[Bug c++/66671] New: Failure to create a new family of templates for template alias

[Bug c++/66671] New: Failure to create a new family of templates for template alias

[thiago at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66671

            Bug ID: 66671
           Summary: Failure to create a new family of templates for
                    template alias
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Whenever you use:

template <typename T>
using Z = Y<T>;

The template Z is different from Y and should be encoded separately. This can
be seen in the example:

====
#include <iostream>

template <template <typename> class>
struct X {
  X() { std::cout << "1"; }
};

template <typename>
struct Y {};

template <typename T>
using Z = Y<T>;

template <>
struct X<Y> {
  X() { std::cout << "2"; }
};

int main() {
  X<Y> x1;
  X<Z> x2;
}
====
(from: http://cppquiz.org/quiz/giveup/117)

With GCC 5.1.1, the above prints "22", whereas it prints "21" with ICC 16,
Clang 3.7 and MSVC 2013. The explanation from the quiz site is given as:

> According to §14.5.7¶1 in the standard, a template alias declaration 
> resolve to a new family of types. The specialization cannot be used, 
> and the first template delcaration is used instead, printing 1.

Disassembly with -O2 -fno-inline shows that Clang and ICC both call different
template specialisations of template class X:

        leaq    16(%rsp), %rdi
        callq   _ZN1XI1YEC2Ev          ; X<Y>::X()
        leaq    8(%rsp), %rdi
        callq   _ZN1XI1ZEC2Ev          ; X<Z>::X()

PS: should X<Z> count as a local type or a global one?
>From gcc-bugs-return-490214-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Thu Jun 25 18:47:23 2015
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From: "thiago at kde dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/66672] New: std::is_same wrong result for captured reference value inside a lambda
Date: Thu, 25 Jun 2015 18:47:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66672

            Bug ID: 66672
           Summary: std::is_same wrong result for captured reference value
                    inside a lambda
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thiago at kde dot org
  Target Milestone: ---

Given the following code:

====
#include <iostream>
#include <type_traits>

using namespace std;

int main()
{
  int i, &j = i;
  [=]
  {
    cout << is_same<decltype((j)), int      &>::value
         << is_same<decltype((j)), int const&>::value;
  }();
}
====
(trimmed from http://cppquiz.org/quiz/question/127)

With GCC 5.1, the code will print 10, but it should be 01. Clang 3.6, ICC 16
and MSVC 2015 do compile this as expected. The output 10 would be correct if
the extra set of parentheses weren't there.

The explanation given on the quiz website is:

> Since the expression for decltype is a parenthesized lvalue 
> expression, §7.1.6.2¶4 has this to say: "The type denoted by 
> decltype(e) is (...) T&, where T is the type of e;" As the
> expression occurs inside a const member function, the expression is
> const, and decltype((j)) denotes int const&. See also the example in 
> §5.1.2¶18.
[NB: it's paragraph 19 as of N4431]

The example in that paragraph of the standard matches almost exactly the code
above.

The output is correct with a functor:

====
struct S
{
  S(int &i) : j(i) {}
  void operator()() const
  {
    cout << is_same<decltype((j)), int      &>::value
         << is_same<decltype((j)), int const&>::value;
  };
  int j;    // captured by value
};
int main()
{
  int i;
  S{i}();
}
====
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From: "joe.carnuccio at qlogic dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/39121] strange behavior in chained operations
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id9121

joe.carnuccio at qlogic dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |joe.carnuccio at qlogic dot com

--- Comment #4 from joe.carnuccio at qlogic dot com ---
I have found the following:

This works: c ^= d ^= c ^= d  (where c and d are not pointers)

This fails: *a ^= *b ^= *a ^= *b  (where a and b are pointers)

When compiling using -Os then the failed case now works.

[Bug c++/66671] Failure to create a new family of templates for template alias

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66671

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-11-10
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |wrong-code
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/66671] Failure to create a new family of templates for template alias

[herring at lanl dot gov]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66671

S. Davis Herring <herring at lanl dot gov> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |herring at lanl dot gov

--- Comment #2 from S. Davis Herring <herring at lanl dot gov> ---
CWG1286 suggests that perhaps such a trivial alias template as Z ought to
actually be interchangeable with Y.