Extend eligible params types to array generic interfaces and span

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

@@ -2572,11 +2572,9 @@ private void CoerceArguments<TMember>(
         private static TypeSymbol GetCorrespondingParameterType(ref MemberAnalysisResult result, ImmutableArray<ParameterSymbol> parameters, int arg)
         {
             int paramNum = result.ParameterFromArgument(arg);
-            var type =
-                (paramNum == parameters.Length - 1 && result.Kind == MemberResolutionKind.ApplicableInExpandedForm) ?
-                ((ArrayTypeSymbol)parameters[paramNum].Type).ElementType :
-                parameters[paramNum].Type;
-            return type;
+            return (paramNum == parameters.Length - 1 && result.Kind == MemberResolutionKind.ApplicableInExpandedForm)
+                ? parameters[paramNum].Type.GetElementTypeOfParamsType()
+                : parameters[paramNum].Type;
         }
 
         private BoundExpression BindArrayCreationExpression(ArrayCreationExpressionSyntax node, DiagnosticBag diagnostics)

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

@@ -815,7 +815,7 @@ public static bool IsValidParams(Symbol member)
             // Note: we need to confirm the "arrayness" on the original definition because
             // it's possible that the type becomes an array as a result of substitution.
             ParameterSymbol final = member.GetParameters().Last();
-            return final.IsParams && ((ParameterSymbol)final.OriginalDefinition).Type.IsSZArray();
+            return final.IsParams && final.OriginalDefinition.Type.IsPossibleParamsType();
         }
 
         private static bool IsOverride(Symbol overridden, Symbol overrider)
@@ -2874,7 +2874,8 @@ private EffectiveParameters GetEffectiveParametersInExpandedForm<TMember>(
             var refs = ArrayBuilder<RefKind>.GetInstance();
             bool anyRef = false;
             var parameters = member.GetParameters();
-            var elementType = ((ArrayTypeSymbol)parameters[parameters.Length - 1].Type).ElementType;
+            var paramsType = parameters[parameters.Length - 1].Type;
+            var elementType = paramsType.GetElementTypeOfParamsType();
             bool hasAnyRefArg = argumentRefKinds.Any();
             hasAnyRefOmittedArgument = false;
 

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

@@ -162,6 +162,8 @@ internal enum MessageID
         IDS_FeatureIndexingMovableFixedBuffers = MessageBase + 12744,
         IDS_FeatureRecursivePatterns = MessageBase + 12745,
         IDS_FeatureNestedStackalloc = MessageBase + 12746,
+
+        IDS_FeatureParamsArrayInterfaceAndSpan = MessageBase + 12747,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -221,6 +223,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 // C# 8.0 features.
                 case MessageID.IDS_FeatureRecursivePatterns:
                 case MessageID.IDS_FeatureNestedStackalloc:
+                case MessageID.IDS_FeatureParamsArrayInterfaceAndSpan: // semantic check
                     return LanguageVersion.CSharp8;
 
                 // C# 7.3 features.

src/Compilers/CSharp/Portable/LanguageVersion.cs

@@ -360,5 +360,10 @@ internal static bool AllowImprovedOverloadCandidates(this LanguageVersion self)
         {
             return self >= MessageID.IDS_FeatureImprovedOverloadCandidates.RequiredVersion();
         }
+
+        internal static bool AllowParamsArrayInterfaceAndSpan(this LanguageVersion self)
+        {
+            return self >= MessageID.IDS_FeatureParamsArrayInterfaceAndSpan.RequiredVersion();
+        }
     }
 }

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

@@ -887,7 +887,12 @@ private BoundExpression BuildParamsArray(
                 }
             }
 
-            var paramArrayType = parameters[paramsParam].Type;
+            var paramsType = parameters[paramsParam].Type;
+            if (paramsType.IsPossibleArrayGenericInterface())
+            {
+                paramsType = ArrayTypeSymbol.CreateSZArray(_compilation.Assembly, paramsType.GetElementTypeOfParamsType()); 
+            }
+
             var arrayArgs = paramArray.ToImmutableAndFree();
 
