[C2y] Implement WG14 N3409

This paper removes UB around use of void expressions. Previously, code
like this had undefined behavior:

  void foo(void) {
    (void)(void)1;
    extern void x;
    x;
  }

and this is now well-defined in C2y. Functionally, this now means that
it is valid to use `void` as a `_Generic` association.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -114,6 +114,10 @@ C2y Feature Support
 - Implemented N3411 which allows a source file to not end with a newline
   character. This is still reported as a conforming extension in earlier
   language modes.
+- Implement `WG14 N3409 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3409.pdf>`_
+  which removes UB around use of ``void`` expressions. In practice, this means
+  that ``_Generic`` selection associations may now have ``void`` type, but it
+  also removes UB with code like ``(void)(void)1;``.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10425,8 +10425,13 @@ def warn_type_safety_null_pointer_required : Warning<
   "specified %0 type tag requires a null pointer">, InGroup<TypeSafety>;
 
 // Generic selections.
-def err_assoc_type_incomplete : Error<
-  "type %0 in generic association incomplete">;
+def ext_assoc_type_incomplete : Extension<
+  "ISO C requires a complete type in a '_Generic' association; %0 is an "
+  "incomplete type">;
+def warn_c2y_compat_assoc_type_incomplete : Warning<
+  "use of an incomplete type in a '_Generic' association is incompatible with "
+  "C standards before C2y; %0 is an incomplete type">,
+  InGroup<CPre2yCompat>, DefaultIgnore;
 def err_assoc_type_nonobject : Error<
   "type %0 in generic association not an object type">;
 def err_assoc_type_variably_modified : Error<

clang/lib/Sema/SemaExpr.cpp

@@ -1748,9 +1748,14 @@ ExprResult Sema::CreateGenericSelectionExpr(
         //
         // C11 6.5.1.1p2 "The type name in a generic association shall specify a
         // complete object type other than a variably modified type."
+        // C2y removed the requirement that an expression form must
+        // use a complete type, though it's still as-if the type has undergone
+        // lvalue conversion. We support this as an extension in C23 and
+        // earlier because GCC does so.
         unsigned D = 0;
         if (ControllingExpr && Types[i]->getType()->isIncompleteType())
-          D = diag::err_assoc_type_incomplete;
+          D = LangOpts.C2y ? diag::warn_c2y_compat_assoc_type_incomplete
+                           : diag::ext_assoc_type_incomplete;
         else if (ControllingExpr && !Types[i]->getType()->isObjectType())
           D = diag::err_assoc_type_nonobject;
         else if (Types[i]->getType()->isVariablyModifiedType())

clang/test/C/C2y/n3409.c

@@ -0,0 +1,32 @@
+// 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}}
+                                      */
+}
+