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.
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