C++ declaration of template parameter shadows template parameter error

Question

I have the following C++ code which I am compiling as below:

#include <cstddef>
#include <cassert>
template<typename T, std::size_t N> struct A;
template<typename T> struct Base {
    T &operator [](std::size_t i);
private:
    template<typename T, std::size_t N> friend struct A;
    #if !defined(NDEBUG)
        size_t n;
    #endif
};
template<typename T, std::size_t N> struct A : public Base<T> {
    A();
private:
    friend class Base<T>;
    T a[N];
};
template<typename T, std::size_t N> inline A<T,N>::A() { n = N; }
template<typename T> inline T& Base<T>::operator [](std::size_t i) {
    assert(i < n);
    return ((A<T,1>*)this)->a[i];
}
int main() {
    A<int, 3> a3;
    a3[1] = 1;
}

compiled using:

g++ -std=c++17 -O2 -Wall -pedantic main.cpp && ./a.out

and gives the following 3 compiler errors:

*main.cpp:7:14: error: declaration of template parameter 'T' shadows template parameter
    7 |     template<typename T, std::size_t N> friend struct A;

main.cpp:4:10: note: template parameter 'T' declared here
    4 | template<typename T> struct Base {

main.cpp: In constructor 'A<T, N>::A()':
main.cpp:18:58: error: 'n' was not declared in this scope
   18 | template<typename T, std::size_t N> inline A<T,N>::A() { n = N; }*

Gone through other similar errors posted here, but could not get the idea much to fix it.

---

After following all your answers there is only one error now by fixing the code with all your suggestions.

#include <cstddef>
using namespace std;
template <typename, std::size_t N> struct A;
template <typename T> struct Base { // template inheritance, compiler errors
    T& operator [](std::size_t i);
private:
    template<typename, std::size_t N> friend struct A;
    size_t n;
};
template <typename T, std::size_t N> struct A : public Base<T> {
    A();
private:
    friend class Base<T>;
    T a[N];
};
template <typename T, std::size_t N> inline A<T,N>::A() { n = N; }
template <typename T> inline T& Base<T>::operator[](std::size_t i) {
    return ((A<T,1>*)this)->a[i];
}
int main() { A<int, 3> a; a[1] = 1; }

Error is:

main.cpp: In constructor 'A< <template-parameter-1-1>, N>::A()':
main.cpp:17:59: error: 'n' was not declared in this scope
17 | template <typename T, std::size_t N> inline A<T,N>::A() { n = N; }

Answer 1

You are using the same name i.e., typename T in the outer and inner template:

Outer:

template<typename T> struct Base {
...

Inner:

    template<typename T, std::size_t N> friend struct A;
    #if !defined(NDEBUG)

Change either one to fix this.

Inner fixed:

    template<typename U, std::size_t N> friend struct A;
    #if !defined(NDEBUG)

Additionally, there's another error here:

template<typename T, std::size_t N> inline A<T,N>::A() { n = N; }

n is undeclared if NDEBUG is not defined, so you might need to wrap this in #ifdef as well.