             // If this is a zero-length array, rather than using "new T[0]", optimize with "Array.Empty<T>()" 
@@ -896,14 +901,13 @@ private BoundExpression BuildParamsArray(
             // of expression lambdas will appropriately understand Array.Empty<T>().
             if (arrayArgs.Length == 0 && !_inExpressionLambda)
             {
-                ArrayTypeSymbol ats = paramArrayType as ArrayTypeSymbol;
-                if (ats != null) // could be null if there's a semantic error, e.g. the params parameter type isn't an array
+                if (paramsType is ArrayTypeSymbol arrayType) // could be false if there's a semantic error, or if the params parameter type isn't an array
                 {
                     MethodSymbol arrayEmpty = _compilation.GetWellKnownTypeMember(WellKnownMember.System_Array__Empty) as MethodSymbol;
                     if (arrayEmpty != null) // will be null if Array.Empty<T> doesn't exist in reference assemblies
                     {
                         // return an invocation of "Array.Empty<T>()"
-                        arrayEmpty = arrayEmpty.Construct(ImmutableArray.Create(ats.ElementType));
+                        arrayEmpty = arrayEmpty.Construct(ImmutableArray.Create(arrayType.ElementType));
                         return new BoundCall(
                             syntax,
                             null,
@@ -922,25 +926,77 @@ private BoundExpression BuildParamsArray(
                 }
             }
 
-            return CreateParamArrayArgument(syntax, paramArrayType, arrayArgs, this, null);
+            return CreateParamArrayArgument(syntax, paramsType, arrayArgs, this, null);
         }
 
-        private static BoundExpression CreateParamArrayArgument(SyntaxNode syntax,
-            TypeSymbol paramArrayType,
+        private static BoundExpression CreateParamArrayArgument(
+            SyntaxNode syntax,
+            TypeSymbol paramsType,
             ImmutableArray<BoundExpression> arrayArgs,
             LocalRewriter localRewriter,
             Binder binder)
         {
             Debug.Assert(localRewriter == null ^ binder == null);
 
-            TypeSymbol int32Type = (localRewriter != null ? localRewriter._compilation : binder.Compilation).GetSpecialType(SpecialType.System_Int32);
+            CSharpCompilation compilation = localRewriter != null ? localRewriter._compilation : binder.Compilation;
+            TypeSymbol int32Type = compilation.GetSpecialType(SpecialType.System_Int32);
             BoundExpression arraySize = MakeLiteral(syntax, ConstantValue.Create(arrayArgs.Length), int32Type, localRewriter);
 
-            return new BoundArrayCreation(
-                syntax,
-                ImmutableArray.Create(arraySize),
-                new BoundArrayInitialization(syntax, arrayArgs) { WasCompilerGenerated = true },
-                paramArrayType) { WasCompilerGenerated = true };
+            var initializer = new BoundArrayInitialization(syntax, arrayArgs) { WasCompilerGenerated = true };
+
+            if (paramsType.IsSZArray())
+            {
+                return new BoundArrayCreation(syntax, ImmutableArray.Create(arraySize), initializer, paramsType) { WasCompilerGenerated = true };
+            }
+
+            TypeSymbol elementType = paramsType.GetElementTypeOfParamsType();
+            bool isReferenceType = elementType.IsReferenceType;
+
+            BoundExpression[] args;
+            if (isReferenceType)
+            {
+                args = new BoundExpression[]
+                {
+                    new BoundArrayCreation(
+                        syntax,
+                        ImmutableArray.Create(arraySize),
+                        initializer,
+                        ArrayTypeSymbol.CreateSZArray(compilation.Assembly, elementType))
+                    { WasCompilerGenerated = true }
+                };
+            }
+            else
+            {
+                args = new BoundExpression[]
+                {
+                    new BoundConvertedStackAllocExpression(
+                        syntax,
+                        elementType,
+                        arraySize,
+                        initializer,
+                        paramsType)
+                    { WasCompilerGenerated = true },
+                    arraySize,
+                };
+            }
+
+            WellKnownMember constructor = compilation.IsReadOnlySpanType(paramsType)
+                ? isReferenceType ? WellKnownMember.System_ReadOnlySpan_T__ctor2 : WellKnownMember.System_ReadOnlySpan_T__ctor
+                : isReferenceType ? WellKnownMember.System_Span_T__ctor2 : WellKnownMember.System_Span_T__ctor;
+
+            // TODO use TryGetWellKnownTypeMember instead, should test with a missing ctor
+            var spanConstructor = compilation.GetWellKnownTypeMember(constructor) as MethodSymbol;
+            if (spanConstructor == null)
+            {
+                return new BoundBadExpression(
+                    syntax: syntax,
+                    resultKind: LookupResultKind.NotInvocable,
+                    symbols: ImmutableArray<Symbol>.Empty,
+                    childBoundNodes: ImmutableArray<BoundExpression>.Empty,
+                    type: ErrorTypeSymbol.UnknownResultType);
+            }
+
+            return new BoundObjectCreationExpression(syntax, spanConstructor.AsMember((NamedTypeSymbol)paramsType), binder, arguments: args);
         }
 
