change the data through a mutable reference.

The second of the two should be accepted. Having a mutable reference to immutable data should be well-defined (and undefined only if one actually uses this reference to change the data), because otherwise it is too easy to make an error leading to undefined behavior what is bad for software reliability.

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