Add support for C# 14 first-class span types in the type system.

ICSharpCode.Decompiler.Tests/Helpers/Tester.cs

@@ -721,7 +721,7 @@ namespace System.Runtime.CompilerServices
 					CompilerOptions.UseRoslyn1_3_2 => CSharp.LanguageVersion.CSharp6,
 					CompilerOptions.UseRoslyn2_10_0 => CSharp.LanguageVersion.CSharp7_3,
 					CompilerOptions.UseRoslyn3_11_0 => CSharp.LanguageVersion.CSharp9_0,
-					_ => cscOptions.HasFlag(CompilerOptions.Preview) ? CSharp.LanguageVersion.Latest : CSharp.LanguageVersion.CSharp13_0,
+					_ => cscOptions.HasFlag(CompilerOptions.Preview) ? CSharp.LanguageVersion.Latest : CSharp.LanguageVersion.CSharp14_0,
 				};
 				DecompilerSettings settings = new(langVersion) {
 					// Never use file-scoped namespaces

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -1438,7 +1438,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			var conversions = CSharpConversions.Get(expressionBuilder.compilation);
 			IType targetType = method.ReturnType;
 			var conv = conversions.ImplicitConversion(argument.Type, targetType);
-			if (!(conv.IsUserDefined && conv.IsValid && conv.Method.Equals(method)))
+			if (!(conv.IsUserDefined && conv.IsValid && conv.Method.Equals(method, NormalizeTypeVisitor.TypeErasure)))
 			{
 				// implicit conversion to targetType isn't directly possible, so first insert a cast to the argument type
 				argument = argument.ConvertTo(method.Parameters[0].Type, expressionBuilder);

ICSharpCode.Decompiler/CSharp/Resolver/CSharpConversions.cs

@@ -247,6 +247,10 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				if (IdentityConversion(elementType, spanElementType))
 					return Conversion.InlineArrayConversion;
 			}
+			if (IsImplicitSpanConversion(fromType, toType))
+			{
+				return Conversion.ImplicitSpanConversion;
+			}
 			return Conversion.None;
 		}
 
@@ -1229,6 +1233,49 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		}
 		#endregion
 
+		#region Implicit Span Conversion
+
+		bool IsImplicitSpanConversion(IType fromType, IType toType)
+		{
+			if (!compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes))
+			{
+				return false;
+			}
+
+			// An implicit span conversion permits array_types, System.Span<T>, System.ReadOnlySpan<T>,
+			// and string to be converted between each other 
+			// see https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-14.0/first-class-span-types#span-conversions
+
+			switch (fromType)
+			{
+				case ArrayType { Dimensions: 1, ElementType: var elementType }:
+					if (toType.IsKnownType(KnownTypeCode.SpanOfT))
+					{
+						return IdentityConversion(elementType, toType.TypeArguments[0]);
+					}
+					if (toType.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+					{
+						return IdentityConversion(elementType, toType.TypeArguments[0])
+							|| IsImplicitReferenceConversion(elementType, toType.TypeArguments[0]);
+					}
+					break;
+				case ParameterizedType pt when pt.IsKnownType(KnownTypeCode.SpanOfT) || pt.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					if (toType.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+					{
+						return IdentityConversion(pt.TypeArguments[0], toType.TypeArguments[0])
+							|| IsImplicitReferenceConversion(pt.TypeArguments[0], toType.TypeArguments[0]);
+					}
+					break;
+				case var s when s.IsKnownType(KnownTypeCode.String):
+					return toType.IsKnownType(KnownTypeCode.ReadOnlySpanOfT)
+						&& toType.TypeArguments[0].IsKnownType(KnownTypeCode.Char);
+			}
+
+			return false;
+		}
+
+		#endregion
+
 		#region AnonymousFunctionConversion
 		Conversion AnonymousFunctionConversion(ResolveResult resolveResult, IType toType)
 		{
@@ -1487,11 +1534,32 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 
 		#region BetterConversion
 		/// <summary>
-		/// Gets the better conversion (C# 4.0 spec, §7.5.3.3)
+		/// Gets the better conversion (from expression) (C# 8.0 spec, §12.6.4.5)
 		/// </summary>
 		/// <returns>0 = neither is better; 1 = t1 is better; 2 = t2 is better</returns>
 		public int BetterConversion(ResolveResult resolveResult, IType t1, IType t2)
 		{
+			bool t1Exact = IsExactlyMatching(resolveResult, t1);
+			bool t2Exact = IsExactlyMatching(resolveResult, t2);
+			if (t1Exact && !t2Exact)
+				return 1;
+			if (t2Exact && !t1Exact)
+				return 2;
+			if (!t1Exact && !t2Exact)
+			{
+				bool c1ImplicitSpanConversion = IsImplicitSpanConversion(resolveResult.Type, t1);
+				bool c2ImplicitSpanConversion = IsImplicitSpanConversion(resolveResult.Type, t2);
+				if (c1ImplicitSpanConversion && !c2ImplicitSpanConversion)
+					return 1;
+				if (c2ImplicitSpanConversion && !c1ImplicitSpanConversion)
+					return 2;
+			}
+			if (t1Exact == t2Exact)
+			{
+				int r = BetterConversionTarget(t1, t2);
+				if (r != 0)
+					return r;
+			}
 			LambdaResolveResult lambda = resolveResult as LambdaResolveResult;
 			if (lambda != null)
 			{
@@ -1542,20 +1610,56 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		}
 
