Don't warn about deprecated fields in implicitly generated special member functions

[HolyBlackCat]

This gives a warning:

struct A
{
    [[deprecated]] int x = 42;
};

int main()
{
    A a;
    A b = a;
    a = b;
}

<source>:1:8: warning: 'A::x' is deprecated [-Wdeprecated-declarations]
    1 | struct A
      |        ^
<source>:3:24: note: declared here
    3 |     [[deprecated]] int x = 42;
      |                        ^
<source>:1:8: warning: 'A::x' is deprecated [-Wdeprecated-declarations]
    1 | struct A
      |        ^
<source>:3:24: note: declared here
    3 |     [[deprecated]] int x = 42;
      |                        ^
<source>: In function 'int main()':
<source>:8:7: note: synthesized method 'constexpr A::A()' first required here
    8 |     A a;
      |       ^

IMO this isn't very useful. Only the manual uses of this field should warn, not the implicit ones.

GCC has the same issue, but MSVC doesn't. I've tested on Clang 20 and trunk.

[shashforge]

Hello @HolyBlackCat/everyone,

Thank you for raising this issue—it looks like a valuable improvement to Clang’s diagnostics. I’d be honored to contribute my first PR by implementing a patch that suppresses deprecation warnings in implicit special‐member functions (default/copy constructors and copy‐assignment operators) via an early bail‐out in Sema::DiagnoseUseOfDecl. I can also add appropriate regression tests under test/SemaCXX/.

If it’s alright, could someone kindly assign this issue to me? I look forward to collaborating on this!

Thank you for your consideration. 😊