Template with parameter pack compiles using avr-gcc, but not using Arduino IDE

Question

I'm trying to write a generic template for Print for an Arduino project. In doing so, I encountered a compiler error I didn't understand, so I made a minimum reproducible example.

template <typename T, typename... T2>
void print_all(Print* const (&ps)[2], T v1, T2... v2) {
    ps[0]->print(v1);
    ps[0]->flush();
    ps[1]->print(v1);
    ps[1]->flush();
    print_all(ps, v2...);
}
void print_all(Print* const (&ps)[2]) {
    return;
}

void setup() {
    print_all({&Serial1, &Serial2}, 1.0, "abc", 5ul);
}

void loop() {
}

The error I get is error: expected ')' before ';' token with no line number information. If I insert an empty line at the start of the file, the compiler error changes to

error: expected ')' before ';' token
    1 | 
      |                                                                       ^
      |                                                                       )

I tried this with avr-gcc version 7.3.0 (official arduino avr board) and 14.1.0 (using arch linux arduino avr boards) and get the same error either way.

I experimented for a while, and eventually put the code on Compiler Explorer using avr-gcc instead of Arduino IDE to compile. I had to provide some stubs to make everything compile, but it seems to work as I would expect. So why does my code work on compiler explorer, but not in the Arduino IDE?

code with supporting method stubs

class Print{
    public:
    virtual void print(unsigned long i) {
    }
    virtual void print(double i) {
    }
    virtual void print(const char* i) {
    }
    virtual void flush() {

    }
};

class Serial: public Print {};

Serial serial1 = Serial{};
Serial serial2 = Serial{};

template <typename T, typename... T2>
void print_all(Print* const (&ps)[2], T v1, T2... v2) {
    ps[0]->print(v1);
    ps[0]->flush();
    ps[1]->print(v1);
    ps[1]->flush();
    print_all(ps, v2...);
}
void print_all(Print* const (&ps)[2]) {
    return;
}

int main() {
    Print* const ps[] = {&serial1, &serial2};
    print_all(ps, 1.0, "abc", 5ul);
}

Answer 1

From the docs:

First, the Arduino development software performs some minor pre-processing to turn your sketch into a C++ program

So your Arduino code is not really a C++ program. It is an input for the Arduino preprocessor which outputs a C++ program.

What does the preprocessor do? Among other things:

Prototypes are generated for all function definitions in .ino/.pde files that don't already have prototypes. In some rare cases, prototype generation may fail for some functions. To work around this, you can provide your own prototypes for these functions

Well, congratulations I guess? you have stumbled upon one of those "rare" cases. Unfortunately, providing a separate template doesn't help. However there is another workaround: place the template header on one line. Yes, you've read it right.

// this works
template <typename T, typename... T2> void print_all(Print* const (&ps)[2], T v1, T2... v2) {

// this doesn't
template <typename T, typename... T2>
void print_all(Print* const (&ps)[2], T v1, T2... v2) {