Update SourceThisParameterSymbol.cs

src/Draco.Compiler/Internal/Symbols/Source/SourceThisParameterSymbol.cs

@@ -1,19 +1,36 @@
+using System;
 using System.Collections.Immutable;
+using System.Linq;
 using Draco.Compiler.Api.Syntax;
 using Draco.Compiler.Internal.Binding;
 
 namespace Draco.Compiler.Internal.Symbols.Source;
 
-internal sealed class SourceThisParameterSymbol(FunctionSymbol containingSymbol, ThisParameterSyntax syntax) : ParameterSymbol, ISourceSymbol
+/// <summary>
+/// The 'this' parameter of a function defined in-source.
+/// </summary>
+internal sealed class SourceThisParameterSymbol : ParameterSymbol, ISourceSymbol
 {
     public override string Name => "this";
-    public override FunctionSymbol ContainingSymbol { get; } = containingSymbol;
+    public override FunctionSymbol ContainingSymbol { get; }
     public override bool IsVariadic => false;
     public override bool IsThis => true;
-    public override ThisParameterSyntax DeclaringSyntax { get; } = syntax;
+    public override ThisParameterSyntax DeclaringSyntax { get; }
 
     public override ImmutableArray<AttributeInstance> Attributes => [];
-    public override TypeSymbol Type { get; } = (containingSymbol.ContainingSymbol as TypeSymbol)!;
+    public override TypeSymbol Type { get; }
+
+    public SourceThisParameterSymbol(FunctionSymbol containingSymbol, ThisParameterSyntax syntax)
+    {
+        var containingType = containingSymbol.ContainingSymbol?.AncestorChain
+            .OfType<TypeSymbol>()
+            .FirstOrDefault()
+            ?? throw new ArgumentException("the containing symbol of a source this parameter must be a function within a type");
+
+        this.ContainingSymbol = containingSymbol;
+        this.DeclaringSyntax = syntax;
+        this.Type = containingType;
+    }
 
     public void Bind(IBinderProvider binderProvider) { }
 }