https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96673
Bug ID: 96673
Summary: Friend class with templates and default constructor
not recognized in C++14 or later
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: andebjor at gmail dot com
Target Milestone: ---
This code fails when compiled as C++14 or C++17 with the error message "error:
'C<T>::C(A<T>&) [with T = int]' is private within this context".
---8<---
template <class T>
class A {};
template <class T>
class B;
template <class T>
class C {
private:
friend class B<T>;
explicit C(A<T>&) {};
};
template <class T>
class B {
public:
B() = default;
//B() {}; // << This implementation of the constructor makes it work
A<T> a = {};
C<T> c = C<T>{a};
};
int main() {
auto b = B<int>{};
auto &c = b.c;
}
--->8---
Compiler explorer link: https://godbolt.org/z/4ofMdM
This code builds with `clang` but fails with most versions of GCC.
* Version 4.9 works
* Version 5.x: internal compiler error
* Version 6.x up until trunk version: "C<T>::C(A<T>&) [with T = int]' is
private within this context"
There are several ways to make the code build:
* Removing templating. Without templates the error about the constructor being
private does not appear.
* Changing the constructor of `B` from `B() = default;` to `B() {};`
* Building with C++11.
