Fix direction of variance check in delegate assignment (#4945)

* Fix direction of variance check in delegate assignment

* Add tests

src/ILVerify/src/ILImporter.Verify.cs

@@ -1003,12 +1003,12 @@ bool IsDelegateAssignable(MethodDesc ftn, TypeDesc delegateType)
 
             for (int i = 0; i < ftnSignature.Length; i++)
             {
-                if (!IsAssignable(ftnSignature[i], delegateSignature[i]))
+                if (!IsAssignable(delegateSignature[i], ftnSignature[i]))
                     return false;
             }
 
             // Compare return type
-            return IsAssignable(delegateSignature.ReturnType, ftnSignature.ReturnType);
+            return IsAssignable(ftnSignature.ReturnType, delegateSignature.ReturnType);
         }
 
         ILOpcode GetOpcodeAt(int instructionOffset)

src/ILVerify/src/ILVerify.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <OutputType>Exe</OutputType>
     <TargetFrameworks>netcoreapp2.0;net46</TargetFrameworks>
+    <PlatformTarget>AnyCPU</PlatformTarget>
     <CLSCompliant>false</CLSCompliant>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <EnableDefaultCompileItems>false</EnableDefaultCompileItems>

src/ILVerify/tests/ILMethodTester.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Reflection.Metadata.Ecma335;
 using Internal.IL;
 using Internal.TypeSystem.Ecma;
@@ -60,7 +61,8 @@ void TestMethodsWithInvalidIL(InvalidILTestCase invalidIL)
 
                 foreach (var item in invalidIL.ExpectedVerifierErrors)
                 {
-                    Assert.True(verifierErrors.Contains(item));
+                    var actual = verifierErrors.Select(e => e.ToString());
+                    Assert.True(verifierErrors.Contains(item), $"Actual errors where: {string.Join(',', actual)}");
                 }
             }
         }

src/ILVerify/tests/ILTests/DelegateTests.il

@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+.assembly extern System.Runtime
+{
+}
+
+.assembly DelegateTests
+{
+}
+
+.class private auto ansi beforefieldinit C
+       extends [System.Runtime]System.Object
+{
+    // assignment from Func<int, string> to Func<int, object> is valid
+    .method private hidebysig instance class [System.Runtime]System.Func`2<int32,object>
+          DelegateAssignmentReturn_Valid(class [System.Runtime]System.Func`2<int32,string> input) cil managed
+    {
+        ldarg.1
+        ret
+    }
+
+    // assignment from Func<object, int> to Func<string, int> is valid
+    .method private hidebysig instance class [System.Runtime]System.Func`2<string,int32>
+          DelegateAssignmentParameter_Valid(class [System.Runtime]System.Func`2<object,int32> input) cil managed
+    {
+        ldarg.1
+        ret
+    }
+
+    // assignment from Func<string, int> to Func<object, int> is invalid
+    .method private hidebysig instance class [System.Runtime]System.Func`2<object,int32>
+          DelegateAssignmentParameter_Invalid_StackUnexpected(class [System.Runtime]System.Func`2<string,int32> input) cil managed
+    {
+        ldarg.1
+        ret
+    }
+
+    // assignment from Func<int, object> to Func<int, string> is invalid
+    .method private hidebysig instance class [System.Runtime]System.Func`2<int32,string>
+          DelegateAssignment_Invalid_StackUnexpected(class [System.Runtime]System.Func`2<int32,object> input) cil managed
+    {
+        ldarg.1
+        ret
+    }
+
+    .method public hidebysig specialname rtspecialname
+          instance void  .ctor() cil managed
+    {
+        ldarg.0
+        call       instance void [System.Runtime]System.Object::.ctor()
+        ret
+    }
+}