[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

1 files changed, 10 insertions, 3 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 233f092..2b386c8 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/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: