[C23] Complete support for WG14 N2508 (#71398)

In Clang 16, we implemented the ability to add a label at the end of a
compound statement. These changes complete the implementation by
allowing a label to be followed by a declaration in C.

Note, this seems to have fixed an issue with some OpenMP stand-alone
directives not being properly diagnosed as per:
https://www.openmp.org/spec-html/5.1/openmpsu19.html#x34-330002.1.3
(The same requirement exists in OpenMP 5.2 as well.)

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -209,6 +209,12 @@ C23 Feature Support
 - Clang now supports ``<stdckdint.h>`` which defines several macros for performing
   checked integer arithmetic. It is also exposed in pre-C23 modes.
 
+- Completed the implementation of
+  `N2508 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2508.pdf>`_. We
+  previously implemented allowing a label at the end of a compound statement,
+  and now we've implemented allowing a label to be followed by a declaration
+  instead of a statement.
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
@@ -566,6 +572,23 @@ Bug Fixes in This Version
   Fixes (`#67687 <https://github.com/llvm/llvm-project/issues/67687>`_)
 - Fix crash from constexpr evaluator evaluating uninitialized arrays as rvalue.
   Fixes (`#67317 <https://github.com/llvm/llvm-project/issues/67317>`_)
+- Clang now properly diagnoses use of stand-alone OpenMP directives after a
+  label (including ``case`` or ``default`` labels).
+
+  Before:
+
+  .. code-block:: c++
+
+    label:
+    #pragma omp barrier // ok
+
+  After:
+
+  .. code-block:: c++
+
+    label:
+    #pragma omp barrier // error: '#pragma omp barrier' cannot be an immediate substatement
+
 - Fixed an issue that a benign assertion might hit when instantiating a pack expansion
   inside a lambda. (`#61460 <https://github.com/llvm/llvm-project/issues/61460>`_)
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -299,6 +299,12 @@ def note_missing_selector_name : Note<
 def note_force_empty_selector_name : Note<
   "or insert whitespace before ':' to use %0 as parameter name "
   "and have an empty entry in the selector">;
+def ext_c_label_followed_by_declaration : ExtWarn<
+  "label followed by a declaration is a C23 extension">,
+  InGroup<C23>;
+def warn_c23_compat_label_followed_by_declaration : Warning<
+  "label followed by a declaration is incompatible with C standards before "
+  "C23">, InGroup<CPre23Compat>, DefaultIgnore;
 def ext_c_label_end_of_compound_statement : ExtWarn<
   "label at end of compound statement is a C23 extension">,
    InGroup<C23>;

clang/include/clang/Parse/Parser.h

@@ -415,18 +415,15 @@ class Parser : public CodeCompletionHandler {
 
   /// Flags describing a context in which we're parsing a statement.
   enum class ParsedStmtContext {
-    /// This context permits declarations in language modes where declarations
-    /// are not statements.
-    AllowDeclarationsInC = 0x1,
     /// This context permits standalone OpenMP directives.
-    AllowStandaloneOpenMPDirectives = 0x2,
+    AllowStandaloneOpenMPDirectives = 0x1,
     /// This context is at the top level of a GNU statement expression.
-    InStmtExpr = 0x4,
+    InStmtExpr = 0x2,
 
     /// The context of a regular substatement.
     SubStmt = 0,
     /// The context of a compound-statement.
-    Compound = AllowDeclarationsInC | AllowStandaloneOpenMPDirectives,
+    Compound = AllowStandaloneOpenMPDirectives,
 
     LLVM_MARK_AS_BITMASK_ENUM(InStmtExpr)
   };

clang/lib/Parse/ParseOpenMP.cpp

@@ -2670,7 +2670,7 @@ StmtResult Parser::ParseOpenMPDeclarativeOrExecutableDirective(
   }
   case OMPD_threadprivate: {
     // FIXME: Should this be permitted in C++?
-    if ((StmtCtx & ParsedStmtContext::AllowDeclarationsInC) ==
+    if ((StmtCtx & ParsedStmtContext::AllowStandaloneOpenMPDirectives) ==
         ParsedStmtContext()) {
       Diag(Tok, diag::err_omp_immediate_directive)
           << getOpenMPDirectiveName(DKind) << 0;
@@ -2689,7 +2689,7 @@ StmtResult Parser::ParseOpenMPDeclarativeOrExecutableDirective(
   }
   case OMPD_allocate: {
     // FIXME: Should this be permitted in C++?
-    if ((StmtCtx & ParsedStmtContext::AllowDeclarationsInC) ==
+    if ((StmtCtx & ParsedStmtContext::AllowStandaloneOpenMPDirectives) ==
         ParsedStmtContext()) {
       Diag(Tok, diag::err_omp_immediate_directive)
           << getOpenMPDirectiveName(DKind) << 0;

