Include nullability in CheckConstraints

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

@@ -1166,7 +1166,8 @@ private NamedTypeSymbol ConstructNamedType(
 
             if (ShouldCheckConstraints)
             {
-                type.CheckConstraintsForNonTuple(this.Conversions, typeSyntax, typeArgumentsSyntax, this.Compilation, basesBeingResolved, diagnostics);
+                bool includeNullability = Compilation.IsFeatureEnabled(MessageID.IDS_FeatureStaticNullChecking);
+                type.CheckConstraintsForNonTuple(this.Conversions.WithNullability(includeNullability), typeSyntax, typeArgumentsSyntax, this.Compilation, basesBeingResolved, diagnostics);
             }
 
             type = (NamedTypeSymbol)TupleTypeSymbol.TransformToTupleIfCompatible(type);

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/Conversions.cs

@@ -13,26 +13,27 @@ internal sealed class Conversions : ConversionsBase
         private readonly Binder _binder;
 
         public Conversions(Binder binder)
-            : this(binder, currentRecursionDepth: 0, includeNullability: false)
+            : this(binder, currentRecursionDepth: 0, includeNullability: false, otherNullabilityOpt: null)
         {
         }
 
-        private Conversions(Binder binder, int currentRecursionDepth, bool includeNullability)
-            : base(binder.Compilation.Assembly.CorLibrary, currentRecursionDepth, includeNullability)
+        private Conversions(Binder binder, int currentRecursionDepth, bool includeNullability, Conversions otherNullabilityOpt)
+            : base(binder.Compilation.Assembly.CorLibrary, currentRecursionDepth, includeNullability, otherNullabilityOpt)
         {
             _binder = binder;
         }
 
         protected override ConversionsBase CreateInstance(int currentRecursionDepth)
         {
-            return new Conversions(_binder, currentRecursionDepth, IncludeNullability);
+            return new Conversions(_binder, currentRecursionDepth, IncludeNullability, otherNullabilityOpt: null);
         }
 
         private CSharpCompilation Compilation { get { return _binder.Compilation; } }
 
-        internal Conversions WithNullability(bool includeNullability)
+        protected override ConversionsBase WithNullabilityCore(bool includeNullability)
         {
-            return (IncludeNullability == includeNullability) ? this : new Conversions(_binder, currentRecursionDepth, includeNullability);
+            Debug.Assert(IncludeNullability != includeNullability);
+            return new Conversions(_binder, currentRecursionDepth, includeNullability, this);
         }
 
         public override Conversion GetMethodGroupConversion(BoundMethodGroup source, TypeSymbol destination, ref HashSet<DiagnosticInfo> useSiteDiagnostics)

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/ConversionsBase.cs

@@ -20,15 +20,45 @@ internal abstract partial class ConversionsBase
 
         internal readonly bool IncludeNullability;
 
-        protected ConversionsBase(AssemblySymbol corLibrary, int currentRecursionDepth, bool includeNullability)
+        /// <summary>
+        /// An optional clone of this instance with distinct IncludeNullability.
+        /// Used to avoid unnecessary allocations when calling WithNullability() repeatedly.
+        /// </summary>
+        private ConversionsBase _lazyOtherNullability;
+
+        protected ConversionsBase(AssemblySymbol corLibrary, int currentRecursionDepth, bool includeNullability, ConversionsBase otherNullabilityOpt)
         {
             Debug.Assert((object)corLibrary != null);
+            Debug.Assert(otherNullabilityOpt == null || includeNullability != otherNullabilityOpt.IncludeNullability);
+            Debug.Assert(otherNullabilityOpt == null || currentRecursionDepth == otherNullabilityOpt.currentRecursionDepth);
 
             this.corLibrary = corLibrary;
             this.currentRecursionDepth = currentRecursionDepth;
             IncludeNullability = includeNullability;
+            _lazyOtherNullability = otherNullabilityOpt;
         }
 
