https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88907
Bug ID: 88907
Summary: Variadic template function deduction failure.
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: Peter.Georg at physik dot uni-regensburg.de
Target Milestone: ---
Created attachment 45462
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45462&action=edit
Code to reproduce bug including possible work-arounds
GCC fails to compile the following code due to alleged ambiguous function call.
```
#include <utility>
template<class T, int... Ns>
class Base
{
};
template<class... Ts, int... Ns>
void error(Base<Ts, Ns...> const &... args)
{
}
template<class... Ts,
class = std::enable_if_t<
std::conjunction<std::is_class<std::remove_reference_t<Ts>>...>{}>>
void error(Ts &&... args)
{
}
int main()
{
Base<int, 4, 4> const a;
error(a);
}
```
The code is also attached or see: https://godbolt.org/z/GuW-YC
I.e. call to error(Base<int, 4, 4> const &) is ambiguous for GCC.
In my opinion, and e.g. clang agrees with me, the first function is clearly
more specialized and thus should be chosen.
There are two possible work-arounds (both are included in the attached code):
#1: Add an extra function error that takes exactly one parameter of Type
Base<T, Ns...> const &
#2: Remove the default template parameter (SFINAE) from the second error
function
I started I discussion about this (I wasn't sure it is a bug at first) and a
related clang bug (https://bugs.llvm.org/show_bug.cgi?id=40305) here:
https://stackoverflow.com/questions/54236545/why-is-it-an-ambigious-function-call-using-gcc-template-deduction-failing
