[libc++] Warn if an unsupported compiler is used

[ldionne]

Summary

This makes it obvious that libc++ is used in an unsupported configuration, and the compiler probably has to be updated. It often happens that people try to use libc++ and don't realize that their compiler is too old.

[Mordante]

Note the patch is a bit premature. Until LLVM-17 is released we still support LLVM-15.
@emaste Once LLVM 17 is released libc++ will drop support for Clang 15. Can you update the FreeBSD CI to Clang 16 or Clang 17. If you prefer to do the update after the LLVM 17 release can you let us know?

libcxx/include/__config

39

[emaste]

Once LLVM 17 is released libc++ will drop support for Clang 15. Can you update the FreeBSD CI to Clang 16 or Clang 17

Updating to Clang 16 now makes sense, and I'll plan to do it next week.
(Clang 16 is what we use in the FreeBSD base system as well.)

I'll update to 17 a little after it is released.

[Mordante]

In D158214#4608324, @emaste wrote:

Once LLVM 17 is released libc++ will drop support for Clang 15. Can you update the FreeBSD CI to Clang 16 or Clang 17

Updating to Clang 16 now makes sense, and I'll plan to do it next week.
(Clang 16 is what we use in the FreeBSD base system as well.)

I'll update to 17 a little after it is released.

Great, thanks!

[ldionne]

LGTM but you should probably address your own TODO :-)

This should pass after a rebase now.

[philnik]

Address comments

[ldionne]

We should probably drop the Clang 15 configuration from the CI now. WDYT @Mordante ?

[Mordante]

In D158214#4643513, @ldionne wrote:

We should probably drop the Clang 15 configuration from the CI now. WDYT @Mordante ?

Then we should have a separate PR to do that. I had it on my todo list for after the release, but I'm not objecting to do it a week before the release. Especially since I'm not aware of patches we want to backport to the release branch. Once that patch has landed this one can be rebased.

[ldionne]

The patch removing Clang 15 is https://github.com/llvm/llvm-project/pull/66406. I'll ping here once landed.

[ldionne]

libcxx/include/__config

42

For now, can we do this? It would allow merging this patch and then I can change the number when I land https://github.com/llvm/llvm-project/pull/66406.

[ldionne]

Commandeering cause I want to land this.

[ldionne]

Encode the real version of Clang that we support right now.

libcxx/include/__config

 // Given AppleClang XX.Y.Z, _LIBCPP_APPLE_CLANG_VER is XXYZ (e.g. AppleClang 14.0.3 => 1403)
 #  define _LIBCPP_COMPILER_CLANG_BASED
 #  define _LIBCPP_APPLE_CLANG_VER (__apple_build_version__ / 10000)
 #elif defined(__clang__)
 #  define _LIBCPP_COMPILER_CLANG_BASED
 #  define _LIBCPP_CLANG_VER (__clang_major__ * 100 + __clang_minor__)
 #elif defined(__GNUC__)
 #  define _LIBCPP_COMPILER_GCC
+#  define _LIBCPP_GCC_VER (__GNUC__ * 100 + __GNUC_MINOR__)
 #endif
 
 #ifdef __cplusplus
 
+// Warn if a compiler version is used that is not supported anymore
+#  if defined(_LIBCPP_CLANG_VER)
+#    if _LIBCPP_CLANG_VER < 1600
+#      warning "Libc++ only supports Clang 16 and later"
+#    endif
+#  elif defined(_LIBCPP_APPLE_CLANG_VER)
+#    if _LIBCPP_APPLE_CLANG_VER < 1400
+#      warning "Libc++ only supports AppleClang 14 and later"
+#    endif
+#  elif defined(_LIBCPP_GCC_VER)
+#    if _LIBCPP_GCC_VER < 1300
+#      warning "Libc++ only supports GCC 13 and later"
+#    endif
+#  endif
+
 // The attributes supported by clang are documented at https://clang.llvm.org/docs/AttributeReference.html
 
 // _LIBCPP_VERSION represents the version of libc++, which matches the version of LLVM.
 // Given a LLVM release LLVM XX.YY.ZZ (e.g. LLVM 17.0.1 == 17.00.01), _LIBCPP_VERSION is
 // defined to XXYYZZ.
 #  define _LIBCPP_VERSION 180000
 
 #  define _LIBCPP_CONCAT_IMPL(_X, _Y) _X##_Y