+        /// <summary>
+        /// Returns this instance if includeNullability is correct, and returns a
+        /// cached clone of this instance with distinct includeNullability otherwise.
+        /// </summary>
+        internal ConversionsBase WithNullability(bool includeNullability)
+        {
+            if (IncludeNullability == includeNullability)
+            {
+                return this;
+            }
+            if (_lazyOtherNullability == null)
+            {
+                _lazyOtherNullability = WithNullabilityCore(includeNullability);
+            }
+            Debug.Assert(_lazyOtherNullability.IncludeNullability == includeNullability);
+            Debug.Assert(_lazyOtherNullability._lazyOtherNullability == this);
+            return _lazyOtherNullability;
+        }
+
+        protected abstract ConversionsBase WithNullabilityCore(bool includeNullability);
+
         public abstract Conversion GetMethodGroupConversion(BoundMethodGroup source, TypeSymbol destination, ref HashSet<DiagnosticInfo> useSiteDiagnostics);
 
         public abstract Conversion GetStackAllocConversion(BoundStackAllocArrayCreation sourceExpression, TypeSymbol destination, ref HashSet<DiagnosticInfo> useSiteDiagnostics);

src/Compilers/CSharp/Portable/Binder/Semantics/Conversions/TypeConversions.cs

@@ -4,24 +4,31 @@
 using Roslyn.Utilities;
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
     internal sealed class TypeConversions : ConversionsBase
     {
         public TypeConversions(AssemblySymbol corLibrary)
-            : this(corLibrary, currentRecursionDepth: 0, includeNullability: false)
+            : this(corLibrary, currentRecursionDepth: 0, includeNullability: false, otherNullabilityOpt: null)
         {
         }
 
-        private TypeConversions(AssemblySymbol corLibrary, int currentRecursionDepth, bool includeNullability)
-            : base(corLibrary, currentRecursionDepth, includeNullability)
+        private TypeConversions(AssemblySymbol corLibrary, int currentRecursionDepth, bool includeNullability, TypeConversions otherNullabilityOpt)
+            : base(corLibrary, currentRecursionDepth, includeNullability, otherNullabilityOpt)
         {
         }
 
         protected override ConversionsBase CreateInstance(int currentRecursionDepth)
         {
-            return new TypeConversions(this.corLibrary, currentRecursionDepth, IncludeNullability);
+            return new TypeConversions(this.corLibrary, currentRecursionDepth, IncludeNullability, otherNullabilityOpt: null);
+        }
+
+        protected override ConversionsBase WithNullabilityCore(bool includeNullability)
+        {
+            Debug.Assert(IncludeNullability != includeNullability);
+            return new TypeConversions(corLibrary, currentRecursionDepth, includeNullability, this);
         }
 
         public override Conversion GetMethodGroupConversion(BoundMethodGroup source, TypeSymbol destination, ref HashSet<DiagnosticInfo> useSiteDiagnostics)

src/Compilers/CSharp/Portable/Binder/Semantics/OverloadResolution/OverloadResolution.cs

@@ -386,6 +386,7 @@ private bool FailsConstraintChecks(MethodSymbol method, out ArrayBuilder<TypePar
                 this.Conversions,
                 this.Compilation,
                 diagnosticsBuilder,
+                warningsBuilderOpt: null,
                 ref useSiteDiagnosticsBuilder);
 
             if (!constraintsSatisfied)

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

@@ -98,7 +98,7 @@ public TypeSymbolWithAnnotations GetInferredReturnType(ref HashSet<DiagnosticInf
                 var diagnostics = DiagnosticBag.GetInstance();
                 var delegateType = Type.GetDelegateType();
                 var compilation = Binder.Compilation;
-                var conversions = Binder.Conversions.WithNullability(includeNullability: true);
+                var conversions = (Conversions)Binder.Conversions.WithNullability(includeNullability: true);
                 NullableWalker.Analyze(compilation, lambda: this, diagnostics, delegateInvokeMethod: delegateType?.DelegateInvokeMethod, returnTypes: returnTypes, initialState: nullableState);
                 diagnostics.Free();
                 var inferredReturnType = InferReturnType(returnTypes, compilation, conversions, delegateType, Symbol.IsAsync);

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5363,6 +5363,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_AnnotationDisallowedInObjectCreation" xml:space="preserve">
     <value>Cannot use a nullable reference type in object creation.</value>
   </data>
+  <data name="WRN_NullabilityMismatchInTypeParameterConstraint" xml:space="preserve">
+    <value>The type '{3}' cannot be used as type parameter '{2}' in the generic type or method '{0}'. Nullability of type argument '{3}' doesn't match constraint type '{1}'.</value>
+  </data>
+  <data name="WRN_NullabilityMismatchInTypeParameterConstraint_Title" xml:space="preserve">
+    <value>The type cannot be used as type parameter in the generic type or method. Nullability of type argument doesn't match constraint type.</value>
+  </data>
   <data name="ERR_ExplicitNullableAttribute" xml:space="preserve">
     <value>Explicit application of 'System.Runtime.CompilerServices.NullableAttribute' is not allowed.</value>
   </data>

