[clang] [C2y] Implement WG14 N3409 (PR #130299)

[via cfe-commits]

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+// RUN: %clang_cc1 -verify -std=c2y -pedantic %s
+// RUN: %clang_cc1 -verify=pre-c2y -std=c2y -Wpre-c2y-compat %s
+// RUN: %clang_cc1 -verify=ext -std=c23 -pedantic %s
+// expected-no-diagnostics
+
+/* WG14 N3409: Clang 21
+ * Slay Some Earthly Demons X
+ *
+ * Removes the requirement that an expression with type void cannot be used in
+ * any way. This was making it UB to use a void expression in a _Generic
+ * selection expression for no good reason, as well as making it UB to cast a
+ * void expression to void, etc.
+ */
+
+extern void x;
+void foo() {
+  // FIXME: this is technically an extension before C2y and should be diagnosed
+  // under -pedantic.
+  (void)(void)1;
+  // FIXME: same with this.
+  x;
+  _Generic(x, void: 1);      /* pre-c2y-warning {{use of an incomplete type in a '_Generic' association is incompatible with C standards before C2y; 'void' is an incomplete type}}
+                                ext-warning {{ISO C requires a complete type in a '_Generic' association; 'void' is an incomplete type}}
+                              */
+  _Generic(x, typeof(x): 1); /* pre-c2y-warning {{use of an incomplete type in a '_Generic' association is incompatible with C standards before C2y; 'typeof (x)' (aka 'void') is an incomplete type}}
+                                ext-warning {{ISO C requires a complete type in a '_Generic' association; 'typeof (x)' (aka 'void') is an incomplete type}}
+                              */
+  (void)_Generic(void, default : 1); /* pre-c2y-warning {{passing a type argument as the first operand to '_Generic' is incompatible with C standards before C2y}}
+                                        ext-warning {{passing a type argument as the first operand to '_Generic' is a C2y extension}}
+                                      */
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Sirraide wrote:

What about `return x;` in this function (since it returns `void`)?

https://github.com/llvm/llvm-project/pull/130299