Why Does Using `std::enable_if_t` With Default Values Result in a Redefinition Error?

Redefinition Issue with enable_if_t Template Arguments

In the provided code, the attempt to convert the std::enable_if type constraint using the new syntax (typename = std::enable_if_t...) results in a redefinition error. This is because the two template functions:

template<typename T, typename = std::enable_if_t<std::is_same<int, T>::value>>>
void g() { }

and

template<typename T, typename = std::enable_if_t<std::is_same<double, T>::value>>>
void g() { }

are both of type templatevoid(). The fact that the second type argument has different default values is irrelevant, as the compiler considers the two templates to be identical.

To address this issue, it is necessary to remove the default values from the enable_if_t constraint. This results in the following code:

template<typename T, std::enable_if_t<std::is_same<int, T>::value, int>*>
void g() { }

template<typename T, std::enable_if_t<std::is_same<double, T>::value, int>*>
void g() { }

In this case, the second type argument is now a pointer, whose type is dependent on the first. The compiler can substitute in the type T to determine if there is a conflict between the two templates, and it will resolve that there is none.

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