Ensure the base type of generic instance types is instantiated

Cpp2IL.Core.Tests/TypeAnalysisContextTests.cs

@@ -1,3 +1,5 @@
+using Cpp2IL.Core.Model.Contexts;
+
 namespace Cpp2IL.Core.Tests;
 
 public class TypeAnalysisContextTests
@@ -49,4 +51,30 @@ public void StaticClassesHaveObjectBaseType()
             Assert.That(count, Is.GreaterThan(0));
         }
     }
+
+    [Test]
+    public void GenericInstanceTypeHasInstantiatedBaseType()
+    {
+        var appContext = TestGameLoader.LoadSimple2019Game();
+
+        // ObjectComparer<T> inherits from Comparer<T>
+        var objectComparerType = appContext.SystemTypes.SystemObjectType.DeclaringAssembly.GetTypeByFullName("System.Collections.Generic.ObjectComparer`1");
+
+        Assert.That(objectComparerType, Is.Not.Null);
+        Assert.That(objectComparerType.BaseType, Is.Not.Null);
+        using (Assert.EnterMultipleScope())
+        {
+            Assert.That(objectComparerType.GenericParameters, Has.Count.EqualTo(1));
+            Assert.That(objectComparerType.BaseType, Is.InstanceOf<GenericInstanceTypeAnalysisContext>());
+        }
+        Assert.That(((GenericInstanceTypeAnalysisContext)objectComparerType.BaseType).GenericArguments[0], Is.EqualTo(objectComparerType.GenericParameters[0]));
+
+        var objectComparerStringType = objectComparerType.MakeGenericInstanceType(appContext.SystemTypes.SystemStringType);
+        Assert.That(objectComparerStringType.BaseType, Is.Not.Null);
+        Assert.That(objectComparerStringType.BaseType, Is.InstanceOf<GenericInstanceTypeAnalysisContext>());
+
+        var baseType = (GenericInstanceTypeAnalysisContext)objectComparerStringType.BaseType;
+        Assert.That(baseType.GenericArguments, Has.Count.EqualTo(1));
+        Assert.That(baseType.GenericArguments[0], Is.EqualTo(appContext.SystemTypes.SystemStringType));
+    }
 }

Cpp2IL.Core/Model/Contexts/GenericInstanceTypeAnalysisContext.cs

@@ -50,7 +50,7 @@ public GenericInstanceTypeAnalysisContext(TypeAnalysisContext genericType, IEnum
     {
         GenericType = genericType;
         GenericArguments.AddRange(genericArguments);
-        DefaultBaseType = genericType.BaseType;
+        DefaultBaseType = genericType.BaseType is null ? null : GenericInstantiation.Instantiate(genericType.BaseType, [..genericArguments], []);
 
         SetDeclaringType();
     }