C++ : Possible Concepts-related bug yields "multiple dispatch"

Question

What's going on here? It seems the ability to declare testGeneric with an auto& parameter creates a weird situation where the function can be used to implement multiple dispatch.

Tested on gcc 10.1 or newer with "-fconcepts-ts" flag, also works on x86-64 clang (old concepts branch) via Compiler Explorer.

#include <iostream>

void testGeneric(auto &g)
{
    g.print();
};

struct Generic
{
    bool value = false;

    void print() {
        std::cout << value << std::endl;
    };
};

auto main() -> int
{

    auto foo = Generic();

    auto bar = []() {
        struct Generic
        {
            int value;
            Generic(int v) : value(v){};

            void print() {
                std::cout << value << std::endl;
            };
        };
        return Generic{123};
    }();

    auto baz = []() {
        struct Generic
        {
            const char* value;
            Generic(const char* v) : value(v){};

            void print() {
                std::cout << value << std::endl;
            };
        };
        return Generic{"What the... ?"};
    }();


    testGeneric(foo); // prints 0 (false)
    testGeneric(bar); // prints 123
    testGeneric(baz); // prints "What the... ?"
};

Answer 1

testGeneric is an abbreviated function template. It is functionally equivalent to the template declaration

template <class T>
void testGeneric(T &g)
{
    g.print();
};

Your code will have 3 different testGeneric functions, one for each type that you call it with.