[Mono] Fix function pointer check (#80855)

* Fix function pointer check

* Make is_monomorphic_array return false when the element is function pointer

* Move test to a better location

src/mono/mono/metadata/class.c

@@ -4242,11 +4242,11 @@ mono_class_is_assignable_from_general (MonoClass *klass, MonoClass *oklass, gboo
 		}
 
 		if (m_class_get_byval_arg (klass)->type == MONO_TYPE_FNPTR) {
-			/*
-			 * if both klass and oklass are fnptr, and they're equal, we would have returned at the
-			 * beginning.
-			 */
-			/* Is this right? or do we need to look at signature compatibility? */
+			if (mono_metadata_signature_equal (klass_byval_arg->data.method, oklass_byval_arg->data.method)) {
+				*result = TRUE;
+				return;
+			}
+
 			*result = FALSE;
 			return;
 		}

src/mono/mono/metadata/marshal.c

@@ -4693,6 +4693,8 @@ is_monomorphic_array (MonoClass *klass)
 		return FALSE;
 
 	element_class = m_class_get_element_class (klass);
+	if (m_class_get_byval_arg (element_class)->type == MONO_TYPE_FNPTR)
+		return FALSE;
 	return mono_class_is_sealed (element_class) || m_class_is_valuetype (element_class);
 }
 

src/tests/Loader/classloader/Casting/Functionpointer.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace TestFunctionPointer
+{
+    unsafe class TestThings
+    {
+        public static delegate* managed<int>[][] Functions = {
+            new delegate* managed<int>[]
+            {
+                &Function,
+            },
+        };
+
+        public static int Function() => 100;
+
+        public static delegate* unmanaged<int>[][] Functions1 = {
+            new delegate* unmanaged<int>[]
+            {
+                &Function1,
+            },
+        };
+
+        [UnmanagedCallersOnly]
+        public static int Function1() => 100;
+
+        public static delegate* managed<int, int>[][] Functions2 = {
+            new delegate* managed<int, int>[]
+            {
+                &Function2,
+            },
+        };
+
+        public static int Function2(int a) {
+            return a;
+        }
+    }
+
+    unsafe class Program
+    {
+        public static int Main()
+        {
+            return TestThings.Functions2[0][0](TestThings.Functions[0][0]());
+        }
+    }
+}

src/tests/Loader/classloader/Casting/Functionpointer.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="Functionpointer.cs" />
+  </ItemGroup>
+</Project>