SFINAE still produces error while using exception-CodePudding

I am learning about SFINAE in C . So after reading about it, i am trying out different examples to better understand the concept. Below i have given 2 snippets out of which 1 i can understand but the second one where i have used noexcept in the declaration i can't understand.

Example 1: I am able to understand this.

#include <iostream>

template<typename T>
decltype(func(T())) timer(T a) 
{
    std::cout<<"template timer called"<<std::endl;
    return func(T());
}

void timer(...)
{
    std::cout<<"ordinary timer called"<<std::endl;   
}
int main()
{
    timer(5);
    return 0;
}

The output of the above program(as expected) is:

ordinary timer called

I can understand that due to SFINAE the deduction will result in failure and so the ordinary timer will be called.

Example 2: Why do we get error in this example.

#include <iostream>

template<typename T>
void timer(T a) noexcept(func(T()))
{
    std::cout<<"template timer called"<<std::endl;
}

void timer(...)
{
    std::cout<<"ordinary timer called"<<std::endl;   
}
int main()
{
    timer(5);
    return 0;
}

This second example results in an error saying that func was not declared. My question is why just like example 1 here also due to deduction failure the ordinary timer isn't selected?

I expected that here also ordinary timer should have been called but it isn't the case. Can someone explain the reason behind it.

CodePudding user response：

The problem is that exception specification do not participate in template argument deduction(TAD). This is explained in more detail below.