[Bug c++/106847] New: deprecated class data member makes the class generate diagnostics even when the member is not used

[Bug c++/106847] New: deprecated class data member makes the class generate diagnostics even when the member is not used

[avi at scylladb dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106847

            Bug ID: 106847
           Summary: deprecated class data member makes the class generate
                    diagnostics even when the member is not used
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avi at scylladb dot com
  Target Milestone: ---

A class with a deprecated member:

    struct foo {
        [[deprecated]] bool x;
    };

Cannot be used without lots of noise from the compiler:

    foo make_a_foo() {
        foo f;
        return f;
    }

<source>: In function 'foo make_a_foo()':
<source>:10:12: warning: 'f' is used uninitialized [-Wuninitialized]
   10 |     return f;
      |            ^
<source>:9:9: note: 'f' declared here
    9 |     foo f;
      |         ^
Compiler returned: 0


Although I didn't touch x, the compiler thinks I did. IMO synthesized
constructors (and assignment operators) should suppress the warning and only
explicit access by the user should trigger the warning.

[Bug c++/106847] deprecated class data member makes the class generate diagnostics even when the member is not used

[avi at scylladb dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106847

--- Comment #1 from Avi Kivity <avi at scylladb dot com> ---
Clang equivalent: https://github.com/llvm/llvm-project/issues/55774

[Bug c++/106847] deprecated class data member makes the class generate diagnostics even when the member is not used

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106847

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Well, you did use it by copying it?  Specify a copy CTOR that does not copy 'x'
and you'll be fine?

[Bug c++/106847] deprecated class data member makes the class generate diagnostics even when the member is not used

[avi at scylladb dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106847

--- Comment #3 from Avi Kivity <avi at scylladb dot com> ---
But I need it to be copied. It's a struct of options:


    struct options {
         bool frob = false;
         [[derecated]] bool quux = false; // will be removed soon
         bool grue = false;
     };


The fact that it's deprecated doesn't mean it can be coerced to false.

Sure, I can create custom constructors and assignment operators, but I lose
designated initializers.

btw, the destructor doesn't trigger the deprecation warning, even though it
does use the deprecated member.


I think it's more reasonable to only emit the warning when the member is
explicitly used.