Better use of std::enable_if

Replace the occurances of `class = std::enable_if_t<Cond>` and `typename = std::enable_if_t<Cond>` that have been identified in the previous commit with `std::enable_if_t<Cond, bool> = true`.

This commit is inspired by #1174, which changed one occurance in the owner header. This commit is aimed to fix all remaining occurances.

Author: Werner Henze

docs/headers.md

@@ -91,16 +91,16 @@ See [SL.str.5: Use `std::byte` to refer to byte values that do not necessarily r
 ### Non-member functions
 
 ```cpp
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept;
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator<<(byte b, IntegerType shift) noexcept;
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept;
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator>>(byte b, IntegerType shift) noexcept;
 ```
 
@@ -134,7 +134,7 @@ constexpr byte operator~(byte b) noexcept;
 Bitwise negation of a `byte`. Flips all bits. Zeroes become ones, ones become zeroes.
 
 ```cpp
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr IntegerType to_integer(byte b) noexcept;
 ```
 
@@ -310,7 +310,7 @@ auto make_not_null(T&& t) noexcept;
 Creates a `gsl::not_null` object, deducing the target type from the type of the argument.
 
 ```cpp
-template <typename T, typename = std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value>>
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value, bool> = true>
 void swap(not_null<T>& a, not_null<T>& b);
 ```
 

include/gsl/byte

@@ -91,25 +91,25 @@ enum class byte_may_alias byte : unsigned char
 {
 };
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept
 {
     return b = byte(static_cast<unsigned char>(b) << shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator<<(byte b, IntegerType shift) noexcept
 {
     return byte(static_cast<unsigned char>(b) << shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept
 {
     return b = byte(static_cast<unsigned char>(b) >> shift);
 }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr byte operator>>(byte b, IntegerType shift) noexcept
 {
     return byte(static_cast<unsigned char>(b) >> shift);
@@ -147,7 +147,7 @@ constexpr byte operator^(byte l, byte r) noexcept
 
 constexpr byte operator~(byte b) noexcept { return byte(~static_cast<unsigned char>(b)); }
 
-template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
+template <class IntegerType, std::enable_if_t<std::is_integral<IntegerType>::value, bool> = true>
 constexpr IntegerType to_integer(byte b) noexcept
 {
     return static_cast<IntegerType>(b);

include/gsl/pointers

@@ -145,7 +145,7 @@ private:
     T ptr_;
 };
 
-template <typename T, typename = std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value>>
+template <typename T, std::enable_if_t<std::is_move_assignable<T>::value && std::is_move_constructible<T>::value, bool> = true>
 void swap(not_null<T>& a, not_null<T>& b)
 {
     a.swap(b);