 		/// <summary>
-		/// Unpacks the generic Task[T]. Returns null if the input is not Task[T].
+		/// Gets whether an expression E exactly matches a type T (C# 8.0 spec, §12.6.4.6)
 		/// </summary>
-		static IType UnpackTask(IType type)
+		bool IsExactlyMatching(ResolveResult e, IType t)
 		{
-			ParameterizedType pt = type as ParameterizedType;
-			if (pt != null && pt.TypeParameterCount == 1 && pt.Name == "Task" && pt.Namespace == "System.Threading.Tasks")
+			var s = e.Type;
+			if (IdentityConversion(s, t))
+				return true;
+			if (e is LambdaResolveResult lambda)
 			{
-				return pt.GetTypeArgument(0);
+				if (!lambda.IsAnonymousMethod)
+				{
+					t = UnpackExpressionTreeType(t);
+				}
+				IMethod m = t.GetDelegateInvokeMethod();
+				if (m == null)
+					return false;
+				IType[] parameterTypes = new IType[m.Parameters.Count];
+				for (int i = 0; i < parameterTypes.Length; i++)
+					parameterTypes[i] = m.Parameters[i].Type;
+				var x = lambda.GetInferredReturnType(parameterTypes);
+				var y = m.ReturnType;
+				if (IdentityConversion(x, y))
+					return true;
+				if (lambda.IsAsync)
+				{
+					x = UnpackTask(x);
+					y = UnpackTask(y);
+				}
+				if (x != null && y != null)
+					return IsExactlyMatching(new ResolveResult(x), y);
+				return false;
+			}
+			else
+			{
+				return false;
 			}
-			return null;
 		}
 
 		/// <summary>
-		/// Gets the better conversion (C# 4.0 spec, §7.5.3.4)
+		/// Unpacks the generic TaskType[T]. Returns null if the input is not TaskType[T].
+		/// </summary>
+		static IType UnpackTask(IType type)
+		{
+			return (TaskType.IsTask(type) || TaskType.IsCustomTask(type, out _)) && type.TypeParameterCount == 1
+				? type.TypeArguments[0]
+				: null;
+		}
+
+		/// <summary>
+		/// Gets the better conversion (from type) (C# 4.0 spec, §7.5.3.4)
 		/// </summary>
 		/// <returns>0 = neither is better; 1 = t1 is better; 2 = t2 is better</returns>
 		public int BetterConversion(IType s, IType t1, IType t2)
@@ -1570,17 +1674,57 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		}
 
