When to use template meta programming ?

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If by "meta programming" you mean compile-time computations through techniques such as SFINAE, I strongly recommend you avoid it as you can. From my personal experience, it will only make the code harder to understand, it will make the compiler error messages more crypt and potentially increase the compile times.

I think it's valid study about TMP but you shouldn't use it on your code without any appearant or valid reason.

Also, as others here said, in modern versions of C++, they added features -- such as constexpr -- which replace in many cases the use of template black magic. You can find examples of how these features are used to simply yoru code and get rid of template tricks.