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

[James Y Knight via cfe-commits]

================
@@ -0,0 +1,61 @@
+// 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.
+ */
+
+int global_array[12];
+
+void test_parsing_failures() {
+  (void)_Countof;     // expected-error {{expected expression}}
+  (void)_Countof(;    // expected-error {{expected expression}}
+  (void)_Countof();   // expected-error {{expected expression}}
+  (void)_Countof int; // expected-error {{expected expression}}
+}
+
+void test_semantic_failures() {
+  (void)_Countof(1);         // expected-error {{'_Countof' requires an argument of array type; 'int' invalid}}
+  int non_array;
+  (void)_Countof non_array;  // expected-error {{'_Countof' requires an argument of array type; 'int' invalid}}  
+  (void)_Countof(int);       // expected-error {{'_Countof' requires an argument of array type; 'int' invalid}}  
+}
+
+void test_constant_expression_behavior(int n) {
+  static_assert(_Countof(global_array) == 12);
+  static_assert(_Countof global_array == 12);  
+  static_assert(_Countof(int[12]) == 12);
+
+  // Use of a VLA makes it not a constant expression, same as with sizeof.
+  int array[n];
+  static_assert(_Countof(array)); // expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(sizeof(array));   // expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(_Countof(int[n]));// expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(sizeof(int[n]));  // expected-error {{static assertion expression is not an integral constant expression}}
+  
+  // Constant folding works the same way as sizeof, too.
+  const int m = 12;
+  int other_array[m];
+  static_assert(sizeof(other_array));   // expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(_Countof(other_array)); // expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(sizeof(int[m]));        // expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(_Countof(int[m]));      // expected-error {{static assertion expression is not an integral constant expression}}
+  
+  // Note that this applies to each array dimension.
+  int another_array[n][7];
+  static_assert(_Countof(another_array)); // expected-error {{static assertion expression is not an integral constant expression}}
+  static_assert(_Countof(*another_array) == 7);
+}
+
+void test_with_function_param(int array[12], int (*array_ptr)[12]) {
----------------
jyknight wrote:

I presume `void fn(int array[static 12])` also doesn't work, as it also doesn't get an array type then (which always seemed really odd to me but _shrug_.)

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