Fix up after removal of notnulls

csharp/src/Google.Protobuf.Test/WellKnownTypes/AnyTest.cs

@@ -177,7 +177,7 @@ public void Unpack_TypeRegistry()
                 UnittestProto3Reflection.Descriptor);
             var unpacked = anyMessages.Select(any => any.Unpack(registry)).ToList();
             var expected = (IMessage[]) messages.Clone();
-            expected[4] = null;
+            expected[4] = null!;
             Assert.AreEqual(expected, unpacked);
         }
     }

csharp/src/Google.Protobuf/Collections/MapField.cs

@@ -79,7 +79,9 @@ public sealed class MapField<TKey, TValue> : IDeepCloneable<MapField<TKey, TValu
         private static readonly EqualityComparer<TKey> KeyEqualityComparer = ProtobufEqualityComparers.GetEqualityComparer<TKey>();
 
         // TODO: Don't create the map/list until we have an entry. (Assume many maps will be empty.)
+#pragma warning disable CS8714 // "Nullability of type argument doesn't match 'notnull' constraint" - see TODO on notnull above.
         private readonly Dictionary<TKey, LinkedListNode<KeyValuePair<TKey, TValue>>> map = new (KeyEqualityComparer);
+#pragma warning restore CS8714
         private readonly LinkedList<KeyValuePair<TKey, TValue>> list = new ();
 
         /// <summary>
@@ -371,7 +373,7 @@ public override int GetHashCode()
             int hash = 0;
             foreach (KeyValuePair<TKey, TValue> pair in list)
             {
-                hash ^= keyComparer.GetHashCode(pair.Key) * 31 + valueComparer.GetHashCode(pair.Value);
+                hash ^= keyComparer.GetHashCode(pair.Key!) * 31 + valueComparer.GetHashCode(pair.Value!);
             }
             return hash;
         }

csharp/src/Google.Protobuf/Collections/RepeatedField.cs

@@ -143,7 +143,7 @@ public void AddEntriesFrom(ref ParseContext ctx, FieldCodec<T> codec)
                             // Only FieldCodecs with a fixed size can reach here, and they are all known
                             // types that don't allow the user to specify a custom reader action.
                             // reader action will never return null.
-                            array[count++] = reader(ref ctx);
+                            array[count++] = reader(ref ctx)!;
                         }
                     }
                     else
@@ -382,7 +382,7 @@ public bool Remove(T item)
             }            
             Array.Copy(array, index + 1, array, index, count - index - 1);
             count--;
-            array[count] = default;
+            array[count] = default!;
             return true;
         }
 
@@ -598,7 +598,7 @@ public void RemoveAt(int index)
             }
             Array.Copy(array, index + 1, array, index, count - index - 1);
             count--;
-            array[count] = default;
+            array[count] = default!;
         }
 
         /// <summary>

csharp/src/Google.Protobuf/ParsingPrimitivesMessages.cs

@@ -33,8 +33,6 @@
 using System;
 using System.Buffers;
 using System.Collections.Generic;
-using System.IO;
-using System.Runtime.CompilerServices;
 using System.Security;
 using Google.Protobuf.Collections;
 
@@ -139,8 +137,10 @@ public static void ReadMessage(ref ParseContext ctx, IMessage message)
         }
 
         public static KeyValuePair<TKey, TValue> ReadMapEntry<TKey, TValue>(ref ParseContext ctx, MapField<TKey, TValue>.Codec codec)
-            where TKey : notnull
-            where TValue : notnull
+            // TODO: Enable non-null constraints on a major version number change to avoid breaking any code on a minor version
+            //   that declared key or value as nullable.
+            // where TKey : notnull
+            // where TValue : notnull
         {
             int length = ParsingPrimitives.ParseLength(ref ctx.buffer, ref ctx.state);
             if (ctx.state.recursionDepth >= ctx.state.recursionLimit)

csharp/src/Google.Protobuf/WellKnownTypes/AnyPartial.cs

@@ -89,7 +89,7 @@ public bool Is(MessageDescriptor descriptor)
         {
             // Note: this doesn't perform as well is it might. We could take a MessageParser<T> in an alternative overload,
             // which would be expected to perform slightly better... although the difference is likely to be negligible.
-            T target = new T();
+            var target = new T();
             if (GetTypeName(TypeUrl) != target.Descriptor.FullName)
             {
                 throw new InvalidProtocolBufferException(
@@ -109,10 +109,10 @@ public bool Is(MessageDescriptor descriptor)
         {
             // Note: deliberately avoid writing anything to result until the end, in case it's being
             // monitored by other threads. (That would be a bug in the calling code, but let's not make it worse.)
-            T target = new T();
+            var target = new T();
             if (GetTypeName(TypeUrl) != target.Descriptor.FullName)
             {
-                result = default(T); // Can't use null as there's no class constraint, but this always *will* be null in real usage.
+                result = default; // Can't use null as there's no class constraint, but this always *will* be null in real usage.
                 return false;
             }
             target.MergeFrom(Value);
@@ -126,16 +126,16 @@ public bool Is(MessageDescriptor descriptor)
         /// </summary>
         /// <param name="registry">The type registry to consult for messages.</param>
         /// <returns>The unpacked message, or <c>null</c> if no matching message was found.</returns>
-        public IMessage Unpack(TypeRegistry registry)
+        public IMessage? Unpack(TypeRegistry registry)
         {
             string typeName = GetTypeName(TypeUrl);
-            MessageDescriptor descriptor = registry.Find(typeName);
+            MessageDescriptor? descriptor = registry.Find(typeName);
             if (descriptor == null)
             {
                 return null;
             }
 
-            var message = descriptor.Parser.CreateTemplate();
+            var message = descriptor.Parser!.CreateTemplate();
             message.MergeFrom(Value);
             return message;
         }