 		/// <summary>
-		/// Gets the better conversion target (C# 4.0 spec, §7.5.3.5)
+		/// Gets the better conversion target (C# 9.0 spec, §12.6.4.7)
 		/// </summary>
 		/// <returns>0 = neither is better; 1 = t1 is better; 2 = t2 is better</returns>
 		int BetterConversionTarget(IType t1, IType t2)
 		{
-			bool t1To2 = ImplicitConversion(t1, t2).IsValid;
-			bool t2To1 = ImplicitConversion(t2, t1).IsValid;
-			if (t1To2 && !t2To1)
-				return 1;
-			if (t2To1 && !t1To2)
-				return 2;
+			if (t1.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+			{
+				if (t2.IsKnownType(KnownTypeCode.SpanOfT))
+				{
+					if (IdentityConversion(t1.TypeArguments[0], t2.TypeArguments[0]))
+						return 1;
+				}
+				if (t2.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+				{
+					bool t1To2 = ImplicitConversion(t1.TypeArguments[0], t2.TypeArguments[0]).IsValid;
+					bool t2To1 = ImplicitConversion(t2.TypeArguments[0], t1.TypeArguments[0]).IsValid;
+					if (t1To2 && !t2To1)
+						return 1;
+				}
+			}
+
+			if (t2.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+			{
+				if (t1.IsKnownType(KnownTypeCode.SpanOfT))
+				{
+					if (IdentityConversion(t2.TypeArguments[0], t1.TypeArguments[0]))
+						return 2;
+				}
+				if (t1.IsKnownType(KnownTypeCode.ReadOnlySpanOfT))
+				{
+					bool t1To2 = ImplicitConversion(t1.TypeArguments[0], t2.TypeArguments[0]).IsValid;
+					bool t2To1 = ImplicitConversion(t2.TypeArguments[0], t1.TypeArguments[0]).IsValid;
+					if (t2To1 && !t1To2)
+						return 2;
+				}
+			}
+
+			{
+				bool t1To2 = ImplicitConversion(t1, t2).IsValid;
+				bool t2To1 = ImplicitConversion(t2, t1).IsValid;
+				if (t1To2 && !t2To1)
+					return 1;
+				if (t2To1 && !t1To2)
+					return 2;
+			}
+
+			var s1 = UnpackTask(t1);
+			var s2 = UnpackTask(t2);
+			if (s1 != null && s2 != null)
+				return BetterConversionTarget(s1, s2);
+
 			TypeCode t1Code = ReflectionHelper.GetTypeCode(t1);
 			TypeCode t2Code = ReflectionHelper.GetTypeCode(t2);
 			if (IsBetterIntegralType(t1Code, t2Code))

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -2171,7 +2171,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				thisParameterType = thisParameterType.AcceptVisitor(substitution);
 			}
 			Conversion c = conversions.ImplicitConversion(targetType, thisParameterType);
-			return c.IsValid && (c.IsIdentityConversion || c.IsReferenceConversion || c.IsBoxingConversion);
+			return c.IsValid && (c.IsIdentityConversion || c.IsReferenceConversion || c.IsBoxingConversion || c.IsImplicitSpanConversion);
 		}
 
 		/// <summary>

ICSharpCode.Decompiler/CSharp/Resolver/OverloadResolution.cs

@@ -710,8 +710,8 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				candidate.ArgumentConversions[i] = c;
 				if (IsExtensionMethodInvocation && parameterIndex == 0)
 				{
-					// First parameter to extension method must be an identity, reference or boxing conversion
-					if (!(c == Conversion.IdentityConversion || c == Conversion.ImplicitReferenceConversion || c == Conversion.BoxingConversion))
+					// First parameter to extension method must be an identity, reference, boxing or span conversion
+					if (!(c == Conversion.IdentityConversion || c == Conversion.ImplicitReferenceConversion || c == Conversion.BoxingConversion || c == Conversion.ImplicitSpanConversion))
 						candidate.AddError(OverloadResolutionErrors.ArgumentTypeMismatch);
 				}
 				else