clang/lib/Parse/ParseStmt.cpp

@@ -235,10 +235,7 @@ StmtResult Parser::ParseStatementOrDeclarationAfterAttributes(
     auto IsStmtAttr = [](ParsedAttr &Attr) { return Attr.isStmtAttr(); };
     bool AllAttrsAreStmtAttrs = llvm::all_of(CXX11Attrs, IsStmtAttr) &&
                                 llvm::all_of(GNUAttrs, IsStmtAttr);
-    if ((getLangOpts().CPlusPlus || getLangOpts().MicrosoftExt ||
-         (StmtCtx & ParsedStmtContext::AllowDeclarationsInC) !=
-             ParsedStmtContext()) &&
-        ((GNUAttributeLoc.isValid() && !(HaveAttrs && AllAttrsAreStmtAttrs)) ||
+    if (((GNUAttributeLoc.isValid() && !(HaveAttrs && AllAttrsAreStmtAttrs)) ||
          isDeclarationStatement())) {
       SourceLocation DeclStart = Tok.getLocation(), DeclEnd;
       DeclGroupPtrTy Decl;
@@ -701,6 +698,18 @@ StmtResult Parser::ParseSEHLeaveStatement() {
   return Actions.ActOnSEHLeaveStmt(LeaveLoc, getCurScope());
 }
 
+static void DiagnoseLabelFollowedByDecl(Parser &P, const Stmt *SubStmt) {
+  // When in C mode (but not Microsoft extensions mode), diagnose use of a
+  // label that is followed by a declaration rather than a statement.
+  if (!P.getLangOpts().CPlusPlus && !P.getLangOpts().MicrosoftExt &&
+      isa<DeclStmt>(SubStmt)) {
+    P.Diag(SubStmt->getBeginLoc(),
+           P.getLangOpts().C23
+               ? diag::warn_c23_compat_label_followed_by_declaration
+               : diag::ext_c_label_followed_by_declaration);
+  }
+}
+
 /// ParseLabeledStatement - We have an identifier and a ':' after it.
 ///
 ///       label:
@@ -715,9 +724,10 @@ StmtResult Parser::ParseLabeledStatement(ParsedAttributes &Attrs,
   assert(Tok.is(tok::identifier) && Tok.getIdentifierInfo() &&
          "Not an identifier!");
 
-  // The substatement is always a 'statement', not a 'declaration', but is
-  // otherwise in the same context as the labeled-statement.
-  StmtCtx &= ~ParsedStmtContext::AllowDeclarationsInC;
+  // [OpenMP 5.1] 2.1.3: A stand-alone directive may not be used in place of a
+  // substatement in a selection statement, in place of the loop body in an
+  // iteration statement, or in place of the statement that follows a label.
+  StmtCtx &= ~ParsedStmtContext::AllowStandaloneOpenMPDirectives;
 
   Token IdentTok = Tok;  // Save the whole token.
   ConsumeToken();  // eat the identifier.
@@ -766,6 +776,8 @@ StmtResult Parser::ParseLabeledStatement(ParsedAttributes &Attrs,
   if (SubStmt.isInvalid())
     SubStmt = Actions.ActOnNullStmt(ColonLoc);
 
+  DiagnoseLabelFollowedByDecl(*this, SubStmt.get());
+
   LabelDecl *LD = Actions.LookupOrCreateLabel(IdentTok.getIdentifierInfo(),
                                               IdentTok.getLocation());
   Actions.ProcessDeclAttributeList(Actions.CurScope, LD, Attrs);
@@ -784,9 +796,10 @@ StmtResult Parser::ParseCaseStatement(ParsedStmtContext StmtCtx,
                                       bool MissingCase, ExprResult Expr) {
   assert((MissingCase || Tok.is(tok::kw_case)) && "Not a case stmt!");
 
-  // The substatement is always a 'statement', not a 'declaration', but is
-  // otherwise in the same context as the labeled-statement.
-  StmtCtx &= ~ParsedStmtContext::AllowDeclarationsInC;
+  // [OpenMP 5.1] 2.1.3: A stand-alone directive may not be used in place of a
+  // substatement in a selection statement, in place of the loop body in an
+  // iteration statement, or in place of the statement that follows a label.
+  StmtCtx &= ~ParsedStmtContext::AllowStandaloneOpenMPDirectives;
 
