Add Get method to Iterator

Necessary for C++ interoperability with iterators.

proposals/p1885.md

@@ -39,6 +39,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -   [Alternatives considered](#alternatives-considered)
     -   [Indexable](#indexable)
     -   [Using interface type parameters](#using-interface-type-parameters)
+    -   [Relying only on `Next()` from `Iterator` to move and get value](#relying-only-on-next-from-iterator-to-move-and-get-value)
     -   [Other](#other)
 
 <!-- tocstop -->
@@ -275,7 +276,8 @@ definition. The usage highlighted here is hypothetical.
 // Basic immutable iterator
 interface Iterator {
     let ElementType:! Type;
-    fn Next[me: Self]() -> Optional(ElementType);
+    fn Get[me: Self]() -> Optional(ElementType);
+    fn Next[addr me: *Self]() -> Optional(ElementType);
 }
 
 // Iterable type
@@ -294,10 +296,14 @@ class MyContainer() {
         class IteratorType { // Associated type as member type
             impl as Iterator where .ElementType = i32 {
                 ...
-
+                fn Get[me: Self]() -> Optional(ElementType) {
+                    return if atEnd then .Empty else currItem;
+                }
                 fn Next[addr me: *Self]() -> Optional(ElementType) {
+                    // move to next item position
                     ...
-                    return if atEnd then .Empty else currItem;
+
+                    return me->Get();
                 }
             }
         }
@@ -407,6 +413,16 @@ Interfaces can use
 as `interface Iterator(ItemType:! Type)` instead of associated types. This would
 however be inconvenient as they require specifying the type again to be used.
 
+### Relying only on `Next()` from `Iterator` to move and get value
+
+The `Iterator` interface currently exposes both `Get` and `Next` methods. While
+`Next` could be sufficient to support iterating and retrieving data (returned),
+this makes the C++ interoperability complex, by not allowing straightforward
+dereferencing (C++'s `operator*`) without side-effects.
+
+Exposing a `Get` method allow for a simple mapping of iterator dereferencing,
+which is needed to provide proper interoperability.
+
 ### Other
 
 -   New keyword or syntax