[Issue 16056] immutable delegate can mutate through context pointer

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16056

--- Comment #5 from ag0aep6g at gmail.com ---
(In reply to Eyal Lotem from comment #4)
> immutable void delegate() pure
> 
> vs.
> 
> immutable void delegate() immutable pure

When the delegate is part of an immutable struct instance, it has the former
type, too:

----
struct S { void delegate() pure f; }
immutable s = S();
pragma(msg, typeof(s.f)); /* immutable(void delegate() pure) */
----

I suppose your point is that it should be the latter type, with two
"immutable"s.

Fair enough, but I don't think that distinction is worth having. D doesn't have
head const with pointers. Why should we make delegates a special case?

So in my opinion, `immutable void delegate()` should imply an immutable context
pointer, making it the same type as `immutable void delegate() immutable`. Just
like `immutable(int*)` is the same as `immutable(immutable(int)*)`.

The compiler agrees (to some extent):

----
alias A = immutable int* delegate();
alias B = immutable int* delegate() immutable;
static assert(is(A == B)); /* passes */
----

But then there's this:

----
void main()
{
    int x = 1;
    const void delegate() dg1 = { ++x; };
    const void delegate() const dg2 = { ++x; };
}
----

That compiles, but when you take the dg1 line away, then the dg2 line isn't
accepted anymore. And when you swap them around, both lines are rejected.
There's obviously something wrong here. I've filed a separate issue:
https://issues.dlang.org/show_bug.cgi?id=16058

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