Error for using positional member that is hidden

docs/compilers/CSharp/Compiler Breaking Changes - post DotNet 5.md

@@ -34,3 +34,17 @@
         return a ?? (b ? 1 : 2);
     }
     ```
+
+3. https://github.com/dotnet/roslyn/issues/52630 In C# 9 (.NET 5, Visual Studio 16.9), it is possible that a record uses a hidden member from a base type as a positional member. In Visual Studio 16.10, this is now an error:
+```csharp
+record Base
+{
+    public int I { get; init; }
+}
+record Derived(int I) // The positional member 'Base.I' found corresponding to this parameter is hidden.
+    : Base
+{
+    public int I() { return 0; }
+}
+```
+

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6606,4 +6606,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_InheritingFromRecordWithSealedToString" xml:space="preserve">
     <value>Inheriting from a record with a sealed 'Object.ToString' is not supported in C# {0}. Please use language version '{1}' or greater.</value>
   </data>
+  <data name="ERR_HiddenPositionalMember" xml:space="preserve">
+    <value>The positional member '{0}' found corresponding to this parameter is hidden.</value>
+  </data>
 </root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1934,7 +1934,8 @@ internal enum ErrorCode
 
         #region diagnostics introduced for C# 10.0
 
-        ERR_InheritingFromRecordWithSealedToString = 8912
+        ERR_InheritingFromRecordWithSealedToString = 8912,
+        ERR_HiddenPositionalMember = 8913,
 
         #endregion
 

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -3410,8 +3410,11 @@ private void AddSynthesizedRecordMembersIfNecessary(MembersAndInitializersBuilde
             var memberSignatures = s_duplicateRecordMemberSignatureDictionary.Allocate();
             var membersSoFar = builder.GetNonTypeMembers(declaredMembersAndInitializers);
             var members = ArrayBuilder<Symbol>.GetInstance(membersSoFar.Count + 1);
+            var memberNames = PooledHashSet<string>.GetInstance();
             foreach (var member in membersSoFar)
             {
+                memberNames.Add(member.Name);
+
                 switch (member)
                 {
                     case FieldSymbol:
@@ -3467,6 +3470,7 @@ private void AddSynthesizedRecordMembersIfNecessary(MembersAndInitializersBuilde
             addToStringMethod(printMembers);
 
             memberSignatures.Free();
+            memberNames.Free();
 
             // We put synthesized record members first so that errors about conflicts show up on user-defined members rather than all
             // going to the record declaration
@@ -3735,6 +3739,11 @@ ImmutableArray<PropertySymbol> addProperties(ImmutableArray<ParameterSymbol> rec
                         {
                             // Deconstruct() is specified to simply assign from this property to the corresponding out parameter.
                             existingOrAddedMembers.Add(prop);
+
+                            if (isInherited)
+                            {
+                                validateNotHidden(prop, param);
+                            }
                         }
                     }
                     else
@@ -3762,6 +3771,14 @@ void addProperty(SynthesizedRecordPropertySymbol property)
                 }
 
                 return existingOrAddedMembers.ToImmutableAndFree();
+
+                void validateNotHidden(Symbol symbol, ParameterSymbol param)
+                {
+                    if (memberNames.Contains(symbol.Name) || !this.GetTypeMembers(symbol.Name).IsEmpty)
+                    {
+                        diagnostics.Add(ErrorCode.ERR_HiddenPositionalMember, param.Locations[0], symbol);
+                    }
+                }
             }
 
             void addObjectEquals(MethodSymbol thisEquals)