c++ - Why clang missing default argument on parameter pack error?

Question

When I use clang (10.0.1) to compile that:

#include <iostream>

template <typename ...Args>
void f( int a = 4, Args&&... aArgs )
{
    std::cout << a << std::endl;
}

int main( int argc, char *argv[] )
{
    f( 1, 2 );
    return 0;
}

I get:

main.cpp:4:30: error: missing default argument on parameter 'aArgs'

But standard say about default arguments:

In a function declaration, after a parameter with a default argument, all subsequent parameters must have a default argument supplied in this or a previous declaration from the same scope (since c++11) ...unless the parameter was expanded from a parameter pack or be a function parameter pack.

It is a clang bug?

hint: I try it on gcc and works fine

Answer 1

Yes, it's a bug: https://bugs.llvm.org/show_bug.cgi?id=23029.

It's fixed in clang 11: https://reviews.llvm.org/rGb5f2c4e45b8d54063051e6955cef0bbb7b6ab0f8