ICSharpCode.Decompiler/CSharp/Resolver/TypeInference.cs

@@ -659,17 +659,31 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				return;
 			}
 			// Handle array types:
-			ArrayType arrU = U as ArrayType;
-			ArrayType arrV = V as ArrayType;
-			if (arrU != null && arrV != null && arrU.Dimensions == arrV.Dimensions)
+			U = U.TupleUnderlyingTypeOrSelf();
+			V = V.TupleUnderlyingTypeOrSelf();
+			switch ((U, V))
 			{
-				MakeExactInference(arrU.ElementType, arrV.ElementType);
-				return;
+				case (ArrayType arrU, ArrayType arrV) when arrU.Dimensions == arrV.Dimensions:
+					MakeExactInference(arrU.ElementType, arrV.ElementType);
+					return;
+				case (ArrayType arrU, ParameterizedType spanV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && spanV.IsKnownType(KnownTypeCode.SpanOfT):
+					MakeExactInference(arrU.ElementType, spanV.TypeArguments[0]);
+					return;
+				case (ParameterizedType spanU, ParameterizedType spanV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && spanU.IsKnownType(KnownTypeCode.SpanOfT) && spanV.IsKnownType(KnownTypeCode.SpanOfT):
+					MakeExactInference(spanU.TypeArguments[0], spanV.TypeArguments[0]);
+					return;
+				case (ArrayType arrU, ParameterizedType rosV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && rosV.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					MakeExactInference(arrU.ElementType, rosV.TypeArguments[0]);
+					return;
+				case (ParameterizedType spanU, ParameterizedType rosV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && spanU.IsKnownType(KnownTypeCode.SpanOfT) && rosV.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					MakeExactInference(spanU.TypeArguments[0], rosV.TypeArguments[0]);
+					return;
+				case (ParameterizedType rosU, ParameterizedType rosV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && rosU.IsKnownType(KnownTypeCode.ReadOnlySpanOfT) && rosV.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					MakeExactInference(rosU.TypeArguments[0], rosV.TypeArguments[0]);
+					return;
 			}
 			// Handle parameterized type:
-			ParameterizedType pU = U.TupleUnderlyingTypeOrSelf() as ParameterizedType;
-			ParameterizedType pV = V.TupleUnderlyingTypeOrSelf() as ParameterizedType;
-			if (pU != null && pV != null
+			if (U is ParameterizedType pU && V is ParameterizedType pV
 				&& object.Equals(pU.GenericType, pV.GenericType)
 				&& pU.TypeParameterCount == pV.TypeParameterCount)
 			{
@@ -751,21 +765,33 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				return;
 			}
 			// Handle array types:
-			ArrayType arrU = U as ArrayType;
-			ArrayType arrV = V as ArrayType;
-			ParameterizedType pV = V.TupleUnderlyingTypeOrSelf() as ParameterizedType;
-			if (arrU != null && arrV != null && arrU.Dimensions == arrV.Dimensions)
+			V = V.TupleUnderlyingTypeOrSelf();
+			switch ((U, V))
 			{
-				MakeLowerBoundInference(arrU.ElementType, arrV.ElementType);
-				return;
-			}
-			else if (arrU != null && pV.IsArrayInterfaceType() && arrU.Dimensions == 1)
-			{
-				MakeLowerBoundInference(arrU.ElementType, pV.GetTypeArgument(0));
-				return;
+				case (ArrayType arrU, ArrayType arrV) when arrU.Dimensions == arrV.Dimensions:
+					MakeLowerBoundInference(arrU.ElementType, arrV.ElementType);
+					return;
+				case (ArrayType arrU, ParameterizedType spanV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && spanV.IsKnownType(KnownTypeCode.SpanOfT):
+					MakeLowerBoundInference(arrU.ElementType, spanV.TypeArguments[0]);
+					return;
+				case (ParameterizedType spanU, ParameterizedType spanV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && spanU.IsKnownType(KnownTypeCode.SpanOfT) && spanV.IsKnownType(KnownTypeCode.SpanOfT):
+					MakeLowerBoundInference(spanU.TypeArguments[0], spanV.TypeArguments[0]);
+					return;
+				case (ArrayType arrU, ParameterizedType rosV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && rosV.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					MakeLowerBoundInference(arrU.ElementType, rosV.TypeArguments[0]);
+					return;
+				case (ParameterizedType spanU, ParameterizedType rosV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && spanU.IsKnownType(KnownTypeCode.SpanOfT) && rosV.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					MakeLowerBoundInference(spanU.TypeArguments[0], rosV.TypeArguments[0]);
+					return;
+				case (ParameterizedType rosU, ParameterizedType rosV) when compilation.TypeSystemOptions.HasFlag(TypeSystemOptions.FirstClassSpanTypes) && rosU.IsKnownType(KnownTypeCode.ReadOnlySpanOfT) && rosV.IsKnownType(KnownTypeCode.ReadOnlySpanOfT):
+					MakeLowerBoundInference(rosU.TypeArguments[0], rosV.TypeArguments[0]);
+					return;
+				case (ArrayType arrU, ParameterizedType arrIntfV) when arrIntfV.IsArrayInterfaceType() && arrU.Dimensions == 1:
+					MakeLowerBoundInference(arrU.ElementType, arrIntfV.TypeArguments[0]);
+					return;
 			}
 			// Handle parameterized types:
-			if (pV != null)
+			if (V is ParameterizedType pV)
 			{
 				ParameterizedType uniqueBaseType = null;
 				foreach (IType baseU in U.GetAllBaseTypes())

