Why doesn't universal reference apply for arrays?

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#include <type_traits>  template<typename T> void f(const T&) {     static_assert(std::is_array_v<T>); // ok }  template<typename T> void g(T&&) {     static_assert(std::is_array_v<T>); // error }  int main() {     char arr[8];     f(arr); // ok     g(arr); // error } 

My compiler is clang 7.0 with -std=c++17.

Why doesn't universal reference apply for arrays?


First of all, these are officially called "forwarding references", not "universal references".

Your static_assert fails due to the fact that T is deduced as T& when passing an lvalue to a function taking a "forwarding reference" - this is one of the special rules of "forwarding references" that apply during template argument deduction.

You can fix your assert by stripping any reference out first:


live example on godbolt.org

std::remove_cvref_t is a bleeding edge C++20 feature - you might want to use std::remove_reference_t instead if your compiler doesn't support it.


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