[Bug]: class variable assumed local if passed to C function

[mppf]

Summary of Problem

Description:
Passing a variable of type unmanaged C? to an extern proc causes the compiler to assume that the class is local.

Is this issue currently blocking your progress?
No. Presumably the reason this has not come up much is that Chapel classes can't really be passed to C code (since we don't have a way of giving C the details of the type). They can be passed if cast to C void pointers, but the code looks different with such a cast (and works correctly).

We noticed this problem when making adjustments to a future in #26417 and #26476.

Steps to Reproduce

Source Code:

use CTypes;

extern proc sizeof(x):c_size_t;

class C {
  var x:int;
}

proc main() {
  var c:unmanaged C?;
  on Locales[numLocales-1] {
    writeln("allocating c on locale ", here);
    c = new unmanaged C(1);
  }
  sizeof(c); // it works if this line is commented out
  writeln(c.locale);
}

chpl program.chpl && ./program -nl 2

produces

allocating c on locale LOCALE1
LOCALE0

but I would expect it to produce

allocating c on locale LOCALE1
LOCALE1

Compile command:
see above

Execution command:
see above

Associated Future Test(s):
TODO

Configuration Information

A multi-locale configuration of Chapel. When writing this issue I was using quickstart+gasnet (local oversubscribed) and 2.4.0 pre-release.