[llvm-bugs] [Bug 48538] New: Template contsructor isntantiates invalid copy constructor

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48538

            Bug ID: 48538
           Summary: Template contsructor isntantiates invalid copy
                    constructor
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: stinkingmadgod at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

#include<type_traits>

template<typename T>
struct S
{
    template<typename U = int,
        std::enable_if_t<std::is_const_v<T>, U> = 0>
    S(S<std::remove_const_t<T>>) {}
};

void foo(S<char>) {}

Will yield the error

<source>:8:32: error: copy constructor must pass its first argument by
reference
    S(S<std::remove_const_t<T>>) {}
                               ^
                                const &
<source>:11:17: note: in instantiation of template class 'S<char>' requested
here
void foo(S<char>)

I believe the snippet is well-formed due to either [class.copy.ctor]/5

> A declaration of a constructor for a class X is ill-formed if its first parameter is of type cv X and
> either there are no other parameters or else all other parameters have default arguments.
> A member function template is never instantiated to produce such a constructor signature.

Or simply because it should SFINAE away, the constructor is dependent.

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