Fix enum value's __int__ returning non-int when underlying type is bool or of char type (#1334)

* Use equivalent_integer for enum's Scalar decision

* Add test for char underlying enum

* Support translating bool type in enum's Scalar

* Add test for bool underlying enum

* Fix comment in test

* Switch from `PYBIND11_CPP20` macro to `PYBIND11_HAS_U8STRING`

* Refine tests
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

include/pybind11/pybind11.h

@@ -1814,6 +1814,19 @@ struct enum_base {
     handle m_parent;
 };
 
+template <bool is_signed, size_t length> struct equivalent_integer {};
+template <> struct equivalent_integer<true,  1> { using type = int8_t;   };
+template <> struct equivalent_integer<false, 1> { using type = uint8_t;  };
+template <> struct equivalent_integer<true,  2> { using type = int16_t;  };
+template <> struct equivalent_integer<false, 2> { using type = uint16_t; };
+template <> struct equivalent_integer<true,  4> { using type = int32_t;  };
+template <> struct equivalent_integer<false, 4> { using type = uint32_t; };
+template <> struct equivalent_integer<true,  8> { using type = int64_t;  };
+template <> struct equivalent_integer<false, 8> { using type = uint64_t; };
+
+template <typename IntLike>
+using equivalent_integer_t = typename equivalent_integer<std::is_signed<IntLike>::value, sizeof(IntLike)>::type;
+
 PYBIND11_NAMESPACE_END(detail)
 
 /// Binds C++ enumerations and enumeration classes to Python
@@ -1824,13 +1837,17 @@ public:
     using Base::attr;
     using Base::def_property_readonly;
     using Base::def_property_readonly_static;
-    using Scalar = typename std::underlying_type<Type>::type;
+    using Underlying = typename std::underlying_type<Type>::type;
+    // Scalar is the integer representation of underlying type
+    using Scalar = detail::conditional_t<detail::any_of<
+        detail::is_std_char_type<Underlying>, std::is_same<Underlying, bool>
+    >::value, detail::equivalent_integer_t<Underlying>, Underlying>;
 
     template <typename... Extra>
     enum_(const handle &scope, const char *name, const Extra&... extra)
       : class_<Type>(scope, name, extra...), m_base(*this, scope) {
         constexpr bool is_arithmetic = detail::any_of<std::is_same<arithmetic, Extra>...>::value;
-        constexpr bool is_convertible = std::is_convertible<Type, Scalar>::value;
+        constexpr bool is_convertible = std::is_convertible<Type, Underlying>::value;
         m_base.init(is_arithmetic, is_convertible);
 
         def(init([](Scalar i) { return static_cast<Type>(i); }), arg("value"));

tests/test_enum.cpp

@@ -84,4 +84,65 @@ TEST_SUBMODULE(enums, m) {
             .value("ONE", SimpleEnum::THREE)
             .export_values();
     });
+
+    // test_enum_scalar
+    enum UnscopedUCharEnum : unsigned char {};
+    enum class ScopedShortEnum : short {};
+    enum class ScopedLongEnum : long {};
+    enum UnscopedUInt64Enum : std::uint64_t {};
+    static_assert(py::detail::all_of<
+        std::is_same<py::enum_<UnscopedUCharEnum>::Scalar, unsigned char>,
+        std::is_same<py::enum_<ScopedShortEnum>::Scalar, short>,
+        std::is_same<py::enum_<ScopedLongEnum>::Scalar, long>,
+        std::is_same<py::enum_<UnscopedUInt64Enum>::Scalar, std::uint64_t>
+    >::value, "Error during the deduction of enum's scalar type with normal integer underlying");
+
+    // test_enum_scalar_with_char_underlying
+    enum class ScopedCharEnum   : char     { Zero, Positive };
+    enum class ScopedWCharEnum  : wchar_t  { Zero, Positive };
+    enum class ScopedChar32Enum : char32_t { Zero, Positive };
+    enum class ScopedChar16Enum : char16_t { Zero, Positive };
+
+    // test the scalar of char type enums according to chapter 'Character types'
+    // from https://en.cppreference.com/w/cpp/language/types
+    static_assert(py::detail::any_of<
+        std::is_same<py::enum_<ScopedCharEnum>::Scalar, signed char>, // e.g. gcc on x86
+        std::is_same<py::enum_<ScopedCharEnum>::Scalar, unsigned char>  // e.g. arm linux
+    >::value, "char should be cast to either signed char or unsigned char");
+    static_assert(
+        sizeof(py::enum_<ScopedWCharEnum>::Scalar) == 2 ||
+        sizeof(py::enum_<ScopedWCharEnum>::Scalar) == 4
+    , "wchar_t should be either 16 bits (Windows) or 32 (everywhere else)");
+    static_assert(py::detail::all_of<
+        std::is_same<py::enum_<ScopedChar32Enum>::Scalar, std::uint_least32_t>,
+        std::is_same<py::enum_<ScopedChar16Enum>::Scalar, std::uint_least16_t>
+    >::value, "char32_t, char16_t (and char8_t)'s size, signedness, and alignment is determined");
+#if defined(PYBIND11_HAS_U8STRING)
+    enum class ScopedChar8Enum : char8_t { Zero, Positive };
+    static_assert(std::is_same<py::enum_<ScopedChar8Enum>::Scalar, unsigned char>::value);
+#endif
+
+    // test_char_underlying_enum
+    py::enum_<ScopedCharEnum>(m, "ScopedCharEnum")
+        .value("Zero", ScopedCharEnum::Zero)
+        .value("Positive", ScopedCharEnum::Positive);
+    py::enum_<ScopedWCharEnum>(m, "ScopedWCharEnum")
+        .value("Zero", ScopedWCharEnum::Zero)
+        .value("Positive", ScopedWCharEnum::Positive);
+    py::enum_<ScopedChar32Enum>(m, "ScopedChar32Enum")
+        .value("Zero", ScopedChar32Enum::Zero)
+        .value("Positive", ScopedChar32Enum::Positive);
+    py::enum_<ScopedChar16Enum>(m, "ScopedChar16Enum")
+        .value("Zero", ScopedChar16Enum::Zero)
+        .value("Positive", ScopedChar16Enum::Positive);
+
+    // test_bool_underlying_enum
+    enum class ScopedBoolEnum : bool { FALSE, TRUE };
+
+    // bool is unsigned (std::is_signed returns false) and 1-byte long, so represented with u8
+    static_assert(std::is_same<py::enum_<ScopedBoolEnum>::Scalar, std::uint8_t>::value, "");
+
+    py::enum_<ScopedBoolEnum>(m, "ScopedBoolEnum")
+        .value("FALSE", ScopedBoolEnum::FALSE)
+        .value("TRUE", ScopedBoolEnum::TRUE);
 }

