Relax conversion requirements for pattern-matching involving type parameters.

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

@@ -197,8 +197,26 @@ private bool CheckValidPatternType(
                     //case ConversionKind.ImplicitConstant:
                     //case ConversionKind.ImplicitNumeric:
                     default:
-                        Error(diagnostics, ErrorCode.ERR_PatternWrongType, typeSyntax, operandType, patternType);
-                        return true;
+                        if (operandType.ContainsTypeParameter() || patternType.ContainsTypeParameter())
+                        {
+                            LanguageVersion requiredVersion = MessageID.IDS_FeatureGenericPatternMatching.RequiredVersion();
+                            if (requiredVersion > Compilation.LanguageVersion)
+                            {
+                                Error(diagnostics, ErrorCode.ERR_PatternWrongGenericTypeInVersion, typeSyntax,
+                                    operandType, patternType,
+                                    Compilation.LanguageVersion.ToDisplayString(),
+                                    new CSharpRequiredLanguageVersion(requiredVersion));
+                                return true;
+                            }
+
+                            // permit pattern-matching when one of the types is an open type in C# 7.1.
+                            break;
+                        }
+                        else
+                        {
+                            Error(diagnostics, ErrorCode.ERR_PatternWrongType, typeSyntax, operandType, patternType);
+                            return true;
+                        }
                 }
             }
 

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

@@ -52,7 +52,8 @@ internal bool AddLabel(BoundPatternSwitchLabel label, DiagnosticBag diagnostics,
                 // For purposes of subsumption, we do not take into consideration the value
                 // of the input expression. Therefore we consider null possible if the type permits.
                 Syntax = label.Syntax;
-                var subsumedErrorCode = CheckSubsumed(label.Pattern, _subsumptionTree, inputCouldBeNull: true);
+                var inputCouldBeNull = _subsumptionTree.Type.CanContainNull();
+                var subsumedErrorCode = CheckSubsumed(label.Pattern, _subsumptionTree, inputCouldBeNull: inputCouldBeNull);
                 if (subsumedErrorCode != 0 && subsumedErrorCode != ErrorCode.ERR_NoImplicitConvCast)
                 {
                     if (!label.HasErrors)

src/Compilers/CSharp/Portable/BoundTree/DecisionTree.cs

@@ -93,7 +93,7 @@ public static DecisionTree Create(BoundExpression expression, TypeSymbol type, S
                 expression = new BoundLocal(expression.Syntax, temp, null, type);
             }
 
-            if (expression.Type.CanContainNull())
+            if (type.CanContainNull() || type.SpecialType == SpecialType.None)
             {
                 // We need the ByType decision tree to separate null from non-null values.
                 // Note that, for the purpose of the decision tree (and subsumption), we
@@ -104,8 +104,6 @@ public static DecisionTree Create(BoundExpression expression, TypeSymbol type, S
             else
             {
                 // If it is a (e.g. builtin) value type, we can switch on its (constant) values.
-                // If it isn't a builtin, in practice we will only use the Default part of the
-                // ByValue.
                 return new ByValue(expression, type, temp);
             }
         }

src/Compilers/CSharp/Portable/BoundTree/DecisionTreeBuilder.cs

@@ -130,7 +130,7 @@ private DecisionTree AddByValue(DecisionTree.Guarded guarded, BoundConstantPatte
 
         private DecisionTree AddByValue(DecisionTree.ByValue byValue, BoundConstantPattern value, DecisionMaker makeDecision)
         {
-            Debug.Assert(value.Value.Type == byValue.Type);
+            Debug.Assert(value.Value.Type.Equals(byValue.Type, TypeCompareKind.IgnoreDynamicAndTupleNames));
             if (byValue.Default != null)
             {
                 return AddByValue(byValue.Default, value, makeDecision);
@@ -221,7 +221,7 @@ private DecisionTree AddByValue(DecisionTree.ByType byType, BoundConstantPattern
                 var kvp = byType.TypeAndDecision[i];
                 var matchedType = kvp.Key;
                 var decision = kvp.Value;
-                if (matchedType.TupleUnderlyingTypeOrSelf() == value.Value.Type.TupleUnderlyingTypeOrSelf())
+                if (matchedType.Equals(value.Value.Type, TypeCompareKind.IgnoreDynamicAndTupleNames))
                 {
                     forType = decision;
                     break;
@@ -260,21 +260,30 @@ private DecisionTree AddByType(DecisionTree decision, TypeSymbol type, DecisionM
                 case DecisionTree.DecisionKind.ByValue:
                     {
                         var byValue = (DecisionTree.ByValue)decision;
+                        DecisionTree result;
                         if (byValue.Default == null)
                         {
-                            byValue.Default = makeDecision(byValue.Expression, byValue.Type);
-                            if (byValue.Default.MatchIsComplete)
+                            if (byValue.Type.Equals(type, TypeCompareKind.IgnoreDynamicAndTupleNames))
                             {
-                                byValue.MatchIsComplete = true;
+                                result = byValue.Default = makeDecision(byValue.Expression, byValue.Type);
+                            }
+                            else
+                            {
+                                byValue.Default = new DecisionTree.ByType(byValue.Expression, byValue.Type, null);
+                                result = AddByType(byValue.Default, type, makeDecision);
                             }
-
-                            return byValue.Default;
                         }
                         else
                         {
-                            Debug.Assert(byValue.Default.Type == type);
-                            return Add(byValue.Default, makeDecision);
+                            result = AddByType(byValue.Default, type, makeDecision);
                         }
+
+                        if (byValue.Default.MatchIsComplete)
+                        {
+                            byValue.MatchIsComplete = true;
+                        }
+
+                        return result;
                     }
                 case DecisionTree.DecisionKind.Guarded:
                     return AddByType((DecisionTree.Guarded)decision, type, makeDecision);
@@ -321,7 +330,7 @@ private DecisionTree AddByType(DecisionTree.ByType byType, TypeSymbol type, Deci
             if (byType.TypeAndDecision.Count != 0)
             {
                 var lastTypeAndDecision = byType.TypeAndDecision.Last();
-                if (lastTypeAndDecision.Key.TupleUnderlyingTypeOrSelf() == type.TupleUnderlyingTypeOrSelf())
+                if (lastTypeAndDecision.Key.Equals(type, TypeCompareKind.IgnoreDynamicAndTupleNames))
                 {
                     result = Add(lastTypeAndDecision.Value, makeDecision);
                 }
@@ -533,7 +542,7 @@ private DecisionTree Add(DecisionTree.ByType byType, DecisionMaker makeDecision)
             TypeSymbol patternType,
             ref HashSet<DiagnosticInfo> useSiteDiagnostics)
         {
-            if (expressionType == patternType)
+            if ((object)expressionType == (object)patternType)
             {
                 return true;
             }

