[Bug c++/24847] New: Instantiates un-called copy constructor

[Bug c++/24847] New: Instantiates un-called copy constructor

[igodard at pacbell dot net]

enum A{b,c};
template<typename T, int i, A x>
struct foo {
explicit    foo(T& t);
explicit    foo(foo<T, 0, x>);
};

int main() {
    int i;
    foo<int, 5, b> v(i);
    }


gets you:

~/ootbc/members/src$ g++ foo.cc
foo.cc: In instantiation of `foo<int, 0,  b>':
foo.cc:10:   instantiated from here
foo.cc:5: error: invalid constructor; you probably meant `foo<int, 0,  b>
(const foo<int, 0,  b>&)'

The error arises because it thinks I am instantiating "foo<T, 0,A>(foo<T, 0,
A>)", i.e. a copy constructor that is not taking a const reference argument.
But I'm not; I'm instantiating the plain constructor "foo<T, int, A>(int&)". 

The second constructor is intended to catch the case: "foo<T, 5, A>(foo<T, 0,
A>", i.e. to convert an instance of foo with second argument zero to a foo with
any other second argument. It looks like when it is expanding the argument type
of the second constructor (i.e. "foo<T, 0, A>") that it is not just parsing the
resulting template but also applying the "valid copy constructor?" check for
that type. But as that second constructor is never called by anyone it the
compiler should just syntax check it (valid) and not semantic check it (invalid
for "foo(T, 0, A)" but valid for everything else).

Ivan


-- 
           Summary: Instantiates un-called copy constructor
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: igodard at pacbell dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

[Bug c++/24847] Instantiates un-called copy constructor

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2005-11-14 19:32 -------
Comeau also rejects this.  I don't understand why we are trying to instantiate
foo<int, 5, b>::foo(foo<int, 0, b>) except to try to match the constructor, so
maybe this is invalid after all.  Some one else really needs to look at this.

A weird testcase at best.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

[Bug c++/24847] Instantiates un-called copy constructor

[igodard at pacbell dot net]

------- Comment #2 from igodard at pacbell dot net  2005-11-15 00:30 -------
The original was much more sensible - and much bigger :-)

Ivan


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

[Bug c++/24847] Instantiates un-called copy constructor

[bangerth at dealii dot org]

------- Comment #3 from bangerth at dealii dot org  2005-11-23 05:21 -------
The second constructor is definitely bogus, but its mere existence 
does no harm. We should only perform the check for sensibility when
we actually instantiate the second constructor.

W.


-- 

bangerth at dealii dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-11-23 05:21:00
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

[Bug c++/24847] Instantiates un-called copy constructor

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
these compiler reject this code:
GCC
MSVC
ICC

Only clang accepts the code.

All of the ones which reject it, all say the constructor is a copy constructor.

[Bug c++/24847] Instantiates un-called copy constructor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Overload resolution considers both constructors, to decide which one v(i)
should use. Performing overload resolution on foo<int, 5, b>::foo(foo<int, 0,
b>) will cause the instantiation of struct foo<int, 0, b> to see if there is a
valid conversion sequence from int to foo<int, 0, b>. Instantiating that type
causes the instantiation of foo<int, 0, b>::foo(foo<int, 0, b>) which is indeed
an invalid copy constructor.

So the problem is not that it thinks the second constructor is a copy
constructor of foo<int, 5, b>, obviously it's not. But it is an invalid copy
constructor for foo<int, 0, b> which gets implicitly instantiated.

I don't know how Clang accepts this.

[Bug c++/24847] Instantiates un-called copy constructor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24847

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Ivan Godard from comment #0)
> The second constructor is intended to catch the case: "foo<T, 5, A>(foo<T,
> 0, A>", i.e. to convert an instance of foo with second argument zero to a
> foo with any other second argument.

You can define that as:

explicit    foo(const foo<T, 0, x>&);

Or:

template<int N, typename = std::enable_if_t<N == 0 && i != 0>
explicit    foo(foo<T, N, x>);

Or in C++20:

explicit    foo(foo<T, 0, x>) requires (i != 0);