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))

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

@@ -4,6 +4,7 @@
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Roslyn.Utilities;
 using System.Collections.Immutable;
+using System.Collections.Generic;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
@@ -80,7 +81,7 @@ private BoundExpression MakeIsDeclarationPattern(BoundDeclarationPattern lowered
                     return result;
                 }
 
-                Debug.Assert((object)loweredPattern.Variable != null && loweredInput.Type == loweredPattern.Variable.GetTypeOrReturnType());
+                Debug.Assert((object)loweredPattern.Variable != null && loweredInput.Type.Equals(loweredPattern.Variable.GetTypeOrReturnType(), TypeCompareKind.IgnoreDynamicAndTupleNames));
 
                 var assignment = _factory.AssignmentExpression(loweredPattern.VariableAccess, loweredInput);
                 return _factory.MakeSequence(assignment, result);
@@ -93,10 +94,10 @@ private BoundExpression MakeIsDeclarationPattern(BoundDeclarationPattern lowered
 
                 return _factory.Sequence(ImmutableArray.Create(temp),
                          sideEffects: ImmutableArray<BoundExpression>.Empty,
-                         result: MakeIsDeclarationPattern(loweredPattern.Syntax, loweredInput, discard, requiresNullTest: true));
+                         result: MakeIsDeclarationPattern(loweredPattern.Syntax, loweredInput, discard, requiresNullTest: loweredInput.Type.CanContainNull()));
             }
 
-            return MakeIsDeclarationPattern(loweredPattern.Syntax, loweredInput, loweredPattern.VariableAccess, requiresNullTest: true);
+            return MakeIsDeclarationPattern(loweredPattern.Syntax, loweredInput, loweredPattern.VariableAccess, requiresNullTest: loweredInput.Type.CanContainNull());
         }
 
         /// <summary>
@@ -141,13 +142,36 @@ private BoundExpression CompareWithConstant(BoundExpression input, BoundExpressi
                 );
         }
 
+        private bool MatchIsIrrefutable(TypeSymbol sourceType, TypeSymbol targetType, bool requiredNullTest)
+        {
+            // use site diagnostics will already have been reported during binding.
+            HashSet<DiagnosticInfo> ignoredDiagnostics = null;
+            switch (_compilation.Conversions.ClassifyBuiltInConversion(sourceType, targetType, ref ignoredDiagnostics).Kind)
+            {
+                case ConversionKind.Boxing:
+                case ConversionKind.ImplicitReference:
+                case ConversionKind.Identity:
+                    return true;
+                default:
+                    return false;
+            }
+        }
+
         BoundExpression MakeIsDeclarationPattern(SyntaxNode syntax, BoundExpression loweredInput, BoundExpression loweredTarget, bool requiresNullTest)
         {
             var type = loweredTarget.Type;
+            requiresNullTest = requiresNullTest && !loweredInput.Type.CanContainNull();
 
-            // The type here is not a Nullable<T> instance type, as that would have led to the semantic error:
-            // ERR_PatternNullableType: It is not legal to use nullable type '{0}' in a pattern; use the underlying type '{1}' instead.
-            Debug.Assert(!type.IsNullableType());
+            // It is possible that the input value is already of the correct type, in which case the pattern
+            // is irrefutable, and we can just do the assignment and return true (or perform the null test).
+            if (MatchIsIrrefutable(loweredInput.Type, loweredTarget.Type, requiresNullTest))
+            {
+                var convertedInput = _factory.Convert(loweredTarget.Type, loweredInput);
+                var assignment = _factory.AssignmentExpression(loweredTarget, convertedInput);
+                return requiresNullTest
+                    ? _factory.ObjectNotEqual(assignment, _factory.Null(type))
+                    : _factory.MakeSequence(assignment, _factory.Literal(true));
+            }
 
             // a pattern match of the form "expression is Type identifier" is equivalent to
             // an invocation of one of these helpers:
@@ -158,31 +182,15 @@ BoundExpression MakeIsDeclarationPattern(SyntaxNode syntax, BoundExpression lowe
                 //     t = e as T;
                 //     return t != null;
                 // }
-                if (loweredInput.Type == type)
-                {
-                    // CONSIDER: this can be done whenever input.Type is a subtype of type for improved code
-                    var assignment = _factory.AssignmentExpression(loweredTarget, loweredInput);
-                    return requiresNullTest
-                        ? _factory.ObjectNotEqual(assignment, _factory.Null(type))
-                        : _factory.MakeSequence(assignment, _factory.Literal(true));
-                }
-                else
-                {
-                    return _factory.ObjectNotEqual(
-                        _factory.AssignmentExpression(loweredTarget, _factory.As(loweredInput, type)),
-                        _factory.Null(type));
-                }
+                return _factory.ObjectNotEqual(
+                    _factory.AssignmentExpression(loweredTarget, _factory.As(loweredInput, type)),
+                    _factory.Null(type));
             }
             else if (type.IsValueType)
             {
-                // It is possible that the input value is already of the correct type, in which case the pattern
-                // is irrefutable, and we can just do the assignment and return true.
-                if (loweredInput.Type == type)
-                {
-                    return _factory.MakeSequence(
-                        _factory.AssignmentExpression(loweredTarget, loweredInput),
-                        _factory.Literal(true));
-                }
+                // The type here is not a Nullable<T> instance type, as that would have led to the semantic error:
+                // ERR_PatternNullableType: It is not legal to use nullable type '{0}' in a pattern; use the underlying type '{1}' instead.
+                Debug.Assert(!type.IsNullableType());
 
                 // It may be possible to improve this code by only assigning t when returning
                 // true (avoid returning a new default value)
@@ -207,18 +215,26 @@ BoundExpression MakeIsDeclarationPattern(SyntaxNode syntax, BoundExpression lowe
                 Debug.Assert(type.IsTypeParameter());
                 // bool Is<T>(this object i, out T o)
                 // {
-                //     // inefficient because it performs the type test twice.
+                //     // inefficient because it performs the type test twice, and also because it boxes the input.
                 //     bool s = i is T;
-                //     if (s) o = (T)i;
+                //     o = s ? (T)i : default(T);
                 //     return s;
                 // }
-                return _factory.Conditional(_factory.Is(loweredInput, type),
-                    _factory.MakeSequence(_factory.AssignmentExpression(
-                        loweredTarget,
-                        _factory.Convert(type, loweredInput)),
-                        _factory.Literal(true)),
-                    _factory.Literal(false),
-                    _factory.SpecialType(SpecialType.System_Boolean));
+
+                // Because a cast involving a type parameter is not necessarily a valid conversion (or, if it is, it might not
+                // be of a kind appropriate for pattern-matching), we use `object` as an intermediate type for the input expression.
+                var tmpType = _factory.SpecialType(SpecialType.System_Object);
+                var s = _factory.SynthesizedLocal(_factory.SpecialType(SpecialType.System_Boolean), syntax);
+                var i = _factory.SynthesizedLocal(tmpType, syntax); // we copy the input to avoid double evaluation
+                return _factory.Sequence(
+                    ImmutableArray.Create(s, i),
+                    ImmutableArray.Create<BoundExpression>(
+                        _factory.AssignmentExpression(_factory.Local(i), _factory.Convert(tmpType, loweredInput)),
+                        _factory.AssignmentExpression(_factory.Local(s), _factory.Is(_factory.Local(i), type)),
+                        _factory.AssignmentExpression(loweredTarget, _factory.Conditional(_factory.Local(s), _factory.Convert(type, _factory.Local(i)), _factory.Default(type), type))
+                        ),
+                    _factory.Local(s)
+                    );
             }
         }
     }