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -4891,7 +4891,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>The switch case has already been handled by a previous case.</value>
   </data>
   <data name="ERR_PatternWrongType" xml:space="preserve">
-    <value>An expression of type {0} cannot be handled by a pattern of type {1}.</value>
+    <value>An expression of type '{0}' cannot be handled by a pattern of type '{1}'.</value>
   </data>
   <data name="WRN_AttributeIgnoredWhenPublicSigning" xml:space="preserve">
     <value>Attribute '{0}' is ignored when public signing is specified.</value>
@@ -5070,4 +5070,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_VoidInTuple" xml:space="preserve">
     <value>A tuple may not contain a value of type 'void'.</value>
   </data>
+  <data name="ERR_PatternWrongGenericTypeInVersion" xml:space="preserve">
+    <value>An expression of type '{0}' cannot be handled by a pattern of type '{1}' in C# {2}. Please use language version {3} or greater.</value>
+  </data>
 </root>

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

@@ -1476,8 +1476,13 @@ internal enum ErrorCode
         WRN_Experimental = 8305,
         ERR_TupleInferredNamesNotAvailable = 8306,
 
+        #region diagnostics for C# 7.1
+
         ERR_BadDynamicMethodArgDefaultLiteral = 9000,
         ERR_DefaultLiteralNotValid = 9001,
         WRN_DefaultInSwitch = 9002,
+        ERR_PatternWrongGenericTypeInVersion = 9003,
+
+        #endregion diagnostics for C# 7.1
     }
 }

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

@@ -129,6 +129,7 @@ internal enum MessageID
         IDS_ThrowExpression = MessageBase + 12717,
         IDS_FeatureDefaultLiteral = MessageBase + 12718,
         IDS_FeatureInferredTupleNames = MessageBase + 12719,
+        IDS_FeatureGenericPatternMatching = MessageBase + 12720,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -188,6 +189,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 // C# 7.1 features.
                 case MessageID.IDS_FeatureDefaultLiteral:
                 case MessageID.IDS_FeatureInferredTupleNames:
+                case MessageID.IDS_FeatureGenericPatternMatching:
                     return LanguageVersion.CSharp7_1;
 
                 // C# 7 features.

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_PatternSwitchStatement.cs

@@ -177,7 +177,7 @@ private DecisionTree LowerToDecisionTree(
                     }
                 }
 
-                if (defaultLabel != null)
+                if (defaultLabel != null && !loweredDecisionTree.MatchIsComplete)
                 {
                     Add(loweredDecisionTree, (e, t) => new DecisionTree.Guarded(loweredExpression, loweredExpression.Type, default(ImmutableArray<KeyValuePair<BoundExpression, BoundExpression>>), defaultSection, null, defaultLabel));
                 }
@@ -219,7 +219,8 @@ private void LowerDecisionTree(BoundExpression expression, DecisionTree decision
                     // Store the input expression into a temp
                     if (decisionTree.Expression != expression)
                     {
-                        _loweredDecisionTree.Add(_factory.Assignment(decisionTree.Expression, expression));
+                        var convertedExpression = _factory.Convert(decisionTree.Expression.Type, expression);
+                        _loweredDecisionTree.Add(_factory.Assignment(decisionTree.Expression, convertedExpression));
                     }
 
                     if (_declaredTempSet.Add(decisionTree.Temp))