[C2x] Disallow functions without prototypes/functions with identifier lists

WG14 has elected to remove support for K&R C functions in C2x. The
feature was introduced into C89 already deprecated, so after this long
of a deprecation period, the committee has made an empty parameter list
mean the same thing in C as it means in C++: the function accepts no
arguments exactly as if the function were written with (void) as the
parameter list.

This patch implements WG14 N2841 No function declarators without
prototypes (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2841.htm)
and WG14 N2432 Remove support for function definitions with identifier
lists (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2432.pdf).

It also adds The -fno-knr-functions command line option to opt into
this behavior in other language modes.

Differential Revision: https://reviews.llvm.org/D123955

clang/docs/ReleaseNotes.rst

@@ -176,6 +176,11 @@ Non-comprehensive list of changes in this release
 
 New Compiler Flags
 ------------------
+- Added the ``-fno-knr-functions`` flag to allow users to opt into the C2x
+  behavior where a function with an empty parameter list is treated as though
+  the parameter list were ``void``. There is no ``-fknr-functions`` or
+  ``-fno-no-knr-functions`` flag; this feature cannot be disabled in language
+  modes where it is required, such as C++ or C2x.
 
 Deprecated Compiler Flags
 -------------------------
@@ -239,6 +244,8 @@ C2x Feature Support
 - Removed support for implicit function declarations. This was a C89 feature
   that was removed in C99, but cannot be supported in C2x because it requires
   support for functions without prototypes, which no longer exist in C2x.
+- Implemented `WG14 N2841 No function declarators without prototypes <https://www9.open-std.org/jtc1/sc22/wg14/www/docs/n2841.htm>`_
+  and `WG14 N2432 Remove support for function definitions with identifier lists <https://www9.open-std.org/jtc1/sc22/wg14/www/docs/n2432.pdf>`_.
 
 C++ Language Changes in Clang
 -----------------------------

clang/include/clang/Basic/LangOptions.def

@@ -121,6 +121,7 @@ LANGOPT(GNUMode           , 1, 1, "GNU extensions")
 LANGOPT(GNUKeywords       , 1, 1, "GNU keywords")
 VALUE_LANGOPT(GNUCVersion , 32, 0, "GNU C compatibility version")
 BENIGN_LANGOPT(ImplicitInt, 1, 0, "C89 implicit 'int'")
+LANGOPT(DisableKNRFunctions, 1, 0, "require function types to have a prototype")
 LANGOPT(Digraphs          , 1, 0, "digraphs")
 BENIGN_LANGOPT(HexFloats  , 1, 0, "C99 hexadecimal float constants")
 LANGOPT(CXXOperatorNames  , 1, 0, "C++ operator name keywords")

clang/include/clang/Basic/LangOptions.h

@@ -521,6 +521,12 @@ public:
   /// as a string.
   std::string getOpenCLVersionString() const;
 
+  /// Returns true if functions without prototypes or functions with an
+  /// identifier list (aka K&R C functions) are not allowed.
+  bool requiresStrictPrototypes() const {
+    return CPlusPlus || C2x || DisableKNRFunctions;
+  }
+
   /// Check if return address signing is enabled.
   bool hasSignReturnAddress() const {
     return getSignReturnAddressScope() != SignReturnAddressScopeKind::None;

clang/include/clang/Driver/Options.td

@@ -2297,6 +2297,11 @@ defm implicit_modules : BoolFOption<"implicit-modules",
 def fretain_comments_from_system_headers : Flag<["-"], "fretain-comments-from-system-headers">, Group<f_Group>, Flags<[CC1Option]>,
   MarshallingInfoFlag<LangOpts<"RetainCommentsFromSystemHeaders">>;
 
+def fno_knr_functions : Flag<["-"], "fno-knr-functions">, Group<f_Group>,
+  MarshallingInfoFlag<LangOpts<"DisableKNRFunctions">>,
+  HelpText<"Disable support for K&R C function declarations">,
+  Flags<[CC1Option, CoreOption]>;
+
 def fmudflapth : Flag<["-"], "fmudflapth">, Group<f_Group>;
 def fmudflap : Flag<["-"], "fmudflap">, Group<f_Group>;
 def fnested_functions : Flag<["-"], "fnested-functions">, Group<f_Group>;

