ShowDecl: Pretty-print an expression's decltype (C++)

ShowDecl.cpp

// ShowDecl: Pretty-print an expression's decltype.
// Works with popular C++20 compilers.
// by Len Popp
// ref: Arthur O'Dwyer https://quuxplusone.github.io/blog/2018/08/22/puts-pretty-function/

#include <string>
#include <string_view>
#include <utility>
#include <iostream>

#define SHOW_DECL(expr) \
    std::cout << "decltype(" #expr ") == " << getDecl<decltype(expr)>() << '\n'

template<typename T>
consteval std::string_view getDecl()
{
#if defined(__clang__)
    auto declC = __PRETTY_FUNCTION__;
    std::string_view head = "[T = ";
    std::string_view tail = "]";
#elif defined(__GNUC__)
    auto declC = __PRETTY_FUNCTION__;
    std::string_view head = "[with T = ";
    std::string_view tail = "; std::string_view = ";
#elif defined(_MSC_VER)
    auto declC = __FUNCSIG__;
    std::string_view head = "getDecl<";
    std::string_view tail = ">(void)";
#else
    #error Compiler not supported
#endif
    // remove boilerplate text
    std::string_view decl = declC;
    size_t c1 = decl.find(head);
    size_t c2 = decl.rfind(tail);
    if (c1 == std::string::npos || c2 == std::string::npos || c1 > c2) {
        return decl; // just in case
    } else {
        c1 += head.length();
        return decl.substr(c1, c2 - c1);
    }
}

// Examples

void showCallable(auto func)
{
    SHOW_DECL(func);
    SHOW_DECL(func(0));
}

auto sub(int i)
{
    return std::tuple("word", i);
}

int main()
{
    SHOW_DECL(0);
    showCallable([](int i) { return std::tuple("word", i); });
    showCallable(sub);
}

/* Output when compiled with MSC:
decltype(0) == int
decltype(func) == class main::<lambda_1>
decltype(func(0)) == class std::tuple<char const *,int>
decltype(func) == class std::tuple<char const *,int>(__cdecl *)(int)
decltype(func(0)) == class std::tuple<char const *,int>
*/