https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115198
Bug ID: 115198
Summary: Class template argument deduction fails for copy ctor
when used with an alias template if the aliased class
template has explicitly defaulted copy ctor
Product: gcc
Version: 14.1.0
URL: https://sigcpp.godbolt.org/z/jjGT8hK8d
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: s.murthy at outlook dot com
Target Milestone: ---
Class template argument deduction fails when the copy ctor is used with an
alias template if the aliased class template has explicitly defaulted copy
ctor. The error disappears if the explicit defaulted copy ctor is removed (i.e.
have the compiler include the implicitly declared copy ctor).
See repro but I believe this issue manifests at least under the following
conditions:
- All parameters of the alias template have default values
- The copy ctor is used using the alias template without any arguments for the
template parameters and without angle brackets (as you would with a class
directly).
The issue is seen in GCC 10.1 to 14.1 (since C++20s support). I have confirmed
that trunk too has this issue as of this writing. In comparison Clang trunk
(post 18.1) does *not* have this issue.
PS: Please forgive me if this report does not meet expectations. This is my
first time filing an issue here. Also, my search of the issues did not show
another matching issue (and thus my filing).
A repro with different scenarios is at: https://sigcpp.godbolt.org/z/jjGT8hK8d
//repro in short
template<bool B = false, typename T = int>
class C1
{
public:
C1() = default;
C1(const C1& other) = default;
};
template<typename T = int>
using A1 = C1<false, T>;
C1 c1;
A1 a1(c1); //Error
//same as C1 but with implict copy ctor
template<bool B = false, typename T = int>
class C2
{
public:
C2() = default;
};
template<typename T = int>
using A2 = C2<false, T>;
C2 c2;
A2 a2(c2); //OK