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -175,12 +175,13 @@ namespace ICSharpCode.Decompiler
 			if (languageVersion < CSharp.LanguageVersion.CSharp14_0)
 			{
 				extensionMembers = false;
+				firstClassSpanTypes = false;
 			}
 		}
 
 		public CSharp.LanguageVersion GetMinimumRequiredVersion()
 		{
-			if (extensionMembers)
+			if (extensionMembers || firstClassSpanTypes)
 				return CSharp.LanguageVersion.CSharp14_0;
 			if (paramsCollections)
 				return CSharp.LanguageVersion.CSharp13_0;
@@ -2179,6 +2180,24 @@ namespace ICSharpCode.Decompiler
 			}
 		}
 
+		bool firstClassSpanTypes = true;
+
+		/// <summary>
+		/// Gets/Sets whether (ReadOnly)Span&lt;T&gt; should be treated like built-in types.
+		/// </summary>
+		[Category("C# 14.0 / VS 202x.yy")]
+		[Description("DecompilerSettings.FirstClassSpanTypes")]
+		public bool FirstClassSpanTypes {
+			get { return firstClassSpanTypes; }
+			set {
+				if (firstClassSpanTypes != value)
+				{
+					firstClassSpanTypes = value;
+					OnPropertyChanged();
+				}
+			}
+		}
+
 		bool separateLocalVariableDeclarations = false;
 
 		/// <summary>

ICSharpCode.Decompiler/Semantics/Conversion.cs

@@ -92,6 +92,11 @@ namespace ICSharpCode.Decompiler.Semantics
 		/// </summary>
 		public static readonly Conversion InlineArrayConversion = new BuiltinConversion(true, 12);
 
+		/// <summary>
+		/// C# 14 implicit span conversion from an array type to <see cref="System.Span{T}"/> or <see cref="System.ReadOnlySpan{T}"/>.
+		/// </summary>
+		public static readonly Conversion ImplicitSpanConversion = new BuiltinConversion(true, 13);
+
 		public static Conversion UserDefinedConversion(IMethod operatorMethod, bool isImplicit, Conversion conversionBeforeUserDefinedOperator, Conversion conversionAfterUserDefinedOperator, bool isLifted = false, bool isAmbiguous = false)
 		{
 			if (operatorMethod == null)
@@ -250,6 +255,9 @@ namespace ICSharpCode.Decompiler.Semantics
 				get { return type == 11; }
 			}
 
