Allow mixed declaration and assignment in deconstruction

src/Compilers/CSharp/Portable/Binder/Binder_Deconstruct.cs

@@ -42,18 +42,17 @@ internal BoundExpression BindDeconstruction(AssignmentExpressionSyntax node, Dia
                     case SyntaxKind.ExpressionStatement:
                         if (expression != null)
                         {
-                            // We only allow assignment-only or declaration-only deconstructions at this point.
-                            // Issue https://github.com/dotnet/roslyn/issues/15050 tracks allowing mixed deconstructions.
-                            // For now we give an error when you mix.
-                            Error(diagnostics, ErrorCode.ERR_MixedDeconstructionUnsupported, left);
+                            MessageID.IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction
+                                .CheckFeatureAvailability(diagnostics, Compilation, node.Location);
                         }
                         break;
                     case SyntaxKind.ForStatement:
                         if (((ForStatementSyntax)node.Parent).Initializers.Contains(node))
                         {
                             if (expression != null)
                             {
-                                Error(diagnostics, ErrorCode.ERR_MixedDeconstructionUnsupported, left);
+                                MessageID.IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction
+                                    .CheckFeatureAvailability(diagnostics, Compilation, node.Location);
                             }
                         }
                         else

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5290,9 +5290,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ThrowMisplaced" xml:space="preserve">
     <value>A throw expression is not allowed in this context.</value>
   </data>
-  <data name="ERR_MixedDeconstructionUnsupported" xml:space="preserve">
-    <value>A deconstruction cannot mix declarations and expressions on the left-hand-side.</value>
-  </data>
   <data name="ERR_DeclarationExpressionNotPermitted" xml:space="preserve">
     <value>A declaration is not allowed in this context.</value>
   </data>
@@ -6560,6 +6557,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureDiscards" xml:space="preserve">
     <value>discards</value>
   </data>
+  <data name="IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction" xml:space="preserve">
+    <value>Mixed declarations and expressions in deconstruction</value>
+  </data>
   <data name="IDS_FeatureVarianceSafetyForStaticInterfaceMembers" xml:space="preserve">
     <value>variance safety for static interface members</value>
   </data>

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

@@ -1441,7 +1441,7 @@ internal enum ErrorCode
         ERR_NewWithTupleTypeSyntax = 8181,
         ERR_PredefinedValueTupleTypeMustBeStruct = 8182,
         ERR_DiscardTypeInferenceFailed = 8183,
-        ERR_MixedDeconstructionUnsupported = 8184,
+        // ERR_MixedDeconstructionUnsupported = 8184,
         ERR_DeclarationExpressionNotPermitted = 8185,
         ERR_MustDeclareForeachIteration = 8186,
         ERR_TupleElementNamesInDeconstruction = 8187,

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

@@ -214,6 +214,7 @@ internal enum MessageID
         IDS_Parameter = MessageBase + 12789,
         IDS_Return = MessageBase + 12790,
         IDS_FeatureVarianceSafetyForStaticInterfaceMembers = MessageBase + 12791,
+        IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction = MessageBase + 12792,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -320,6 +321,9 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
             // Checks are in the LanguageParser unless otherwise noted.
             switch (feature)
             {
+                // C# preview features.
+                case MessageID.IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction:
+                    return LanguageVersion.Preview;
                 // C# 9.0 features.
                 case MessageID.IDS_FeatureLambdaDiscardParameters: // semantic check
                 case MessageID.IDS_FeatureFunctionPointers: