[C2y] Implement WG14 N3409 (#130299)

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-tools-extra/clangd/IncludeFixer.cpp

@@ -84,7 +84,6 @@ std::vector<Fix> IncludeFixer::fix(DiagnosticsEngine::Level DiagLevel,
   case diag::err_array_incomplete_or_sizeless_type:
   case diag::err_array_size_incomplete_type:
   case diag::err_asm_incomplete_type:
-  case diag::err_assoc_type_incomplete:
   case diag::err_bad_cast_incomplete:
   case diag::err_call_function_incomplete_return:
   case diag::err_call_incomplete_argument:

clang/docs/ReleaseNotes.rst

@@ -111,6 +111,10 @@ C Language Changes
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^
+- 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;``.
 - Implemented `WG14 N3411 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3411.pdf>`_
   which allows a source file to not end with a newline character. This is still
   reported as a conforming extension in earlier language modes.

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<
+  "incomplete type %0 in a '_Generic' association is a C2y extension">,
+  InGroup<C2y>;
+def warn_c2y_compat_assoc_type_incomplete : Warning<
+  "use of incomplete type %0 in a '_Generic' association is incompatible with "
+  "C standards before C2y">,
+  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/lib/Sema/SemaStmt.cpp

@@ -4052,9 +4052,9 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp,
           Diag(ReturnLoc, D) << CurDecl << isa<CXXDestructorDecl>(CurDecl)
                              << RetValExp->getSourceRange();
         }
-        // return (some void expression); is legal in C++.
+        // return (some void expression); is legal in C++ and C2y.
         else if (D != diag::ext_return_has_void_expr ||
-                 !getLangOpts().CPlusPlus) {
+                 (!getLangOpts().CPlusPlus && !getLangOpts().C2y)) {
           NamedDecl *CurDecl = getCurFunctionOrMethodDecl();
 
           int FunctionKind = 0;

clang/test/C/C2y/n3409.c

@@ -0,0 +1,36 @@
+// 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 incomplete type 'void' in a '_Generic' association is incompatible with C standards before C2y}}
+                                ext-warning {{incomplete type 'void' in a '_Generic' association is a C2y extension}}
+                              */
+  _Generic(x, typeof(x): 1); /* pre-c2y-warning {{use of incomplete type 'typeof (x)' (aka 'void') in a '_Generic' association is incompatible with C standards before C2y}}
+                                ext-warning {{incomplete type 'typeof (x)' (aka 'void') in a '_Generic' association is a C2y extension}}
+                              */
+  (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}}
+                                      */
+
+  // This is not sufficiently important of an extension to warrant a "not
+  // compatible with standards before C2y" warning, but it is an extension in
+  // C23 and earlier.
+  return x; // ext-warning {{void function 'foo' should not return void expression}}
+}

clang/test/Sema/generic-selection-type-extension.c

@@ -37,7 +37,7 @@ static_assert(_Generic(ci, int : 1, const int : 0) == 1); // expected-warning {{
 // but the expression operand form still rejects them.
 static_assert(_Generic(struct incomplete, struct incomplete : 1, default : 0) == 1);
 static_assert(_Generic(struct another_incomplete, struct incomplete : 1, default : 0) == 0);
-static_assert(_Generic(1, struct also_incomplete : 1, default : 0) == 0); // expected-error {{type 'struct also_incomplete' in generic association incomplete}}
+static_assert(_Generic(1, struct also_incomplete : 1, default : 0) == 0);
 
 void foo(int);
 static_assert(_Generic(__typeof__(foo), void(int) : 1, default : 0) == 1);

clang/test/Sema/generic-selection.c

@@ -5,7 +5,7 @@ void g(void);
 
 void foo(int n) {
   (void) _Generic(0, // ext-warning {{'_Generic' is a C11 extension}}
-      struct A: 0, // expected-error {{type 'struct A' in generic association incomplete}}
+      struct A: 0, // ext-warning {{incomplete type 'struct A' in a '_Generic' association is a C2y extension}}
       void(): 0,   // expected-error {{type 'void ()' in generic association not an object type}}
       int[n]: 0);  // expected-error {{type 'int[n]' in generic association is a variably modified type}}
 

clang/test/SemaCXX/generic-selection.cpp

@@ -81,10 +81,10 @@ void func(struct S s) {
   // is an elaborated type specifier followed by the association's value and
   // it should work the same as in C.
   (void)_Generic(s, struct S : 1);
+  (void)_Generic(s, struct T : 1);
 
   // The rest of these cases test that we still produce a reasonable diagnostic
   // when referencing an unknown type or trying to define a type in other ways.
-  (void)_Generic(s, struct T : 1);            // expected-error {{type 'struct T' in generic association incomplete}}
   (void)_Generic(s, struct U { int a; } : 1); // expected-error {{'U' cannot be defined in a type specifier}}
   (void)_Generic(s, struct V : S);            // expected-error {{'S' does not refer to a value}}
   (void)_Generic(s, struct W : S { int b; } : 1); // expected-error {{expected '(' for function-style cast or type construction}}