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#include <array>
using namespace std;
 
template<class T, size_t N1, size_t N2> array<T, N1+N2> concat(
    const array<T, N1> &a, const array<T, N2> &b)
{
    array<T, N1+N2> ret;
    /* copy data into ret */
    return ret;
}
 
int main()
{
	typedef int T;
	array<T, 10> a;
	array<T, 5> b;
	array<T, 14> ab1 = concat(a, b); // error
	array<T, 15> ab2 = concat(a, b); // works
	auto ab3 = concat(a, b); // type of ab3 is auto-deduced to array<T, 15>
}

prog.cpp: In function ‘int main()’:
prog.cpp:17:27: error: conversion from ‘array<[...],15>’ to non-scalar type ‘array<[...],14>’ requested
  array<T, 14> ab1 = concat(a, b); // error
                     ~~~~~~^~~~~~

Standard output is empty