[Issue 6741] implicit casting of pure-function delegate result to immutable breaks immutability.

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=6741



--- Comment #2 from Steven Schveighoffer <schveiguy at yahoo.com> 2014-03-20 12:57:35 PDT ---
It's a bug because when the object is cast implicitly to immutable, it does not
affect the context pointer. In essence, the delegate captures the instance when
it was mutable.

Normally, implicitly casting a pure return to immutable is OK because immutable
is transitive. Since a strong pure function cannot have side effects, the
return must be the only reference to it. However, if it contains a reference to
itself via a delegate, that reference does NOT get cast to immutable.

Why do you think the given code is correct behavior? I should never be able to
do this without casts:

immutable a = ...
assert(a.n == 3);
...
assert(a.n == 4);

BTW, I do not say that a class with delegate members should not be allowed to
be immutable, I'm saying that it should not *implicitly* cast to immutable. In
other words, the code would be allowed if you did:

auto tmp = cast(immutable(A))createAnA(3);

In this case, you are telling the compiler "I know what I'm doing."

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