Error with gcc but not with clang when compiling initializer list containing pointers to template function

Question

The following snippet compiles fine in vc++ and clang++, but fails on gcc (inc 9.2) unless i add an explicit cast. Which compiler is right here?

#include <initializer_list>

template<typename T>
void Task(void) {}

int main()
{
    for(auto p_task: {&Task<int>}) {} // error
//  for(auto p_task: {static_cast<void (*)(void)>(&Task<int>)}) {} // ok
}

<source>: In function 'int main()':
<source>:8:33: error: unable to deduce 'std::initializer_list<auto>&&' from '{(& Task<int>)}'
    8 |     for(auto p_task: {&Task<int>}) {} // error
      |                                 ^
<source>:8:33: note:   couldn't deduce template parameter 'auto'

online compiler

Lux

Note: You don't need a range-based loop to cause this problem. auto l = { &Task<int> }; does the same (live demo). – Lutero

This seems like a bug in the gcc compiler. Interestingly, it works if you do +Task<int> which implies the &Task<int> might have a type ambiguity in gcc that's not present for implicit conversion. For instance gcc might simply ignore the & when applied to a global function, which then has an ambiguity of void(*)() vs void() or void(&)(). Or something... I'd send in a ticket. – Fulminate

Filed gcc 93107. – Tague

To add more information, this will compile on gcc: { std::addressof(Task<int>) } which may help you with a possibly more pleasing workaround (though the unary plus is always a crowd pleaser). – Grad

@Tague Fix committed to GCC master on 2020-10-29 20:12:19 UTC; gcc trunk now successfully compiles the live demo. – Southsoutheast

Answer 1

GCC was wrong, but not anymore! Thanks to @Barry reporting it, and Marek Polacek fixing it, the live demo can now be compiled with GCC trunk (future GCC 11).

Southsoutheast