clang/lib/AST/ASTContext.cpp

@@ -4239,6 +4239,13 @@ static bool isCanonicalResultType(QualType T) {
 QualType
 ASTContext::getFunctionNoProtoType(QualType ResultTy,
                                    const FunctionType::ExtInfo &Info) const {
+  // FIXME: This assertion cannot be enabled (yet) because the ObjC rewriter
+  // functionality creates a function without a prototype regardless of
+  // language mode (so it makes them even in C++). Once the rewriter has been
+  // fixed, this assertion can be enabled again.
+  //assert(!LangOpts.requiresStrictPrototypes() &&
+  //       "strict prototypes are disabled");
+
   // Unique functions, to guarantee there is only one function of a particular
   // structure.
   llvm::FoldingSetNodeID ID;
@@ -11235,7 +11242,7 @@ QualType ASTContext::GetBuiltinType(unsigned Id,
 
 
   // We really shouldn't be making a no-proto type here.
-  if (ArgTypes.empty() && Variadic && !getLangOpts().CPlusPlus)
+  if (ArgTypes.empty() && Variadic && !getLangOpts().requiresStrictPrototypes())
     return getFunctionNoProtoType(ResType, EI);
 
   FunctionProtoType::ExtProtoInfo EPI;

clang/lib/Parse/ParseDecl.cpp

@@ -6661,8 +6661,11 @@ void Parser::ParseFunctionDeclarator(Declarator &D,
     else if (RequiresArg)
       Diag(Tok, diag::err_argument_required_after_attribute);
 
-    HasProto = ParamInfo.size() || getLangOpts().CPlusPlus
-                                || getLangOpts().OpenCL;
+    // OpenCL disallows functions without a prototype, but it doesn't enforce
+    // strict prototypes as in C2x because it allows a function definition to
+    // have an identifier list. See OpenCL 3.0 6.11/g for more details.
+    HasProto = ParamInfo.size() || getLangOpts().requiresStrictPrototypes() ||
+               getLangOpts().OpenCL;
 
     // If we have the closing ')', eat it.
     Tracker.consumeClose();
@@ -6799,7 +6802,7 @@ bool Parser::ParseRefQualifier(bool &RefQualifierIsLValueRef,
 /// Note that identifier-lists are only allowed for normal declarators, not for
 /// abstract-declarators.
 bool Parser::isFunctionDeclaratorIdentifierList() {
-  return !getLangOpts().CPlusPlus
+  return !getLangOpts().requiresStrictPrototypes()
          && Tok.is(tok::identifier)
          && !TryAltiVecVectorToken()
          // K&R identifier lists can't have typedefs as identifiers, per C99
@@ -6833,6 +6836,10 @@ bool Parser::isFunctionDeclaratorIdentifierList() {
 void Parser::ParseFunctionDeclaratorIdentifierList(
        Declarator &D,
        SmallVectorImpl<DeclaratorChunk::ParamInfo> &ParamInfo) {
+  // We should never reach this point in C2x or C++.
+  assert(!getLangOpts().requiresStrictPrototypes() &&
+         "Cannot parse an identifier list in C2x or C++");
+
   // If there was no identifier specified for the declarator, either we are in
   // an abstract-declarator, or we are in a parameter declarator which was found
   // to be abstract.  In abstract-declarators, identifier lists are not valid:

clang/lib/Sema/SemaDecl.cpp

@@ -8822,6 +8822,9 @@ static FunctionDecl *CreateNewFunctionDecl(Sema &SemaRef, Declarator &D,
         (D.getDeclSpec().isTypeRep() &&
          D.getDeclSpec().getRepAsType().get()->isFunctionProtoType()) ||
         (!R->getAsAdjusted<FunctionType>() && R->isFunctionProtoType());
+    assert(
+        (HasPrototype || !SemaRef.getLangOpts().requiresStrictPrototypes()) &&
+        "Strict prototypes are required");
 
     NewFD = FunctionDecl::Create(
         SemaRef.Context, DC, D.getBeginLoc(), NameInfo, R, TInfo, SC,

clang/lib/Sema/SemaType.cpp

@@ -5270,8 +5270,11 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
       FunctionType::ExtInfo EI(
           getCCForDeclaratorChunk(S, D, DeclType.getAttrs(), FTI, chunkIndex));
 
