r343285 - [cxx2a] P0641R2: (Some) type mismatches on defaulted functions only

[Stephan Bergmann via cfe-commits]

On 28/09/2018 03:16, Richard Smith via cfe-commits wrote:
> Author: rsmith
> Date: Thu Sep 27 18:16:43 2018
> New Revision: 343285
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=343285&view=rev
> Log:
> [cxx2a] P0641R2: (Some) type mismatches on defaulted functions only
> render the function deleted instead of rendering the program ill-formed.
> 
> This change also adds an enabled-by-default warning for the case where
> an explicitly-defaulted special member function of a non-template class
> is implicitly deleted by the type checking rules. (This fires either due
> to this language change or due to pre-C++20 reasons for the member being
> implicitly deleted). I've tested this on a large codebase and found only
> bugs (where the program means something that's clearly different from
> what the programmer intended), so this is enabled by default, but we
> should revisit this if there are problems with this being enabled by
> default.

Two questions on the new -Wdefaulted-function-deleted:

1  Do you have examples of actual bugs found by that?  I'm running it on 
the LibreOffice code base now and it feels like lots of just noise.  For 
example, I recently added explicitly defaulted decls of the four 
copy/move functions to lots of classes with a virtual dtor (where the 
dtor needs to be user-declared e.g. because it doesn't override any 
virtual base class dtor or because it shall not be defined inline), to 
silence GCC's new -Wdeprecated-copy.  If such a class then for example 
has a const non-static data member, Clang's -Wdefaulted-function-deleted 
now kicks in for the assignment ops.  I'm not sure whether it's better 
to have those functions explicitly deleted (which then needs revisiting 
when some details of the class change), or just explicitly defaulted and 
the compiler figure out whether its deleted or not (but which now causes 
Clang to bark), or simply not user-declared at all (but which now causes 
GCC to bark).

2  With

> $ cat test.cc
> struct S1 { S1 & operator =(S1 const &) = delete; };
> struct S2 {
>     S1 m;
>     S2(S2 const &);
> };
> struct S3: S2 {
>     S3 & operator =(S3 const &) = default;
>     S3 & operator =(S3 &&) = default;
> };
> 
> $ clang++ -fsyntax-only test.cc
> test.cc:7:10: warning: explicitly defaulted copy assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
>     S3 & operator =(S3 const &) = default;
>          ^
> test.cc:6:12: note: copy assignment operator of 'S3' is implicitly deleted because base class 'S2' has a deleted copy assignment operator
> struct S3: S2 {
>            ^
> test.cc:3:8: note: copy assignment operator of 'S2' is implicitly deleted because field 'm' has a deleted copy assignment operator
>     S1 m;
>        ^
> test.cc:1:18: note: 'operator=' has been explicitly marked deleted here
> struct S1 { S1 & operator =(S1 const &) = delete; };
>                  ^
> test.cc:8:10: warning: explicitly defaulted move assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
>     S3 & operator =(S3 &&) = default;
>          ^
> test.cc:6:12: note: move assignment operator of 'S3' is implicitly deleted because base class 'S2' has a deleted move assignment operator
> struct S3: S2 {
>            ^
> test.cc:3:8: note: copy assignment operator of 'S2' is implicitly deleted because field 'm' has a deleted copy assignment operator
>     S1 m;
>        ^
> test.cc:1:18: note: 'operator=' has been explicitly marked deleted here
> struct S1 { S1 & operator =(S1 const &) = delete; };
>                  ^
> 2 warnings generated.

the final two notes for the second warning are somewhat confusing I 
think, as there's a mismatch from the preceding note's "'S2' has a 
deleted move assignment operator" to the second-last note's "copy 
assignment operator of 'S2' is implicitly deleted".  Not sure though 
whether this is an issue specific to this new 
-Wdefaulted-function-deleted, or a general issue with generic code 
producing such "deleted because" notes.