   // It is very common for code to contain many case statements recursively
   // nested, as in (but usually without indentation):
@@ -912,6 +925,7 @@ StmtResult Parser::ParseCaseStatement(ParsedStmtContext StmtCtx,
     // Broken sub-stmt shouldn't prevent forming the case statement properly.
     if (SubStmt.isInvalid())
       SubStmt = Actions.ActOnNullStmt(SourceLocation());
+    DiagnoseLabelFollowedByDecl(*this, SubStmt.get());
     Actions.ActOnCaseStmtBody(DeepestParsedCaseStmt, SubStmt.get());
   }
 
@@ -927,9 +941,10 @@ StmtResult Parser::ParseCaseStatement(ParsedStmtContext StmtCtx,
 StmtResult Parser::ParseDefaultStatement(ParsedStmtContext StmtCtx) {
   assert(Tok.is(tok::kw_default) && "Not a default stmt!");
 
-  // The substatement is always a 'statement', not a 'declaration', but is
-  // otherwise in the same context as the labeled-statement.
-  StmtCtx &= ~ParsedStmtContext::AllowDeclarationsInC;
+  // [OpenMP 5.1] 2.1.3: A stand-alone directive may not be used in place of a
+  // substatement in a selection statement, in place of the loop body in an
+  // iteration statement, or in place of the statement that follows a label.
+  StmtCtx &= ~ParsedStmtContext::AllowStandaloneOpenMPDirectives;
 
   SourceLocation DefaultLoc = ConsumeToken();  // eat the 'default'.
 
@@ -963,6 +978,7 @@ StmtResult Parser::ParseDefaultStatement(ParsedStmtContext StmtCtx) {
   if (SubStmt.isInvalid())
     SubStmt = Actions.ActOnNullStmt(ColonLoc);
 
+  DiagnoseLabelFollowedByDecl(*this, SubStmt.get());
   return Actions.ActOnDefaultStmt(DefaultLoc, ColonLoc,
                                   SubStmt.get(), getCurScope());
 }

clang/test/C/C2x/n2508.c

@@ -1,23 +1,50 @@
-// RUN: %clang_cc1 -verify -std=c2x %s
+// RUN: %clang_cc1 -verify -std=c23 %s
+// RUN: %clang_cc1 -verify=pedantic -std=c11 -pedantic %s
+// RUN: %clang_cc1 -verify=compat -std=c23 -Wpre-c23-compat %s
 
 // expected-no-diagnostics
 
 /* WG14 N2508: yes
  * Free positioning of labels inside compound statements
  */
-void test() {
+void test(void) {
   {
   inner:
-  }
+  } /* pedantic-warning {{label at end of compound statement is a C23 extension}}
+       compat-warning {{label at end of compound statement is incompatible with C standards before C23}}
+     */
 
   switch (1) {
   case 1:
-  }
+  } /* pedantic-warning {{label at end of compound statement is a C23 extension}}
+       compat-warning {{label at end of compound statement is incompatible with C standards before C23}}
+     */
 
   {
   multiple: labels: on: a: line:
-  }
+  } /* pedantic-warning {{label at end of compound statement is a C23 extension}}
+       compat-warning {{label at end of compound statement is incompatible with C standards before C23}}
+     */
 
 final:
-}
+} /* pedantic-warning {{label at end of compound statement is a C23 extension}}
+     compat-warning {{label at end of compound statement is incompatible with C standards before C23}}
+   */
 
+void test_labels(void) {
+label:
+  int i = 0; /* pedantic-warning {{label followed by a declaration is a C23 extension}}
+                compat-warning {{label followed by a declaration is incompatible with C standards before C23}}
+              */
+
+  switch (i) {
+  case 1:
+    _Static_assert(1, ""); /* pedantic-warning {{label followed by a declaration is a C23 extension}}
+                              compat-warning {{label followed by a declaration is incompatible with C standards before C23}}
+                            */
+  default:
+    _Static_assert(1, ""); /* pedantic-warning {{label followed by a declaration is a C23 extension}}
+                              compat-warning {{label followed by a declaration is incompatible with C standards before C23}}
+                            */
+  }
+}

clang/test/OpenMP/barrier_ast_print.cpp

@@ -17,13 +17,13 @@ T tmain(T argc) {
   static T a;
 #pragma omp barrier
   switch (argc) {
-  case 0:
+  case 0: {
 #pragma omp barrier
-    break;
-  default:
+  } break;
+  default: {
 #pragma omp barrier
 #pragma omp barrier
-    break;
+  } break;
   }
   return a + argc;
 }
@@ -35,11 +35,15 @@ T tmain(T argc) {
 // CHECK-NEXT: #pragma omp barrier
 // CHECK-NEXT: switch (argc) {
 // CHECK-NEXT: case 0:
+// CHECK-NEXT: {
 // CHECK-NEXT: #pragma omp barrier
+// CHECK-NEXT: }
 // CHECK-NEXT: break;
 // CHECK-NEXT: default:
+// CHECK-NEXT: {
 // CHECK-NEXT: #pragma omp barrier
 // CHECK-NEXT: #pragma omp barrier
+// CHECK-NEXT: }
 // CHECK-NEXT: break;
 // CHECK-NEXT: }
 