-      if (!FTI.NumParams && !FTI.isVariadic && !LangOpts.CPlusPlus
-                                            && !LangOpts.OpenCL) {
+      // OpenCL disallows functions without a prototype, but it doesn't enforce
+      // strict prototypes as in C2x because it allows a function definition to
+      // have an identifier list. See OpenCL 3.0 6.11/g for more details.
+      if (!FTI.NumParams && !FTI.isVariadic &&
+          !LangOpts.requiresStrictPrototypes() && !LangOpts.OpenCL) {
         // Simple void foo(), where the incoming T is the result type.
         T = Context.getFunctionNoProtoType(T, EI);
       } else {
@@ -5290,8 +5293,10 @@ static TypeSourceInfo *GetFullTypeForDeclarator(TypeProcessingState &state,
           S.Diag(FTI.Params[0].IdentLoc,
                  diag::err_ident_list_in_fn_declaration);
           D.setInvalidType(true);
-          // Recover by creating a K&R-style function type.
-          T = Context.getFunctionNoProtoType(T, EI);
+          // Recover by creating a K&R-style function type, if possible.
+          T = (!LangOpts.requiresStrictPrototypes() && !LangOpts.OpenCL)
+                  ? Context.getFunctionNoProtoType(T, EI)
+                  : Context.IntTy;
           break;
         }
 

clang/lib/StaticAnalyzer/Core/MemRegion.cpp

@@ -1033,8 +1033,10 @@ const VarRegion *MemRegionManager::getVarRegion(const VarDecl *D,
             T = TSI->getType();
           if (T.isNull())
             T = getContext().VoidTy;
-          if (!T->getAs<FunctionType>())
-            T = getContext().getFunctionNoProtoType(T);
+          if (!T->getAs<FunctionType>()) {
+            FunctionProtoType::ExtProtoInfo Ext;
+            T = getContext().getFunctionType(T, None, Ext);
+          }
           T = getContext().getBlockPointerType(T);
 
           const BlockCodeRegion *BTR =

clang/test/Driver/no-knr-functions.c

@@ -0,0 +1,11 @@
+// Ensure that we cannot disable strict prototypes, no matter how hard we try.
+
+// RUN: not %clang -fno-no-knr-functions -x c++ %s 2>&1 | FileCheck --check-prefixes=NONO %s
+// RUN: not %clang -fno-no-knr-functions -x c %s 2>&1 | FileCheck --check-prefixes=NONO %s
+// RUN: not %clang -fno-no-knr-functions -std=c89 -x c %s 2>&1 | FileCheck --check-prefixes=NONO %s
+// RUN: not %clang -fknr-functions -x c++ %s 2>&1 | FileCheck --check-prefixes=POS %s
+// RUN: not %clang -fknr-functions -x c %s 2>&1 | FileCheck --check-prefixes=POS %s
+// RUN: not %clang -fknr-functions -std=c89 -x c %s 2>&1 | FileCheck --check-prefixes=POS %s
+
+// NONO: error: unknown argument: '-fno-no-knr-functions'
+// POS: error: unknown argument: '-fknr-functions'

clang/test/Frontend/no-knr-functions.c

@@ -0,0 +1,11 @@
+// Ensure that we cannot disable strict prototypes, no matter how hard we try.
+
+// RUN: not %clang_cc1 -fno-no-knr-functions -x c++ %s 2>&1 | FileCheck --check-prefixes=NONO %s
+// RUN: not %clang_cc1 -fno-no-knr-functions -x c %s 2>&1 | FileCheck --check-prefixes=NONO %s
+// RUN: not %clang_cc1 -fno-no-knr-functions -std=c89 -x c %s 2>&1 | FileCheck --check-prefixes=NONO %s
+// RUN: not %clang_cc1 -fknr-functions -x c++ %s 2>&1 | FileCheck --check-prefixes=POS %s
+// RUN: not %clang_cc1 -fknr-functions -x c %s 2>&1 | FileCheck --check-prefixes=POS %s
+// RUN: not %clang_cc1 -fknr-functions -std=c89 -x c %s 2>&1 | FileCheck --check-prefixes=POS %s
+
+// NONO: error: unknown argument: '-fno-no-knr-functions'
+// POS: error: unknown argument: '-fknr-functions'

clang/test/Parser/c2x-attributes.c

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -fsyntax-only -fdouble-square-bracket-attributes -verify -Wno-strict-prototypes %s
-// RUN: %clang_cc1 -fsyntax-only -std=gnu2x -verify -Wno-strict-prototypes %s
+// RUN: %clang_cc1 -fsyntax-only -fdouble-square-bracket-attributes -verify=expected,notc2x -Wno-strict-prototypes %s
+// RUN: %clang_cc1 -fsyntax-only -std=gnu2x -verify=expected,c2x %s
 
 enum [[]] E {
   One [[]],
@@ -59,7 +59,10 @@ void f4(void) [[]];
 
 void f5(int i [[]], [[]] int j, int [[]] k);
 
