[Dlang-internal] DIP1000 discussion and testing

[Mathias Lang via Dlang-internal]

On Sunday, 23 October 2016 at 06:42:41 UTC, Walter Bright wrote:
> On 10/22/2016 7:34 PM, Walter Bright wrote:
>> 2. You're right that the above should error, I'll look into it.
>
> Added the fix to the PR.

Somehow my previous (mailed yesterday afternoon) message didn't 
go through, so I'm going to paste it here again:

Just looked at it.

The diff shows:
+fail_compilation/retscope.d(198): Error: scope variable e may 
not be returned
+void* escapeDg1(scope void* d) @safe
+{
+    Escaper e;
+    e.DG = d;
+    return e.DG; // L198
+}

So it means that the storage class of 'e' changes during semantic 
analysis of the function. I don't think that's something we want 
to ever do.
Crafting a test case that exploit this was trivial:
void* escape3 (scope void* p) @safe
{
     Escaper e;
     scope dg = () { return e.e; };
     e.e = p;
     return dg();
}

Not to mention, if we ever relax the restriction on taking the 
address of a local variable, as the DIP seems to hint as a goal, 
you can just do:
Escaper e;
Escaper* pe = &e;
And you'll have the same issue.