[C2y] Implement WG14 N3344 (llvm#115313)

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3344.pdf

This paper disallows a single `void` parameter from having qualifiers or
storage class specifiers. Clang has diagnosed most of these as an error
for a long time, but `register void` was previously accepted in all C
language modes and is now being rejected in all C language modes.

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -280,6 +280,12 @@ C2y Feature Support
   this is now a C2y extension in C. ``-Wgnu-case-range`` still applies in C++
   modes.
 
+- Clang implemented support for `N3344 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3344.pdf>`_
+  which disallows a ``void`` parameter from having a qualifier or storage class
+  specifier. Note that ``register void`` was previously accepted in all C
+  language modes but is now rejected (all of the other qualifiers and storage
+  class specifiers were previously rejected).
+
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaDecl.cpp

@@ -15002,6 +15002,12 @@ Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D,
   const DeclSpec &DS = D.getDeclSpec();
 
   // Verify C99 6.7.5.3p2: The only SCS allowed is 'register'.
+  // C2y 6.7.7.4p4: A parameter declaration shall not specify a void type,
+  // except for the special case of a single unnamed parameter of type void
+  // with no storage class specifier, no type qualifier, and no following
+  // ellipsis terminator.
+  // Clang applies the C2y rules for 'register void' in all C language modes,
+  // same as GCC, because it's questionable what that could possibly mean.
 
   // C++03 [dcl.stc]p2 also permits 'auto'.
   StorageClass SC = SC_None;
@@ -15010,10 +15016,16 @@ Decl *Sema::ActOnParamDeclarator(Scope *S, Declarator &D,
     // In C++11, the 'register' storage class specifier is deprecated.
     // In C++17, it is not allowed, but we tolerate it as an extension.
     if (getLangOpts().CPlusPlus11) {
+      Diag(DS.getStorageClassSpecLoc(), getLangOpts().CPlusPlus17
+                                            ? diag::ext_register_storage_class
+                                            : diag::warn_deprecated_register)
+          << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+    } else if (!getLangOpts().CPlusPlus &&
+               DS.getTypeSpecType() == DeclSpec::TST_void) {
       Diag(DS.getStorageClassSpecLoc(),
-           getLangOpts().CPlusPlus17 ? diag::ext_register_storage_class
-                                     : diag::warn_deprecated_register)
-        << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+           diag::err_invalid_storage_class_in_func_decl)
+          << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+      D.getMutableDeclSpec().ClearStorageClassSpecs();
     }
   } else if (getLangOpts().CPlusPlus &&
              DS.getStorageClassSpec() == DeclSpec::SCS_auto) {

clang/test/C/C2y/n3344.c

@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic %s
+// RUN: %clang_cc1 -verify -Wall -pedantic %s
+
+/* WG14 N3344: Yes
+ * Slay Some Earthly Demons VI
+ *
+ * A 'void' parameter cannot have any qualifiers, storage class specifiers, or
+ * be followed by an ellipsis.
+ *
+ * Note: Clang treats 'register void' as being a DR and rejects it in all
+ * language modes; there's no evidence that this will break users and it's not
+ * clear what the programmer intended if they wrote such code anyway. This
+ * matches GCC's behavior.
+ */
+
+void baz(volatile void);         // expected-error {{'void' as parameter must not have type qualifiers}}
+void bar(const void);            // expected-error {{'void' as parameter must not have type qualifiers}}
+void foo(register void);         // expected-error {{invalid storage class specifier in function declarator}}
+void quux(static void);          // expected-error {{invalid storage class specifier in function declarator}}
+void quobble(auto void);         // expected-error {{invalid storage class specifier in function declarator}}
+void quubble(extern void);       // expected-error {{invalid storage class specifier in function declarator}}
+// FIXME: it's odd that these aren't diagnosed as storage class specifiers.
+#if __STDC_VERSION__ >= 202311L
+void quibble(constexpr void);    // expected-error {{function parameter cannot be constexpr}}
+#endif
+void quabble(_Thread_local void); // expected-error {{'_Thread_local' is only allowed on variable declarations}}
+void bing(void, ...);            // expected-error {{'void' must be the first and only parameter if specified}}
+

clang/www/c_status.html

@@ -201,7 +201,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons VI</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3344.pdf">N3344</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="unreleased" align="center">Clang 20</td>
     </tr>
     <tr>
       <td>Slay Some Earthly Demons VII</td>