Is std::is_unsigned<bool>::value well defined?

Solution 1:

There is no concept of signedness for bool. From [basic.fundamental]/6:

Values of type bool are either true of false. [Note: There are no signed, unsigned, short, or long bool types or values. — end note] Values of type bool participate in integral promotions (4.5).

By contrast, signedness is explicitly called out for the signed integer types (paragraph 2) and unsigned integer types (paragraph 3).

Now for the is_signed and is_unsigned traits. First off, the traits are always well-defined, but only interesting for arithmetic types. bool is an arithmetic type, and is_signed<T>::value is defined (see Table 49) as T(-1) < T(0). By using the rules of boolean conversion and standard arithmetic conversions, we see that this is is false for T = bool (because bool(-1) is true, which converts to 1). Similarly, is_unsigned<T>::value is defined as T(0) < T(-1), which is true for T = bool.

Solution 2:

is_unsigned is defined in [meta.unary.comp]/2 as

If is_arithmetic<T>::value is true, the same result as
bool_constant<T(0) < T(-1)>::value; otherwise, false

bool^† is clearly an arithmetic type (being integral). Now consider [conv.bool]/1:

A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

I.e. bool(0) < bool(-1) is equivalent to false < true, and the latter holds since the values are promoted to 0 and 1, respectively.

Thus is_unsigned<bool>::value is true (and, conversely, is_signed is false), due to the fact that boolean values correspond to the unsigned values 0 and 1 during arithmetic operations. However, it doesn't really make sense to assess bool's signedness, much less perform make_unsigned on it, since it doesn't represent integers, but rather states.

Solution 3:

Yes, it is well-defined, as is any other unary type trait.

C++14 (n4140) 20.10.4/2 "Unary type traits" mandates:

Each of these templates shall be a UnaryTypeTrait (20.10.1) with a BaseCharacteristic of true_type if the corresponding condition is true, otherwise false_type.

20.10.1/1:

A UnaryTypeTrait describes a property of a type. It shall be a class template that takes one template type argument and, optionally, additional arguments that help define the property being described. It shall be DefaultConstructible, CopyConstructible, and publicly and unambiguously derived, directly or indirectly, from its BaseCharacteristic, which is a specialization of the template integral_constant (20.10.3), with the arguments to the template integral_constant determined by the requirements for the particular property being described. The member names of the BaseCharacteristic shall not be hidden and shall be unambiguously available in the UnaryTypeTrait.

From this it follows that the construct std::is_unsigned<T>::value has to be well-defined for any type T, whether the concept of "signedness" makes sense for the type or not.

Solution 4:

Yes it is well defined and the result should be std::is_unsigned<bool>::value == true

The documentation for std::is_signed says

If T is a signed arithmetic type, provides the member constant value equal true. For any other type, value is false.

So then if you look at std::is_arithmetic

If T is an arithmetic type (that is, an integral type or a floating-point type), provides the member constant value equal true. For any other type, value is false.

Which finally leads to std::is_integral

Checks whether T is an integral type. Provides the member constant value which is equal to true, if T is the type bool, char, char16_t, char32_t, wchar_t, short, int, long, long long, or any implementation-defined extended integer types, including any signed, unsigned, and cv-qualified variants. Otherwise, value is equal to false.

Interestingly, there is another function std::numeric_limits::is_signed that states

The value of std::numeric_limits<T>::is_signed is true for all signed arithmetic types T and false for the unsigned types. This constant is meaningful for all specializations.

Where the specialization for bool is listed as false, which also confirms that bool is considered unsigned.