Ensure GetCorElementType returns value type for enums on Mono runti…

…me (#72685)

`Type.GetTypeCode` on Mono was calling `GetCorElementType`, which, when invoked on an enum nested in a generic class, would return `TypeCode.Object` instead of `TypeCode.Int32`. Matching to CoreClr, we change `GetCorElementType` to return `VALUETYPE` for enums to fix the issue. The original issue would also provoke Enum parsing to trigger an assert in a debug runtime and go through a slower, rare path for parsing as well.

Fixes #72543

src/libraries/System.Runtime/tests/System/Type/TypeTests.cs

@@ -1138,6 +1138,36 @@ internal abstract class C4 : I4
             }
         }
 #endregion
+
+        [Fact]
+        public void GetEnumTypeCode()
+        {
+            Assert.True(Type.GetTypeCode(typeof(TestEnum)) == TypeCode.Int32);
+        }
+
+        [Fact]
+        public void GetEnumNestedInGenericClassTypeCode()
+        {
+            Assert.True(Type.GetTypeCode(typeof(TestGenericClass<TestClass>.NestedEnum)) == TypeCode.Int32);
+        }
+
+        public enum TestEnum
+        {
+            A,
+            B,
+            C
+        }
+
+        public class TestClass { }
+        public class TestGenericClass<T>
+        {
+            public enum NestedEnum
+            {
+                A,
+                B,
+                C
+            }
+        }
     }
 
     public class NonGenericClass { }

src/mono/mono/metadata/icall.c

@@ -2765,6 +2765,10 @@ ves_icall_RuntimeTypeHandle_GetCorElementType (MonoQCallTypeHandle type_handle)
 {
 	MonoType *type = type_handle.type;
 
+	// Enums in generic classes should still return VALUETYPE
+	if (type->type == MONO_TYPE_GENERICINST && m_class_is_enumtype (type->data.generic_class->container_class) && !m_type_is_byref (type))
+		return MONO_TYPE_VALUETYPE;
+
 	if (m_type_is_byref (type))
 		return MONO_TYPE_BYREF;
 	else