[clang] [Clang] Define __cpp_pack_indexing (PR #101956)

[via cfe-commits]

https://github.com/Sirraide created https://github.com/llvm/llvm-project/pull/101956

Following the discussion on [#101448](https://github.com/llvm/llvm-project/pull/101448#pullrequestreview-2217511926), this defines `__cpp_pack_indexing`. Since pack indexing is currently supported in all language modes, the feature test macro is also defined in all language modes.

>From eb93820676ab6c6683a76399910c692d76f86b53 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Mon, 5 Aug 2024 12:29:24 +0200
Subject: [PATCH] [Clang] Define __cpp_pack_indexing

---
 clang/docs/LanguageExtensions.rst       | 1 +
 clang/lib/Frontend/InitPreprocessor.cpp | 1 +
 clang/test/Lexer/cxx-features.cpp       | 4 ++++
 3 files changed, 6 insertions(+)

diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index 9dcb4ac9b75ca..be07f81cc41b0 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -1503,6 +1503,7 @@ Conditional ``explicit``                     __cpp_conditional_explicit       C+
 ``static operator()``                        __cpp_static_call_operator       C++23         C++03
 Attributes on Lambda-Expressions                                              C++23         C++11
 Attributes on Structured Bindings            __cpp_structured_bindings        C++26         C++03
+Pack Indexing                                __cpp_pack_indexing              C++26         C++03
 ``= delete ("should have a reason");``       __cpp_deleted_function           C++26         C++03
 -------------------------------------------- -------------------------------- ------------- -------------
 Designated initializers (N494)                                                C99           C89
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 17b9ca7cb9910..8e62461d8a181 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -763,6 +763,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   Builder.defineMacro("__cpp_placeholder_variables", "202306L");
 
   // C++26 features supported in earlier language modes.
+  Builder.defineMacro("__cpp_pack_indexing", "202311L");
   Builder.defineMacro("__cpp_deleted_function", "202403L");
 
   if (LangOpts.Char8)
diff --git a/clang/test/Lexer/cxx-features.cpp b/clang/test/Lexer/cxx-features.cpp
index 4c2aa3ae2c544..08b732132228b 100644
--- a/clang/test/Lexer/cxx-features.cpp
+++ b/clang/test/Lexer/cxx-features.cpp
@@ -38,6 +38,10 @@
 #error "wrong value for __cpp_deleted_function"
 #endif
 
+#if check(pack_indexing, 202311, 202311, 202311, 202311, 202311, 202311, 202311)
+#error "wrong value for __cpp_pack_indexing"
+#endif
+
 #if check(placeholder_variables, 202306, 202306, 202306, 202306, 202306, 202306, 202306)
 #error "wrong value for __cpp_placeholder_variables"
 #endif