+			public override bool IsInlineArrayConversion => type == 12;
+			public override bool IsImplicitSpanConversion => type == 13;
+
 			public override string ToString()
 			{
 				string name = null;
@@ -284,6 +292,10 @@ namespace ICSharpCode.Decompiler.Semantics
 						return "interpolated string";
 					case 11:
 						return "throw-expression conversion";
+					case 12:
+						return "inline array conversion";
+					case 13:
+						return "implicit span conversion";
 				}
 				return (isImplicit ? "implicit " : "explicit ") + name + " conversion";
 			}
@@ -621,6 +633,16 @@ namespace ICSharpCode.Decompiler.Semantics
 		/// </summary>
 		public virtual bool IsInterpolatedStringConversion => false;
 
+		/// <summary>
+		/// Gets whether this is an inline array conversion to <see cref="System.Span{T}"/> or <see cref="System.ReadOnlySpan{T}"/>.
+		/// </summary>
+		public virtual bool IsInlineArrayConversion => false;
+
+		/// <summary>
+		/// Gets whether this is an implicit span conversion from an array type to <see cref="System.Span{T}"/> or <see cref="System.ReadOnlySpan{T}"/>.
+		/// </summary>
+		public virtual bool IsImplicitSpanConversion => false;
+
 		/// <summary>
 		/// For a tuple conversion, gets the individual tuple element conversions.
 		/// </summary>

ICSharpCode.Decompiler/TypeSystem/DecompilerTypeSystem.cs

@@ -145,12 +145,17 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		ParamsCollections = 0x20000,
 		/// <summary>
-		/// Default settings: typical options for the decompiler, with all C# languages features enabled.
+		/// If this option is active, span types (Span&lt;T&gt; and ReadOnlySpan&lt;T&gt;) are treated like
+		/// built-in types and language rules of C# 14 and later are applied.
+		/// </summary>
+		FirstClassSpanTypes = 0x40000,
+		/// <summary>
+		/// Default settings: typical options for the decompiler, with all C# language features enabled.
 		/// </summary>
 		Default = Dynamic | Tuple | ExtensionMethods | DecimalConstants | ReadOnlyStructsAndParameters
 			| RefStructs | UnmanagedConstraints | NullabilityAnnotations | ReadOnlyMethods
 			| NativeIntegers | FunctionPointers | ScopedRef | NativeIntegersWithoutAttribute
-			| RefReadOnlyParameters | ParamsCollections
+			| RefReadOnlyParameters | ParamsCollections | FirstClassSpanTypes
 	}
 
 	/// <summary>
@@ -194,6 +199,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				typeSystemOptions |= TypeSystemOptions.RefReadOnlyParameters;
 			if (settings.ParamsCollections)
 				typeSystemOptions |= TypeSystemOptions.ParamsCollections;
+			if (settings.FirstClassSpanTypes)
+				typeSystemOptions |= TypeSystemOptions.FirstClassSpanTypes;
 			return typeSystemOptions;
 		}
 
@@ -213,16 +220,18 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				throw new ArgumentNullException(nameof(mainModule));
 			if (assemblyResolver == null)
 				throw new ArgumentNullException(nameof(assemblyResolver));
-			var ts = new DecompilerTypeSystem();
-			await ts.InitializeAsync(mainModule, assemblyResolver, typeSystemOptions)
+			var ts = new DecompilerTypeSystem(typeSystemOptions);
+			await ts.InitializeAsync(mainModule, assemblyResolver)
 				.ConfigureAwait(false);
 			return ts;
 		}
 
 		private MetadataModule mainModule;
+		private TypeSystemOptions typeSystemOptions;
 
