enable_if compilation question void = nullptr

** nullptr and 0 are not the same. **

For a very clear explanation please see the following:

https://hackernoon.com/what-exactly-is-nullptr-in-c-94d63y6t

@brian has provided a very good technical answer, but I felt it necessary to add this answer since we should no longer be trying to use 0 for pointer values.

Technically speaking, this is because a non-type template argument must be a "converted constant expression" of the parameter type. This means that the argument itself must be a constant expression, and its conversion to the required parameter type must use only the conversions specified in [expr.const]/4.

According to [expr.const]/4, null pointer conversions are only allowed from std::nullptr_t. In other words, the conversion from 0 to a null pointer value is not allowed as part of the implicit conversion sequence in a converted constant expression.

Yet it's perfectly legitimate to specify static_cast<T*>(0) as a template argument to a non-type template parameter of type T*. In other words, a null pointer conversion from 0 is allowed as part of a constant expression. It's only when the conversion is done at a certain point---after computing the argument and while converting the argument type to the parameter type---that the standard forbids it.

I have no idea about the rationale for this rule.