tests/test_enum.py

@@ -218,10 +218,16 @@ def test_binary_operators():
 def test_enum_to_int():
     m.test_enum_to_int(m.Flags.Read)
     m.test_enum_to_int(m.ClassWithUnscopedEnum.EMode.EFirstMode)
+    m.test_enum_to_int(m.ScopedCharEnum.Positive)
+    m.test_enum_to_int(m.ScopedBoolEnum.TRUE)
     m.test_enum_to_uint(m.Flags.Read)
     m.test_enum_to_uint(m.ClassWithUnscopedEnum.EMode.EFirstMode)
+    m.test_enum_to_uint(m.ScopedCharEnum.Positive)
+    m.test_enum_to_uint(m.ScopedBoolEnum.TRUE)
     m.test_enum_to_long_long(m.Flags.Read)
     m.test_enum_to_long_long(m.ClassWithUnscopedEnum.EMode.EFirstMode)
+    m.test_enum_to_long_long(m.ScopedCharEnum.Positive)
+    m.test_enum_to_long_long(m.ScopedBoolEnum.TRUE)
 
 
 def test_duplicate_enum_name():
@@ -230,6 +236,28 @@ def test_duplicate_enum_name():
     assert str(excinfo.value) == 'SimpleEnum: element "ONE" already exists!'
 
 
+def test_char_underlying_enum():  # Issue #1331/PR #1334:
+    assert type(m.ScopedCharEnum.Positive.__int__()) is int
+    assert int(m.ScopedChar16Enum.Zero) == 0  # int() call should successfully return
+    assert hash(m.ScopedChar32Enum.Positive) == 1
+    assert m.ScopedCharEnum.Positive.__getstate__() == 1  # return type is long in py2.x
+    assert m.ScopedWCharEnum(1) == m.ScopedWCharEnum.Positive
+    with pytest.raises(TypeError):
+        # Enum should construct with a int, even with char underlying type
+        m.ScopedWCharEnum("0")
+
+
+def test_bool_underlying_enum():
+    assert type(m.ScopedBoolEnum.TRUE.__int__()) is int
+    assert int(m.ScopedBoolEnum.FALSE) == 0
+    assert hash(m.ScopedBoolEnum.TRUE) == 1
+    assert m.ScopedBoolEnum.TRUE.__getstate__() == 1
+    assert m.ScopedBoolEnum(1) == m.ScopedBoolEnum.TRUE
+    # Enum could construct with a bool
+    # (bool is a strict subclass of int, and False will be converted to 0)
+    assert m.ScopedBoolEnum(False) == m.ScopedBoolEnum.FALSE
+
+
 def test_docstring_signatures():
     for enum_type in [m.ScopedEnum, m.UnscopedEnum]:
         for attr in enum_type.__dict__.values():