[Bug c++/66537] New: An explicit default constructor is accepted when initializing from empty braces

[Bug c++/66537] New: An explicit default constructor is accepted when initializing from empty braces

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66537

            Bug ID: 66537
           Summary: An explicit default constructor is accepted when
                    initializing from empty braces
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ville.voutilainen at gmail dot com
                CC: jason at redhat dot com
  Target Milestone: ---

Test snippet:

struct Bob
{
    explicit Bob() {}
};

Bob f() 
{
    return {}; // #1
}

void f2(Bob) {}

int main()
{
    f2({}); // #2
}

gcc accepts both #1 and #2, clang rejects both.

I have submitted a patch that makes the standard library tag types
non-default-constructible, but an alternative solution to certain kinds
of ambiguity problems with tuple would be to make the tag types' default
constructors explicit rather than suppressing them. That doesn't currently
work because the front-end accepts initializations like in the example above.

[Bug c++/66537] An explicit default constructor is accepted when initializing from empty braces

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66537

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler@googlemail.
                   |                            |com

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
Isn't that similar to bug 54835? As far as I remember Jason interprets the
standard that the code should be valid, see his comments in above mentioned
issue.
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Subject: [Bug c++/66537] An explicit default constructor is accepted when initializing from empty braces
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66537

--- Comment #2 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
(In reply to Daniel Krügler from comment #1)
> Isn't that similar to bug 54835? As far as I remember Jason interprets the
> standard that the code should be valid, see his comments in above mentioned
> issue.

I think there's a difference. The _temporary_ initialized from {}
is expected to not be allowed when the default constructor is explicit. What
happens after that in returning from a function or passing an argument
is separate.

Implementations disagree, so perhaps the specification needs to be clarified.
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Subject: [Bug c/66425] (void) cast doesn't suppress __attribute__((warn_unused_result))
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf425

--- Comment #19 from rusty at rustcorp dot com.au ---
I like WUR as a sanity-check, and it is useful that more and more library
authors are using it (generally quite well).  As Andrew points out, this has
taken 10 years!  The downside is that false positives are becoming more common.

For example, gnulib also includes a routine to handle this, since 2008:

http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/ignore-value.h;hh521edb38f50cb2230c8560a125864be212db6a;hb=HEAD

I believe that changing conditions call for a re-evaluation.  And yes, I'd like
to see a comment on any cast-to-void, not just for WUR.

Thanks for your patience!

[Bug c++/66537] An explicit default constructor is accepted when initializing from empty braces

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66537

--- Comment #3 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Oh well, I guess CWG 1518 as referenced in the other bug should solve this.
I'm fine either way, if explicit default constructors are decided to
work with the example, we can close this bug as invalid, and I should pursue
harder making library tag types non-default-constructible.

[Bug c++/66537] [DR 1518] An explicit default constructor is accepted when initializing from empty braces

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66537

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|---                         |DUPLICATE
            Summary|An explicit default         |[DR 1518] An explicit
                   |constructor is accepted     |default constructor is
                   |when initializing from      |accepted when initializing
                   |empty braces                |from empty braces

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
This seems the same as 54835.

*** This bug has been marked as a duplicate of bug 54835 ***