         /// <summary>

src/Compilers/CSharp/Portable/Symbols/MemberSignatureComparer.cs

@@ -96,7 +96,8 @@ internal class MemberSignatureComparer : IEqualityComparer<Symbol>
             considerTypeConstraints: false,
             considerCallingConvention: false,
             considerRefKindDifferences: false,
-            considerCustomModifiers: false); //shouldn't actually matter for source members
+            considerCustomModifiers: false, //shouldn't actually matter for source members
+            considerParamsElementTypes: true);
 
         /// <summary>
         /// This instance is used to determine if a partial method implementation matches the definition.
@@ -302,6 +303,8 @@ internal class MemberSignatureComparer : IEqualityComparer<Symbol>
         // Ignore the names of the tuple elements
         private readonly bool _ignoreTupleNames;
 
+        private readonly bool _considerParamsElementTypes;
+
         private MemberSignatureComparer(
             bool considerName,
             bool considerExplicitlyImplementedInterfaces,
@@ -310,6 +313,7 @@ private MemberSignatureComparer(
             bool considerCallingConvention,
             bool considerRefKindDifferences,
             bool considerCustomModifiers,
+            bool considerParamsElementTypes = false,
             bool ignoreDynamic = true,
             bool ignoreTupleNames = true)
         {
@@ -324,6 +328,7 @@ private MemberSignatureComparer(
             _considerCustomModifiers = considerCustomModifiers;
             _ignoreDynamic = ignoreDynamic;
             _ignoreTupleNames = ignoreTupleNames;
+            _considerParamsElementTypes = considerParamsElementTypes;
         }
 
         #region IEqualityComparer<Symbol> Members
@@ -374,10 +379,21 @@ public bool Equals(Symbol member1, Symbol member2)
                 return false;
             }
 
-            if (member1.GetParameterCount() > 0 && !HaveSameParameterTypes(member1.GetParameters(), typeMap1, member2.GetParameters(), typeMap2,
-                                                                           _considerRefKindDifferences, _considerCustomModifiers, _ignoreDynamic, _ignoreTupleNames))
+            var parameters1 = member1.GetParameters();
+            var parameters2 = member2.GetParameters();
+
+            if (member1.GetParameterCount() > 0)
             {
-                return false;
+                if (!HaveSameParameterTypes(parameters1, typeMap1, parameters2, typeMap2, _considerRefKindDifferences, _considerCustomModifiers, _ignoreDynamic, _ignoreTupleNames))
+                {
+                    return false;
+                }
+
+                if (_considerParamsElementTypes && member1.HasParamsParameter() && member2.HasParamsParameter() &&
+                    !HaveSameParamsElementTypes(parameters1, parameters2))
+                {
+                    return false;
+                }
             }
 
             if (_considerCallingConvention)
@@ -693,6 +709,14 @@ private static bool HaveSameParameterTypes(ImmutableArray<ParameterSymbol> param
             return true;
         }
 
