Update pop_back

Author: 20162026

include/beman/inplace_vector/inplace_vector.hpp

@@ -396,8 +396,10 @@ struct inplace_vector_base : private storage::storage_for<T, N> {
 
   constexpr void pop_back() {
     IV_EXPECT(size() > 0 && "pop_back from empty inplace_vector!");
-    unsafe_destroy(end() - 1, end());
-    unsafe_set_size(size() - 1);
+    if (size() > 0) { // UB fail-safe
+      unsafe_destroy(end() - 1, end());
+      unsafe_set_size(size() - 1);
+    }
   }
 
   constexpr friend bool operator==(const inplace_vector_base &x,

tests/beman/inplace_vector/freestanding.test.cpp

@@ -314,11 +314,17 @@ TEST(Freestanding, usage) {
 
   EXPECT_EQ(device.size(), device.capacity());
 
+  EXPECT_EQ(nullptr, device.try_emplace_back(T{}));
+
+  EXPECT_EQ(device.size(), device.capacity());
+
   for (auto i = int(device.capacity()); i > 0; --i) {
     const auto value = T{i - 1};
     EXPECT_EQ(device.back(), value);
     device.pop_back();
   }
 
+  device.pop_back(); // UB should be safe
+
   EXPECT_EQ(device.size(), 0);
 }