Add test case for dotnet#3112 with pseudo-circular reference when bas…

…e provides iface method (dotnet#3120)

Add test case for dotnet#3112 with pseudo-circular reference with ifaces

Commit migrated from dotnet@2172c79

src/tools/illink/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/BaseProvidesInterfaceEdgeCase/BaseProvidesInterfaceMethodCircularReference.cs

@@ -0,0 +1,38 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+using Mono.Linker.Tests.Cases.Inheritance.Interfaces.BaseProvidesInterfaceEdgeCase.Dependencies;
+
+namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.BaseProvidesInterfaceEdgeCase
+{
+	/// <summary>
+	/// Reproduces the issue found in https://github.com/dotnet/linker/issues/3112.
+	/// <see cref="Derived1"/> derives from <see cref="Base"/> and uses <see cref="Base"/>'s method to implement <see cref="IFoo"/>, 
+	/// creating a psuedo-circular assembly reference (but not quite since <see cref="Base"/> doesn't implement IFoo itself).
+	/// In the linker, IsMethodNeededByInstantiatedTypeDueToPreservedScope would iterate through <see cref="Base"/>'s method's base methods,
+	/// and in the process would trigger the assembly of <see cref="IFoo"/> to be processed. Since that assembly also has <see cref="Derived2"/> that 
+	/// inherits from <see cref="Base"/> and implements <see cref="IBar"/> using <see cref="Base"/>'s methods, the linker adds 
+	/// <see cref="IBar"/>'s method as a base to <see cref="Base"/>'s method, which modifies the collection as it's being iterated, causing an exception.
+	/// </summary>
+	[SetupCompileBefore ("base.dll", new[] { "Dependencies/Base.cs" })] // Base Implements IFoo.Method (psuedo-reference to ifoo.dll)
+	[SetupCompileBefore ("ifoo.dll", new[] { "Dependencies/IFoo.cs" }, references: new[] { "base.dll" })] // Derived2 references base.dll (circular reference)
+	[SetupCompileBefore ("derived1.dll", new[] { "Dependencies/Derived1.cs" }, references: new[] { "ifoo.dll", "base.dll" })]
+	public class BaseProvidesInterfaceMethodCircularReference
+	{
+		[Kept]
+		public static void Main ()
+		{
+			_ = new Derived1 ();
+			Foo ();
+		}
+
+		[Kept]
+		public static void Foo ()
+		{
+			((IFoo) null).Method ();
+			object x = null;
+		}
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/BaseProvidesInterfaceEdgeCase/Dependencies/Base.cs

@@ -0,0 +1,15 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.BaseProvidesInterfaceEdgeCase.Dependencies
+{
+	public class Base
+	{
+		public virtual void Method ()
+		{
+			throw new NotImplementedException ();
+		}
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/BaseProvidesInterfaceEdgeCase/Dependencies/Derived1.cs

@@ -0,0 +1,9 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.BaseProvidesInterfaceEdgeCase.Dependencies
+{
+	public class Derived1 : Base, IFoo
+	{
+	}
+}

src/tools/illink/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/BaseProvidesInterfaceEdgeCase/Dependencies/IFoo.cs

@@ -0,0 +1,17 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.BaseProvidesInterfaceEdgeCase.Dependencies
+{
+	public interface IFoo
+	{
+		void Method ();
+	}
+	public interface IBar
+	{
+		void Method ();
+	}
+	public class Derived2 : Base, IBar
+	{
+	}
+}