Fix PR35564 - std::list splice/erase incorrectly throw in debug mode.

There was a bug in the implementation of splice where the container
sizes were updated before decrementing one of the iterators. Afterwards,
the result of decrementing the iterator was flagged as UB by the debug
implementation because the container was reported to be empty.

This patch fixes that bug by delaying the updating of the container
sizes until after the iterators have been correctly constructed.

llvm-svn: 323390

libcxx/include/list

@@ -2058,15 +2058,15 @@ list<_Tp, _Alloc>::splice(const_iterator __p, list& __c, const_iterator __f, con
 #endif
     if (__f != __l)
     {
-        if (this != &__c)
-        {
-            size_type __s = _VSTD::distance(__f, __l);
-            __c.__sz() -= __s;
-            base::__sz() += __s;
-        }
         __link_pointer __first = __f.__ptr_;
         --__l;
         __link_pointer __last = __l.__ptr_;
+        if (this != &__c)
+        {
+            size_type __s = _VSTD::distance(__f, __l) + 1;
+            __c.__sz() -= __s;
+            base::__sz() += __s;
+        }
         base::__unlink_nodes(__first, __last);
         __link_nodes(__p.__ptr_, __first, __last);
 #if _LIBCPP_DEBUG_LEVEL >= 2

libcxx/test/libcxx/debug/containers/db_sequence_container_iterators.pass.cpp

@@ -42,6 +42,7 @@ public:
     Base::run();
     try {
       FrontOnEmptyContainer();
+
       if constexpr (CT != CT_ForwardList) {
         AssignInvalidates();
         BackOnEmptyContainer();
@@ -50,6 +51,8 @@ public:
         InsertIterIterIter();
         EmplaceIterValue();
         EraseIterIter();
+      } else {
+        SpliceFirstElemAfter();
       }
       if constexpr (CT == CT_Vector || CT == CT_Deque || CT == CT_List) {
         PopBack();
@@ -57,12 +60,66 @@ public:
       if constexpr (CT == CT_List || CT == CT_Deque) {
         PopFront(); // FIXME: Run with forward list as well
       }
+      if constexpr (CT == CT_List || CT == CT_ForwardList) {
+        RemoveFirstElem();
+      }
+      if constexpr (CT == CT_List) {
+        SpliceFirstElem();
+      }
     } catch (...) {
       assert(false && "uncaught debug exception");
     }
   }
 
 private:
+  static void RemoveFirstElem() {
+    // See llvm.org/PR35564
+    CHECKPOINT("remove(<first-elem>)");
+    {
+      Container C = makeContainer(1);
+      auto FirstVal = *(C.begin());
+      C.remove(FirstVal);
+      assert(C.empty());
+    }
+    {
+      Container C = {1, 1, 1, 1};
+      auto FirstVal = *(C.begin());
+      C.remove(FirstVal);
+      assert(C.empty());
+    }
+  }
+
+  static void SpliceFirstElem() {
+    // See llvm.org/PR35564
+    CHECKPOINT("splice(<first-elem>)");
+    {
+      Container C = makeContainer(1);
+      Container C2;
+      C2.splice(C2.end(), C, C.begin(), ++C.begin());
+    }
+    {
+      Container C = makeContainer(1);
+      Container C2;
+      C2.splice(C2.end(), C, C.begin());
+    }
+  }
+
+
+  static void SpliceFirstElemAfter() {
+    // See llvm.org/PR35564
+    CHECKPOINT("splice(<first-elem>)");
+    {
+      Container C = makeContainer(1);
+      Container C2;
+      C2.splice_after(C2.begin(), C, C.begin(), ++C.begin());
+    }
+    {
+      Container C = makeContainer(1);
+      Container C2;
+      C2.splice_after(C2.begin(), C, C.begin());
+    }
+  }
+
   static void AssignInvalidates() {
     CHECKPOINT("assign(Size, Value)");
     Container C(allocator_type{});