Optimization does not use a "caching iterator"

Author: patrickroberts

tests/beman/any_view/constexpr.test.cpp

@@ -64,3 +64,26 @@ TEST(ConstexprTest, sort_vector) {
 #endif
     EXPECT_TRUE(sort(std::vector{6, 8, 7, 5, 3, 0, 9}));
 }
+
+constexpr auto set_front(any_view<int, forward> view, int value) {
+    // forward iterator of lvalue reference uses cache object to fuse virtual dispatches
+#ifndef _MSC_VER
+    // TODO use [[msvc::no_unique_address]]
+    static_assert(sizeof(std::ranges::iterator_t<any_view<int, forward>>) ==
+                  sizeof(std::ranges::iterator_t<any_view<int, input>>) + sizeof(int*));
+#endif
+
+    auto& ref = view.front();
+    // even with cache object, lifetime of reference is not tied to lifetime of iterator
+    ref = value;
+
+    return view;
+}
+
+TEST(ConstexprTest, reference_lifetime) {
+#ifndef _MSC_VER
+    // error C2131: expression did not evaluate to a constant
+    static_assert(42 == set_front(std::vector{7}, 42).front());
+#endif
+    EXPECT_EQ(42, set_front(std::vector{7}, 42).front());
+}