Missed guaranteed copy elision during implicit conversion and constructor delegation.

[llvmbot]

Bugzilla Link	34689
Version	5.0
OS	Linux
Attachments	Example source file.
Reporter	LLVM Bugzilla Contributor
CC	@davidstone

Extended Description

Various guaranteed copy elisions are inconsistently missed depending on copy or direct initialization. See the attached file for the sample source file. For reference it compiles on g++ 7.1.0, I didn't test runtime behavior though.

https://godbolt.org/g/c1Aouy

I compiled with clang++-5.0 -std=c++1z -Wall -pedantic -c -o copy_elision.o copy_elision.cpp

Got 7 errors at the marked places, all complaining about the deleted move and copy constructors.

I think the attached source file is well-formed and it should compile.

$ uname -a
Linux lenard-hp 4.9.0-3-amd64 #​1 SMP Debian 4.9.30-2+deb9u3 (2017-08-06) x86_64 GNU/Linux
$ clang++-5.0 --version
clang version 5.0.0-2 (tags/RELEASE_500/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[llvmbot]

This is partly fixed in clang trunk (don't know which revision though).

While investigating the last two failures, I came across this comment from Richard:

  // Per DR (no number yet), this does not apply when initializing a base
  // class or delegating to another constructor from a mem-initializer.

Which is basically what your cant_copy_B constructor is doing. If I'm not mistaken, that would now mean that v31 and v32 shouldn't compile too.

[davidstone]

I don't understand why this should not work in a delegating constructor (which likely suggests a failure of imagination on my part, I'm sure), which is what it appears Richard is saying. As stated in the previous comment, most of the example code works in clang trunk now, but it appears that the one remaining failure can be reduced to this (https://godbolt.org/g/pgeF6Y):

struct S {
    S(S const &) = delete;
    S() = default;
    S(int): S(S()) {}
};

Is it intentional that this does not work, and if so, why? I looked through the current CWG defect page and I could not find anything about this.

[frederick-vs-ja]

The CWG issue is now CWG2403.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [34689](https://llvm.org/bz34689) | | Version | 5.0 | | OS | Linux | | Attachments | [Example source file.](https://user-images.githubusercontent.com/60944935/143756342-34706d74-a0b2-4697-892f-892838b20e78.gz) | | Reporter | LLVM Bugzilla Contributor | | CC | @davidstone |

Extended Description

Various guaranteed copy elisions are inconsistently missed depending on copy or direct initialization. See the attached file for the sample source file. For reference it compiles on g++ 7.1.0, I didn't test runtime behavior though.

https://godbolt.org/g/c1Aouy

I compiled with clang++-5.0 -std=c++1z -Wall -pedantic -c -o copy_elision.o copy_elision.cpp

Got 7 errors at the marked places, all complaining about the deleted move and copy constructors.

I think the attached source file is well-formed and it should compile.

$ uname -a
Linux lenard-hp 4.9.0-3-amd64 #&amp;#<!-- -->8203;1 SMP Debian 4.9.30-2+deb9u3 (2017-08-06) x86_64 GNU/Linux
$ clang++-5.0 --version
clang version 5.0.0-2 (tags/RELEASE_500/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin