Why clang19 support explicit this parameter but doesn't define `__cpp_explicit_this_parameter` macro?

[illusory0x0]

godbolt code snippet

clang18 also has the same issue.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: 猗露 (illusory0x0)

[godbolt code snippet](https://godbolt.org/z/E1ozsc3cf)

[frederick-vs-ja]

See https://reviews.llvm.org/D140828. The feature-test macro was added in Clang 20 by #107451, although there didn't seem critical bugs with explicit object parameters in Clang 18/19.

[AaronBallman]

We often will implement support for a new feature but not set the feature test macro so that we can get some additional feedback on the implementation before claiming full support. The macro was added in Clang 20 once we decided that our support was sufficient for claiming conformance.

Closing this issue as there's nothing here to fix, but if you have further questions or concerns in this area, feel free to reopen with more details.