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

@@ -1615,6 +1615,7 @@ internal enum ErrorCode
         ERR_AnnotationDisallowedInObjectCreation = 8628,
         WRN_MissingNonNullTypesContext = 8629,
         ERR_NonNullTypesNotAvailable = 8630,
+        WRN_NullabilityMismatchInTypeParameterConstraint = 8631,
     }
     // Note: you will need to re-generate compiler code after adding warnings (build\scripts\generate-compiler-code.cmd)
 }

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

@@ -342,6 +342,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate:
                 case ErrorCode.WRN_NullAsNonNullable:
                 case ErrorCode.WRN_NoBestNullabilityConditionalExpression:
+                case ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint:
                 case ErrorCode.WRN_Experimental:
                 case ErrorCode.WRN_AttributesOnBackingFieldsNotAvailable:
                 case ErrorCode.WRN_TupleBinopLiteralNameMismatch:

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -141,7 +141,7 @@ private NullableWalker(
             // If so, are we interested in an InMethodBinder specifically?
             _binder = compilation.GetBinderFactory(node.SyntaxTree).GetBinder(node.Syntax);
             Debug.Assert(!_binder.Conversions.IncludeNullability);
-            _conversions = _binder.Conversions.WithNullability(true);
+            _conversions = (Conversions)_binder.Conversions.WithNullability(true);
             _useMethodSignatureReturnType = (object)methodSignatureOpt != null && useMethodSignatureReturnType;
             _useMethodSignatureParameterTypes = (object)methodSignatureOpt != null && useMethodSignatureParameterTypes;
             _methodSignatureOpt = methodSignatureOpt;
@@ -2047,10 +2047,18 @@ private MethodSymbol VisitArguments(
             // We do a first pass to work through the arguments without making any assumptions
             ImmutableArray<TypeSymbolWithAnnotations> results = VisitArgumentsEvaluate(arguments, refKindsOpt);
 
-            if ((object)method != null && method.IsGenericMethod && HasImplicitTypeArguments(node))
+            if ((object)method != null && method.IsGenericMethod)
             {
-                method = InferMethodTypeArguments((BoundCall)node, method, GetArgumentsForMethodTypeInference(arguments, results));
-                parameters = method.Parameters;
+                if (HasImplicitTypeArguments(node))
+                {
+                    method = InferMethodTypeArguments((BoundCall)node, method, GetArgumentsForMethodTypeInference(arguments, results));
+                    parameters = method.Parameters;
+                }
+                if (!method.IsDefinition)
+                {
+                    var syntax = node.Syntax;
+                    CheckMethodConstraints((syntax as InvocationExpressionSyntax)?.Expression ?? syntax, method);
+                }
             }
 
             // PROTOTYPE(NullableReferenceTypes): Can we handle some error cases?
@@ -2489,7 +2497,6 @@ private MethodSymbol InferMethodTypeArguments(BoundCall node, MethodSymbol metho
                 ref useSiteDiagnostics);
             if (result.Success)
             {
-                // PROTOTYPE(NullableReferenceTypes): Check constraints
                 return definition.Construct(result.InferredTypeArguments);
             }
             return method;
@@ -2532,6 +2539,27 @@ BoundExpression getArgumentForMethodTypeInference(BoundExpression argument, Type
             }
         }
 
+        private void CheckMethodConstraints(SyntaxNode syntax, MethodSymbol method)
+        {
+            var diagnosticsBuilder = ArrayBuilder<TypeParameterDiagnosticInfo>.GetInstance();
+            var warningsBuilder = ArrayBuilder<TypeParameterDiagnosticInfo>.GetInstance();
+            ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder = null;
+            ConstraintsHelper.CheckMethodConstraints(
+                method,
+                _conversions,
+                compilation,
+                diagnosticsBuilder,
+                warningsBuilder,
+                ref useSiteDiagnosticsBuilder);
+            foreach (var pair in warningsBuilder)
+            {
+                Diagnostics.Add(pair.DiagnosticInfo, syntax.Location);
+            }
+            useSiteDiagnosticsBuilder?.Free();
+            warningsBuilder.Free();
+            diagnosticsBuilder.Free();
+        }
+
         private void ReplayReadsAndWrites(LocalFunctionSymbol localFunc,
                                   SyntaxNode syntax,
                                   bool writes)

src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs

@@ -203,6 +203,7 @@ public static bool IsWarning(ErrorCode code)
                 case ErrorCode.WRN_NullAsNonNullable:
                 case ErrorCode.WRN_NoBestNullabilityConditionalExpression:
                 case ErrorCode.WRN_MissingNonNullTypesContext:
+                case ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint:
                     return true;
                 default:
                     return false;