[C++2b] Implement multidimentional subscript operator

Implement P2128R6 in C++23 mode.

Unlike GCC's implementation, this doesn't try to recover when a user
meant to use a comma expression.

Because the syntax changes meaning in C++23, the patch is *NOT*
implemented as an extension. Instead, declaring an array with not
exactly 1 parameter is an error in older languages modes. There is an
off-by-default extension warning in C++23 mode.

Unlike the standard, we supports default arguments;

Ie, we assume, based on conversations in WG21, that the proposed
resolution to CWG2507 will be accepted.

We allow arrays OpenMP sections and C++23 multidimensional array to
coexist:

[a , b] multi dimensional array
[a : b] open mp section
[a, b: c] // error

The rest of the patch is relatively straight forward: we take care to
support an arbitrary number of arguments everywhere.

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index e259ab4..bf8a0b2 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -642,6 +642,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_implicit_move", "202011L");
     Builder.defineMacro("__cpp_size_t_suffix", "202011L");
     Builder.defineMacro("__cpp_if_consteval", "202106L");
+    Builder.defineMacro("__cpp_­multidimensional_­subscript", "202110L");
   }
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "201811L");