std::enable_if works because the compiler applies the SFINAE rule when performing overload resolution. Before we can explain how std::enable_if works, we should have a quick look at what SFINAE is.
When the compiler encounters a function call, it needs to build a set of possible overloads and select the best match for the call based on the arguments for the function call. When building this overload set, the compiler evaluates function templates too and has to perform a substitution for the specified or deduced types into the template arguments. According to SFINAE, when the substitution fails, instead of yielding an error, the compiler should just remove the function template from the overload set and continue.
The standard ...
