On Tue, 7 May 2024 10:36:25 +0200 Sebastian Wittmeier wrote:
>So in your case, you want to have a reference type deduced (by template
argument deduction),

>but currently [expr.type]/1 disagrees (
https://timsong-cpp.github.io/cppwp/expr.type#1): >If an expression
initially has the type ?reference to T? ([dcl.ref], [dcl.init.ref]), the
type is >adjusted to T prior to any further analysis.

>The copy constructor is something special (compared to other constructors).
>Why do you want to handle it by a template
>in the first place?

Yes, I want to have a reference type deduced, but not through the template
parameter deduction mechanism PR87332
<https://github.com/llvm/llvm-project/pull/87332>. The template parameter
deduction mechanism leads to T, not T&. "template <typename T> base(T x)
{}" prevents the compiler from calling the default copy constructor.

There are two ways to prevent it: modifying function matching rules which
seems to confuse the users and breaks existing rules or passing derived
objects by reference which is easy to implement and test but violates
[expr.type]/1. But this violation is just a special case.

Handling it by a template may be the user's choice which is provided by
C++. Although users use it correctly, it still leads to bugs.