Only support ==/!= int on unscoped enums

This makes the Python interface mirror the C++ interface:
pybind11-exported scoped enums aren't directly comparable to the
underlying integer values.

example/example-constants-and-functions.py

@@ -22,7 +22,26 @@ print(test_function(7))
 print(test_function(EMyEnumeration.EFirstEntry))
 print(test_function(EMyEnumeration.ESecondEntry))
 test_ecenum(ECMyEnum.Three)
-test_ecenum(ECMyEnum.Two)
+z = ECMyEnum.Two
+test_ecenum(z)
+try:
+    z == 2
+    print("Bad: expected a TypeError exception")
+except TypeError:
+    try:
+        z != 3
+        print("Bad: expected a TypeError exception")
+    except TypeError:
+        print("Good: caught expected TypeError exceptions for scoped enum ==/!= int comparisons")
+
+y = EMyEnumeration.ESecondEntry
+try:
+    y == 2
+    y != 2
+    print("Good: no TypeError exception for unscoped enum ==/!= int comparisions")
+except TypeError:
+    print("Bad: caught TypeError exception for unscoped enum ==/!= int comparisons")
+
 print("enum->integer = %i" % int(EMyEnumeration.ESecondEntry))
 print("integer->enum = %s" % str(EMyEnumeration(2)))
 

example/example-constants-and-functions.ref

@@ -12,6 +12,8 @@ test_function(enum=2)
 None
 test_ecenum(ECMyEnum::Three)
 test_ecenum(ECMyEnum::Two)
+Good: caught expected TypeError exceptions for scoped enum ==/!= int comparisons
+Good: no TypeError exception for unscoped enum ==/!= int comparisions
 enum->integer = 2
 integer->enum = EMyEnumeration.ESecondEntry
 A constant = 14

include/pybind11/pybind11.h

@@ -1019,9 +1019,14 @@ public:
         this->def("__init__", [](Type& value, UnderlyingType i) { new (&value) Type((Type) i); });
         this->def("__int__", [](Type value) { return (UnderlyingType) value; });
         this->def("__eq__", [](const Type &value, Type *value2) { return value2 && value == *value2; });
-        this->def("__eq__", [](const Type &value, UnderlyingType value2) { return (UnderlyingType) value == value2; });
         this->def("__ne__", [](const Type &value, Type *value2) { return !value2 || value != *value2; });
+        if (std::is_convertible<Type, UnderlyingType>::value) {
+            // Don't provide comparison with the underlying type if the enum isn't convertible,
+            // i.e. if Type is a scoped enum, mirroring the C++ behaviour.  (NB: we explicitly
+            // convert Type to UnderlyingType below anyway because this needs to compile).
+            this->def("__eq__", [](const Type &value, UnderlyingType value2) { return (UnderlyingType) value == value2; });
             this->def("__ne__", [](const Type &value, UnderlyingType value2) { return (UnderlyingType) value != value2; });
+        }
         this->def("__hash__", [](const Type &value) { return (UnderlyingType) value; });
         m_entries = entries;
     }