-		private DecompilerTypeSystem()
+		private DecompilerTypeSystem(TypeSystemOptions typeSystemOptions)
 		{
+			this.typeSystemOptions = typeSystemOptions;
 		}
 
 		public DecompilerTypeSystem(MetadataFile mainModule, IAssemblyResolver assemblyResolver)
@@ -236,12 +245,13 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		}
 
 		public DecompilerTypeSystem(MetadataFile mainModule, IAssemblyResolver assemblyResolver, TypeSystemOptions typeSystemOptions)
+			: this(typeSystemOptions)
 		{
 			if (mainModule == null)
 				throw new ArgumentNullException(nameof(mainModule));
 			if (assemblyResolver == null)
 				throw new ArgumentNullException(nameof(assemblyResolver));
-			InitializeAsync(mainModule, assemblyResolver, typeSystemOptions).GetAwaiter().GetResult();
+			InitializeAsync(mainModule, assemblyResolver).GetAwaiter().GetResult();
 		}
 
 		static readonly string[] implicitReferences = new[] {
@@ -249,7 +259,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			"System.Runtime.CompilerServices.Unsafe"
 		};
 
-		private async Task InitializeAsync(MetadataFile mainModule, IAssemblyResolver assemblyResolver, TypeSystemOptions typeSystemOptions)
+		private async Task InitializeAsync(MetadataFile mainModule, IAssemblyResolver assemblyResolver)
 		{
 			// Load referenced assemblies and type-forwarder references.
 			// This is necessary to make .NET Core/PCL binaries work better.
@@ -410,5 +420,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		}
 
 		public new MetadataModule MainModule => mainModule;
+
+		public override TypeSystemOptions TypeSystemOptions => typeSystemOptions;
 	}
 }

ICSharpCode.Decompiler/TypeSystem/ICompilation.cs

@@ -76,6 +76,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		StringComparer NameComparer { get; }
 
 		CacheManager CacheManager { get; }
+
+		TypeSystemOptions TypeSystemOptions { get; }
 	}
 
 	public interface ICompilationProvider

ICSharpCode.Decompiler/TypeSystem/Implementation/SimpleCompilation.cs

@@ -157,6 +157,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			get { return StringComparer.Ordinal; }
 		}
 
+		public virtual TypeSystemOptions TypeSystemOptions => TypeSystemOptions.Default;
+
 		public override string ToString()
 		{
 			return "[" + GetType().Name + " " + mainModule.AssemblyName + "]";

ILSpy/Properties/Resources.Designer.cs

@@ -19,7 +19,7 @@ namespace ICSharpCode.ILSpy.Properties {
     // class via a tool like ResGen or Visual Studio.
     // To add or remove a member, edit your .ResX file then rerun ResGen
     // with the /str option, or rebuild your VS project.
-    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "17.0.0.0")]
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "18.0.0.0")]
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
     [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
     public class Resources {
@@ -1053,6 +1053,15 @@ namespace ICSharpCode.ILSpy.Properties {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Treat Span&lt;T&gt; and ReadOnlySpan&lt;T&gt; as built-in types.
+        /// </summary>
+        public static string DecompilerSettings_FirstClassSpanTypes {
+            get {
+                return ResourceManager.GetString("DecompilerSettings.FirstClassSpanTypes", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Transform to for, if possible.
         /// </summary>

ILSpy/Properties/Resources.resx

@@ -375,6 +375,9 @@ Are you sure you want to continue?</value>
   <data name="DecompilerSettings.FileScopedNamespaces" xml:space="preserve">
     <value>Use file-scoped namespace declarations</value>
   </data>
+  <data name="DecompilerSettings.FirstClassSpanTypes" xml:space="preserve">
+    <value>Treat Span&lt;T&gt; and ReadOnlySpan&lt;T&gt; as built-in types</value>
+  </data>
   <data name="DecompilerSettings.ForStatement" xml:space="preserve">
     <value>Transform to for, if possible</value>
   </data>