+        private static bool HaveSameParamsElementTypes(ImmutableArray<ParameterSymbol> parameters1,  ImmutableArray<ParameterSymbol> parameters2)
+        {
+            Debug.Assert(parameters1.Length == parameters2.Length);
+            var elementType1 = parameters1.Last().Type.GetElementTypeOfParamsType();
+            var elementType2 = parameters2.Last().Type.GetElementTypeOfParamsType();
+            return elementType1.Equals(elementType2, TypeCompareKind.IgnoreTupleNames);
+        }
+
         private static TypeWithModifiers SubstituteType(TypeMap typeMap, TypeWithModifiers typeSymbol)
         {
             return typeMap == null ? typeSymbol : typeSymbol.SubstituteType(typeMap);

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

@@ -92,7 +92,7 @@ public static ImmutableArray<ParameterSymbol> MakeParameters(
                     addRefReadOnlyModifier,
                     diagnostics);
 
-                ReportParameterErrors(owner, parameterSyntax, parameter, thisKeyword, paramsKeyword, firstDefault, diagnostics);
+                ReportParameterErrors(owner, parameterSyntax, parameter, thisKeyword, paramsKeyword, firstDefault, binder.Compilation, diagnostics);
 
                 builder.Add(parameter);
                 ++parameterIndex;
@@ -269,13 +269,13 @@ private static void CheckParameterModifiers(
             }
         }
 
-        private static void ReportParameterErrors(
-            Symbol owner,
+        private static void ReportParameterErrors(Symbol owner,
             ParameterSyntax parameterSyntax,
             SourceParameterSymbol parameter,
             SyntaxToken thisKeyword,
             SyntaxToken paramsKeyword,
             int firstDefault,
+            CSharpCompilation compilation,
             DiagnosticBag diagnostics)
         {
             TypeSymbol parameterType = parameter.Type;
@@ -295,12 +295,24 @@ private static void ReportParameterErrors(
                 // error CS1670: params is not valid in this context
                 diagnostics.Add(ErrorCode.ERR_IllegalParams, paramsKeyword.GetLocation());
             }
-            else if (parameter.IsParams && !parameterType.IsSZArray())
+            else if (parameter.IsParams)
             {
-                // error CS0225: The params parameter must be a single dimensional array
-                diagnostics.Add(ErrorCode.ERR_ParamsMustBeArray, paramsKeyword.GetLocation());
+                if (!parameterType.IsPossibleParamsType())
+                {
+                    // error CS0225: The params parameter must be assignable from array.
+                    diagnostics.Add(ErrorCode.ERR_ParamsMustBeArray, paramsKeyword.GetLocation());
+                    return;
+                }
+
+                if (!parameterType.IsSZArray() && !compilation.LanguageVersion.AllowParamsArrayInterfaceAndSpan())
+                {
+                    diagnostics.Add(ErrorCode.ERR_ParamsMustBeArray, paramsKeyword.GetLocation(),
+                         new CSharpRequiredLanguageVersion(MessageID.IDS_FeatureParamsArrayInterfaceAndSpan.RequiredVersion()));
+                    return;
+                }
             }
-            else if (parameter.Type.IsStatic && !parameter.ContainingSymbol.ContainingType.IsInterfaceType())
+
+            if (parameter.Type.IsStatic && !parameter.ContainingSymbol.ContainingType.IsInterfaceType())
             {
                 // error CS0721: '{0}': static types cannot be used as parameters
                 diagnostics.Add(ErrorCode.ERR_ParameterIsStaticClass, owner.Locations[0], parameter.Type);

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

@@ -1551,9 +1551,7 @@ private void CheckMemberNameConflicts(DiagnosticBag diagnostics)
                     // That takes care of the first category of conflict; we detect the
                     // second and third categories as follows:
 
-                    var conversion = symbol as SourceUserDefinedConversionSymbol;
-                    var method = symbol as SourceMemberMethodSymbol;
-                    if ((object)conversion != null)
+                    if (symbol is SourceUserDefinedConversionSymbol conversion)
                     {
                         // Does this conversion collide *as a conversion* with any previously-seen
                         // conversion?
@@ -1584,7 +1582,7 @@ private void CheckMemberNameConflicts(DiagnosticBag diagnostics)
                         // Do not add the conversion to the set of previously-seen methods; that set
                         // is only non-conversion methods.
                     }
-                    else if ((object)method != null)
+                    else if (symbol is SourceMemberMethodSymbol method)
                     {
                         // Does this method collide *as a method* with any previously-seen
                         // conversion?