C++: operator<< Overloads not working - c++

Question

Because I am working on a mikrocontroller, where I don't have std::cout available, I want to define several operator<<() functions for my print class.
Because I have several print-backend-classes (one for each output channel e.g. uart, wireless, aso.), I wanted to define my operator<<() functions as templates.
But if I want to call this overloads, they don't work. I get compiler errors like
no match for ‘operator<<’ (operand types are ‘print_t’ and ‘gsl::basic_string_span<const char, -1>’)
I have the following Example:
#include <gsl/string_span>
template <typename T=char, std::size_t N>
constexpr gsl::cstring_span<> gen_cstring_span(const char (&a)[N]) noexcept {
return gsl::cstring_span<>(a, N);
}
class print_t {
public:
inline void append(const gsl::cstring_span<> buf) noexcept {
for(auto c : buf) {
printf("%c", c);
}
}
};
print_t p;
class prefix_t {
public:
static constexpr char prefix[] = "Prefix: ";
template<typename T>
print_t& operator<<(T&& arg) noexcept {
fprintf(stderr, "fwd, ");
p.append(prefix);
p << std::forward<T>(arg);
return p;
}
};
template<typename T>
using span_append_function_exists = std::is_member_function_pointer<decltype(static_cast<void(T::*)(const gsl::cstring_span<>)noexcept>(&T::append))>;
template<typename stream_t, std::enable_if_t<
span_append_function_exists<stream_t>::value, bool>>
stream_t& operator<<(stream_t& sb, const gsl::cstring_span<> str) noexcept {
sb.append(str);
fprintf(stderr, "cstring_span, ");
return sb;
}
template<typename stream_t, typename T=char, std::size_t N>
stream_t& operator<<(stream_t& sb, const char (&str)[N]) noexcept {
fprintf(stderr, "string_literal, ");
auto s = gen_cstring_span(str);
return sb << s;
}
template<typename stream_t>
stream_t& operator<<(stream_t& sb, const bool flag) noexcept {
fprintf(stderr, "bool, ");
if(flag) {
return sb << gen_cstring_span("true");
} else {
return sb << gen_cstring_span("false");
}
}
prefix_t pre;
int main() {
gsl::cstring_span<> span{gen_cstring_span("span")};
p << span << "\n";fprintf(stderr, "\n");
p << "string-literal" << "\n";fprintf(stderr, "\n");
p << true << "\n";fprintf(stderr, "\n");
pre << span << "\n";fprintf(stderr, "\n");
pre << "string-literal" << "\n";fprintf(stderr, "\n");
pre << true << "\n";fprintf(stderr, "\n");
}
This example should be C++17 (and was designed to work on a classic pc, but it should be ported to the MC.) and in this example, the print-backend-class would be print_t.
And each of the operator<<() functions has a template-parameter stream_t, which should become in this example the backend-class print_t.
I have worked with std::enable_if<>, to enable the sream_t param only on classes where the append() function is defined, but I did not succeed well.
Additionally, If I apply the operator on class prefix_t, it should print everything to the backend and additionally add a prefix-value before the output.
The expected STDOUT:
span
string-literal
true
Prefix: span
Prefix: string-literal
Prefix: true
The expected STDERR:
cstring_span,
string_literal, cstring_span,
bool, cstring_span,
fwd, cstring_span,
fwd, string_literal, cstring_span,
fwd, bool, cstring_span,
And a short part of the compiler errors:
tmp3.cpp: In function ‘int main()’:
tmp3.cpp:65:7: error: no match for ‘operator<<’ (operand types are ‘print_t’ and ‘gsl::cstring_span<> {aka gsl::basic_string_span<const char, -1>}’)
p << span << "\n";fprintf(stderr, "\n");
~~^~~~~~~
tmp3.cpp:36:11: note: candidate: template<class stream_t, typename std::enable_if<std::is_member_function_pointer<decltype (static_cast<void (T::*)(gsl::basic_string_span<const char, -1>) noexcept>(& T:: append))>::value, bool>::type <anonymous> > stream_t& operator<<(stream_t&, gsl::cstring_span<>)
stream_t& operator<<(stream_t& sb, const gsl::cstring_span<> str) noexcept {
^~~~~~~~
tmp3.cpp:36:11: note: template argument deduction/substitution failed:
tmp3.cpp:65:10: note: couldn't deduce template parameter ‘<anonymous>’
p << span << "\n";fprintf(stderr, "\n");
^~~~
tmp3.cpp:43:11: note: candidate: template<class stream_t, class T, long unsigned int N> stream_t& operator<<(stream_t&, const char (&)[N])
stream_t& operator<<(stream_t& sb, const char (&str)[N]) noexcept {
^~~~~~~~
tmp3.cpp:43:11: note: template argument deduction/substitution failed:
tmp3.cpp:65:10: note: mismatched types ‘const char [N]’ and ‘gsl::cstring_span<> {aka gsl::basic_string_span<const char, -1>}’
p << span << "\n";fprintf(stderr, "\n");
^~~~
tmp3.cpp:50:11: note: candidate: template<class stream_t> stream_t& operator<<(stream_t&, bool)
stream_t& operator<<(stream_t& sb, const bool flag) noexcept {
^~~~~~~~
tmp3.cpp:50:11: note: template argument deduction/substitution failed:
tmp3.cpp:65:10: note: cannot convert ‘span’ (type ‘gsl::cstring_span<> {aka gsl::basic_string_span<const char, -1>}’) to type ‘bool’
p << span << "\n";fprintf(stderr, "\n");
I am using C++17 with g++7.3.