Concatenate Two C++ Index Sequences at Compile Time

Problem Statement

In modern C++, template metaprogramming often requires manipulating sequences of integers at compile time. Your task is to implement a utility concat_sequences that takes two std::index_sequence types and returns a new std::index_sequence containing all elements from the first sequence followed by all elements from the second.

You must provide:

1. A template alias concat_sequences_t that evaluates to the resulting sequence.
2. The underlying struct template concat_sequences.

Constraints

• You may use C++14 features (std::index_sequence, std::make_index_sequence, std::integer_sequence).
• Do not use recursive template instantiation that exceeds typical compiler depth limits for sequences up to size 100.
• Your solution should handle empty sequences correctly (e.g., concat(empty, {1, 2}) -> {1, 2}).

Example

// Input
using Seq1 = std::index_sequence;
using Seq2 = std::index_sequence;

// Result
using Result = concat_sequences_t;
// Result is std::index_sequence

Implementation Template

#include 

template 
struct concat_sequences;

template 
using concat_sequences_t = typename concat_sequences::type;