[Bug c++/66392] New: rejects-valid: copy-initialization through user-defined conversion sequence fails

[Bug c++/66392] New: rejects-valid: copy-initialization through user-defined conversion sequence fails

[filip.roseen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66392

            Bug ID: 66392
           Summary: rejects-valid: copy-initialization through
                    user-defined conversion sequence fails
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: filip.roseen at gmail dot com
  Target Milestone: ---

Created attachment 35687
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35687&action=edit
testcase.cpp - inaccurately rejected

There are many variants of the supplied testcase, but the following shall
compile due to the wording of `[dcl.init]p17` (and relevant sections) - `gcc`
inaccurately rejects the snippet:

    struct B { };

    struct A { 
      explicit A (A const&);
               A (B const&);
    };  

    int main () {
      A x = B {}; // legal
    }   

---------------

    testcase.cpp: In function ‘int main()’:
    testcase.cpp:9:12: error: no matching function for call to ‘A::A(A)’
       A x = B {}; // legal
                ^
    testcase.cpp:5:12: note: candidate: A::A(const B&)
                A (B const&);
                ^
    testcase.cpp:5:12: note:   no known conversion for argument 1 from ‘A’ to
‘const B&’
    testcase.cpp:5:12: note:   after user-defined conversion: A::A(const B&)

---------------

[ Note: `msvc`, `clang`, `icc` all accept the above snippet ]

---------------

In short the ISO C++ Standard says that if copy-initialization takes place
where the source type is not the same as the destination type, but there is a
user-defined conversion sequence that can convert the source type into the
destination type - such will be used to construct a temporary, and that
temporary will then be used to _direct-initialize_ the destination object.

It should be noted, as directly addressed in the standard wording, that
copy-elision may be used to skip the construction of the temporary object.

[ Note: it is important to note that the wording talks about "user-defined
conversion sequences", as such one must properly take into account usage of
`operator T`, where `T` is the destination type, in the source type which shall
yield the same described behavior as the attached testcase ]

------------------------------------------

Relevant part of `[dcl.init]p17`:

> Otherwise (i.e., for the remaining copy-initialization cases), user-defined
> conversion sequences that can convert from the source type to the destination
> type or (when a conversion function is used) to a derived class thereof are
> enumerated as described in [over.match.copy], and the best one is chosen through
> overload resolution ([over.match]). If the conversion cannot be done or is
> ambiguous, the initialization is ill-formed. The function selected is called
> with the initializer expression as its argument; if the function is a
> constructor, the call initializes a temporary of the cv-unqualified version of
> the destination type. The temporary is a prvalue. The result of the call (which
> is the temporary for the constructor case) is then used to direct-initialize,
> according to the rules above, the object that is the destination of the
> copy-initialization. In certain cases, an implementation is permitted to
> eliminate the copying inherent in this direct-initialization by constructing the
> intermediate result directly into the object being initialized; see
> [class.temporary], [class.copy]
>From gcc-bugs-return-487932-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Wed Jun 03 08:25:26 2015
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id)358

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Author: mpolacek
Date: Wed Jun  3 08:24:50 2015
New Revision: 224062

URL: https://gcc.gnu.org/viewcvs?rev"4062&root=gcc&view=rev
Log:
        PR c/64223
        PR c/29358
        * tree.c (attribute_value_equal): Handle attribute format.
        (cmp_attrib_identifiers): Factor out of lookup_ident_attribute.

        * gcc.dg/pr64223-1.c: New test.
        * gcc.dg/pr64223-2.c: New test.

Added:
    trunk/gcc/testsuite/gcc.dg/pr64223-1.c
    trunk/gcc/testsuite/gcc.dg/pr64223-2.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/tree.c

[Bug c++/66392] rejects-valid: copy-initialization through user-defined conversion sequence fails

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66392

--- Comment #2 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Tue Aug  4 14:54:07 2015
New Revision: 226579

URL: https://gcc.gnu.org/viewcvs?rev=226579&root=gcc&view=rev
Log:
2015-08-04  Paolo Carlini  <paolo.carlini@oracle.com>

        PR c++/66392
        * g++.dg/init/explicit4.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/init/explicit4.C
Modified:
    trunk/gcc/testsuite/ChangeLog

[Bug c++/66392] rejects-valid: copy-initialization through user-defined conversion sequence fails

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66392

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|paolo.carlini at oracle dot com    |
         Resolution|---                         |FIXED
   Target Milestone|---                         |6.0

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Done.