[clang] [C2y] Implement WG14 N3369 and N3469 (_Countof) (PR #133125)

[via cfe-commits]

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+// RUN: %clang_cc1 -fsyntax-only -std=c2y -pedantic -Wall -Wno-comment -verify %s
+
+/* WG14 N3369: Clang 21
+ * _Lengthof operator
+ *
+ * Adds an operator to get the length of an array. Note that WG14 N3469 renamed
+ * this operator to _Countof.
+ */
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cor3ntin wrote:

More tests for you to consider:

 - unspecified arrays length
 - arrays of rank 3
 - `_Countof array`
 - typedefs
 - `static_assert(__is_same_as(typeof(_Countof(foo)), size_t))`
 - Constant folding tests

https://github.com/llvm/llvm-project/pull/133125