@@ -49,21 +53,25 @@ int main(int argc, char **argv) {
 #pragma omp barrier
   // CHECK-NEXT: #pragma omp barrier
   switch (argc) {
-  case 0:
+  case 0: {
 #pragma omp barrier
 #pragma omp barrier
-    break;
-  default:
+  } break;
+  default: {
 #pragma omp barrier
-    break;
+  } break;
   }
 // CHECK-NEXT: switch (argc) {
 // CHECK-NEXT: case 0:
+// CHECK-NEXT: {
 // CHECK-NEXT: #pragma omp barrier
 // CHECK-NEXT: #pragma omp barrier
+// CHECK-NEXT: }
 // CHECK-NEXT: break;
 // CHECK-NEXT: default:
+// CHECK-NEXT: {
 // CHECK-NEXT: #pragma omp barrier
+// CHECK-NEXT: }
 // CHECK-NEXT: break;
 // CHECK-NEXT: }
   return tmain(argc) + tmain(argv[0][0]) + a;

clang/test/OpenMP/barrier_messages.cpp

@@ -38,7 +38,7 @@ T tmain(T argc) {
   switch (argc) {
 #pragma omp barrier
   case 1:
-#pragma omp barrier
+#pragma omp barrier // expected-error {{'#pragma omp barrier' cannot be an immediate substatement}}
     break;
   default: {
 #pragma omp barrier
@@ -50,7 +50,7 @@ T tmain(T argc) {
 #pragma omp barrier
     }
 label:
-#pragma omp barrier
+#pragma omp barrier // expected-error {{'#pragma omp barrier' cannot be an immediate substatement}}
 label1 : {
 #pragma omp barrier
 }
@@ -92,7 +92,7 @@ int main(int argc, char **argv) {
   switch (argc) {
 #pragma omp barrier
   case 1:
-#pragma omp barrier
+#pragma omp barrier // expected-error {{'#pragma omp barrier' cannot be an immediate substatement}}
     break;
   default: {
 #pragma omp barrier
@@ -104,7 +104,7 @@ int main(int argc, char **argv) {
 #pragma omp barrier
     }
 label:
-#pragma omp barrier
+#pragma omp barrier // expected-error {{'#pragma omp barrier' cannot be an immediate substatement}}
 label1 : {
 #pragma omp barrier
 }

clang/test/OpenMP/cancel_messages.cpp

@@ -71,7 +71,8 @@ int main(int argc, char **argv) {
   switch (argc) {
 #pragma omp cancel taskgroup // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
   case 1:
-#pragma omp cancel parallel // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
+#pragma omp cancel parallel // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}} \
+                               expected-error {{'#pragma omp cancel' cannot be an immediate substatement}}
     break;
   default: {
 #pragma omp cancel sections // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
@@ -83,7 +84,8 @@ int main(int argc, char **argv) {
 #pragma omp cancel taskgroup // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
     }
 label:
-#pragma omp cancel parallel // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
+#pragma omp cancel parallel // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}} \
+                               expected-error {{'#pragma omp cancel' cannot be an immediate substatement}}
 label1 : {
 #pragma omp cancel sections // expected-error {{orphaned 'omp cancel' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
 }

clang/test/OpenMP/cancellation_point_messages.cpp

@@ -71,7 +71,8 @@ int main(int argc, char **argv) {
   switch (argc) {
 #pragma omp cancellation point taskgroup // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
   case 1:
-#pragma omp cancellation point parallel // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
+#pragma omp cancellation point parallel // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}} \
+                                           expected-error {{'#pragma omp cancellation point' cannot be an immediate substatement}}
     break;
   default: {
 #pragma omp cancellation point sections // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
@@ -83,7 +84,8 @@ int main(int argc, char **argv) {
 #pragma omp cancellation point taskgroup // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
     }
 label:
-#pragma omp cancellation point parallel // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
+#pragma omp cancellation point parallel // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}} \
+                                           expected-error {{'#pragma omp cancellation point' cannot be an immediate substatement}}
 label1 : {
 #pragma omp cancellation point sections // expected-error {{orphaned 'omp cancellation point' directives are prohibited; perhaps you forget to enclose the directive into a region?}}
 }