Fixed Extent etl::span ambiguous function overloads

With the fix for #753, Function overloading based on the Extent of a fixed extent etl::span no longer works. This was caused by the introduction of etl::span constructors were the parameters are a container with .data() and size() fields.

void f(etl::span<uint8_t, 8> b){
    std::cout << "f with etl::span<uint8_t, 8>" << std::endl;
}

void f(etl::span<uint8_t, 16> b){
    std::cout << "f with etl::span<uint8_t, 16>" << std::endl;
}

int main()
{
    etl::array<uint8_t, 8> arr8;
    etl::array<uint8_t, 16> arr16;
    f(arr8);
    f(arr16);
    return 0;
}

The above code doesn't compile for ETL. If I change to std::span, then compilation is fine.

main.cpp:21:5: error: call to 'f' is ambiguous
    f(arr8);
    ^
main.cpp:9:6: note: candidate function
void f(etl::span<uint8_t, 8> b){
     ^
main.cpp:13:6: note: candidate function
void f(etl::span<uint8_t, 16> b){
     ^
main.cpp:22:5: error: call to 'f' is ambiguous
    f(arr16);
    ^
main.cpp:9:6: note: candidate function
void f(etl::span<uint8_t, 8> b){
     ^
main.cpp:13:6: note: candidate function
void f(etl::span<uint8_t, 16> b){
     ^
2 errors generated.

A fix for this is as follows:

Add a new type_trait etl::is_std_array

template<typename T> struct is_std_array : false_type {};
template<typename T, size_t N> struct is_std_array<etl::array<T, N>> : true_type {};

Use the etl::is_std_array to disable the container based etl::span constructor. e.g.

template <typename TContainer, typename = typename etl::enable_if<!etl::is_pointer<etl::remove_reference_t<TContainer>>::value &&
                                                                  !etl::is_array<etl::remove_reference_t<TContainer>>::value&&
                                                                  !etl::is_std_array<etl::remove_reference_t<TContainer>>::value&&
                                                                  etl::is_same<etl::remove_cv_t<T>, etl::remove_cv_t<typename etl::remove_reference_t<TContainer>::value_type>>::value, void>::type>
  ETL_CONSTEXPR span(TContainer&& a) ETL_NOEXCEPT
  : pbegin(a.data())
{
}

Add back the "from std::array/etl::array" constructors which were removed in #753 .

I don't know if such a fix is acceptable, or would it cause issue for the issue in #753.

This only applies to fixed extent etl::span.

ETLCPP / etl

Fixed Extent etl::span ambiguous function overloads #826