-void f6(a, b) [[]] int a; int b; { // expected-error {{an attribute list cannot appear here}}
+void f6(a, b) [[]] int a; int b; { // notc2x-error {{an attribute list cannot appear here}} \
+                                      c2x-warning 2 {{type specifier missing, defaults to 'int'}} \
+                                      c2x-error {{expected ';' after top level declarator}} \
+                                      c2x-error {{expected identifier or '('}}
 }
 
 // FIXME: technically, an attribute list cannot appear here, but we currently
@@ -67,7 +70,10 @@ void f6(a, b) [[]] int a; int b; { // expected-error {{an attribute list cannot
 // behavior given that we *don't* want to parse it as part of the K&R parameter
 // declarations. It is disallowed to avoid a parsing ambiguity we already
 // handle well.
-int (*f7(a, b))(int, int) [[]] int a; int b; {
+int (*f7(a, b))(int, int) [[]] int a; int b; { // c2x-warning 2 {{type specifier missing, defaults to 'int'}} \
+                                                  c2x-error {{expected ';' after top level declarator}} \
+                                                  c2x-error {{expected identifier or '('}}
+
   return 0;
 }
 

clang/test/Parser/c2x-func-prototype.c

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=c2x -std=c2x %s
+// RUN: %clang_cc1 -Wno-strict-prototypes -fsyntax-only -verify -std=c17 %s
+// expected-no-diagnostics
+
+// Functions with an identifier list are not supported in C2x.
+void ident_list(a) // c2x-error {{expected ';' after top level declarator}} \
+                      c2x-warning {{type specifier missing, defaults to 'int'}}
+  int a;
+{}                 // c2x-error {{expected identifier or '('}}
+
+// Functions with an empty parameter list are supported as though the function
+// was declared with a parameter list of (void). Ensure they still parse.
+void no_param_decl();
+void no_param_defn() {}
+void (*var_of_type_with_no_param)();
+typedef void fn();

clang/test/Sema/attr-c2x.c

@@ -20,7 +20,7 @@ void context_async_okay(void *context [[clang::swift_async_context]]) [[clang::s
 void context_async_okay2(void *context [[clang::swift_async_context]], void *selfType, char **selfWitnessTable) [[clang::swiftasynccall]];
 
 [[clang::ownership_returns(foo)]] void *f1(void);
-[[clang::ownership_returns(foo)]] void *f2(); // expected-warning {{'ownership_returns' attribute only applies to non-K&R-style functions}}
+[[clang::ownership_returns(foo)]] void *f2();
 
 [[clang::unavailable("not available - replaced")]] void foo2(void); // expected-note {{'foo2' has been explicitly marked unavailable here}}
 void bar(void) {

clang/test/Sema/c2x-func-prototype.c

@@ -0,0 +1,24 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=c2x -std=c2x %s
+// RUN: %clang_cc1 -Wno-strict-prototypes -fsyntax-only -verify -std=c17 %s
+// RUN: %clang_cc1 -fsyntax-only -fno-knr-functions -std=c99 -verify=c2x %s
+// expected-no-diagnostics
+
+void func(); // c2x-note {{'func' declared here}}
+typedef void (*fp)();
+
+void other_func(int i);
+
+void call(void) {
+  func(1, 2, 3); // c2x-error {{too many arguments to function call, expected 0, have 3}}
+  fp call_me = func;
+  call_me(1, 2, 3); // c2x-error {{too many arguments to function call, expected 0, have 3}}
+
+  fp nope = other_func; // c2x-warning {{incompatible function pointer types initializing 'fp' (aka 'void (*)(void)') with an expression of type 'void (int)'}}
+}
+
+// Ensure these function declarations do not merge in C2x.
+void redecl1();      // c2x-note {{previous declaration is here}}
+void redecl1(int i); // c2x-error {{conflicting types for 'redecl1'}}
+
+void redecl2(int i); // c2x-note {{previous declaration is here}}
+void redecl2();      // c2x-error {{conflicting types for 'redecl2'}}

clang/www/c_status.html

@@ -725,7 +725,7 @@ conformance.</p>
     <tr>
       <td>Remove support for function definitions with identifier lists</td>
       <td><a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2432.pdf">N2432</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 15</td>
     </tr>
     <!-- Oct 2019 Papers -->
     <tr>