[clang][Parse] Add parsing support for C++ attributes on using-declarations

This is a re-application of dc67299 which was reverted in f63adf5b because
it broke the build. The issue should now be fixed.

Attribution note: The original author of this patch is Erik Pilkington.
I'm only trying to land it after rebasing.

Differential Revision: https://reviews.llvm.org/D91630

clang/docs/LanguageExtensions.rst

@@ -632,6 +632,20 @@ Attributes on the ``enum`` declaration do not apply to individual enumerators.
 
 Query for this feature with ``__has_extension(enumerator_attributes)``.
 
+C++11 Attributes on using-declarations
+======================================
+
+Clang allows C++-style ``[[]]`` attributes to be written on using-declarations.
+For instance:
+
+.. code-block:: c++
+
+  [[clang::using_if_exists]] using foo::bar;
+  using foo::baz [[clang::using_if_exists]];
+
+You can test for support for this extension with
+``__has_extension(cxx_attributes_on_using_declarations)``.
+
 'User-Specified' System Frameworks
 ==================================
 

clang/docs/ReleaseNotes.rst

@@ -113,6 +113,9 @@ Attribute Changes in Clang
 
 - ...
 
+- Added support for C++11-style ``[[]]`` attributes on using-declarations, as a
+  clang extension.
+
 Windows Support
 ---------------
 

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -693,6 +693,9 @@ def ext_using_attribute_ns : ExtWarn<
 def err_using_attribute_ns_conflict : Error<
   "attribute with scope specifier cannot follow default scope specifier">;
 def err_attributes_not_allowed : Error<"an attribute list cannot appear here">;
+def ext_cxx11_attr_placement : ExtWarn<
+  "ISO C++ does not allow an attribute list to appear here">,
+  InGroup<DiagGroup<"cxx-attribute-extension">>;
 def err_attributes_misplaced : Error<"misplaced attributes; expected attributes here">;
 def err_l_square_l_square_not_attribute : Error<
   "C++11 only allows consecutive left square brackets when "

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3957,6 +3957,9 @@ def warn_attribute_sentinel_named_arguments : Warning<
 def warn_attribute_sentinel_not_variadic : Warning<
   "'sentinel' attribute only supported for variadic %select{functions|blocks}0">,
   InGroup<IgnoredAttributes>;
+def warn_deprecated_ignored_on_using : Warning<
+  "%0 currently has no effect on a using declaration">,
+  InGroup<IgnoredAttributes>;
 def err_attribute_sentinel_less_than_zero : Error<
   "'sentinel' parameter 1 less than zero">;
 def err_attribute_sentinel_not_zero_or_one : Error<

clang/include/clang/Basic/Features.def

@@ -259,6 +259,7 @@ EXTENSION(gnu_asm, LangOpts.GNUAsm)
 EXTENSION(gnu_asm_goto_with_outputs, LangOpts.GNUAsm)
 EXTENSION(matrix_types, LangOpts.MatrixTypes)
 EXTENSION(matrix_types_scalar_division, true)
+EXTENSION(cxx_attributes_on_using_declarations, LangOpts.CPlusPlus11)
 
 FEATURE(cxx_abi_relative_vtable, LangOpts.CPlusPlus && LangOpts.RelativeCXXABIVTables)
 

clang/include/clang/Parse/Parser.h

@@ -2635,6 +2635,10 @@ private:
   /// locations where attributes are not allowed.
   void DiagnoseAndSkipCXX11Attributes();
 
+  /// Emit warnings for C++11 and C2x attributes that are in a position that
+  /// clang accepts as an extension.
+  void DiagnoseCXX11AttributeExtension(ParsedAttributesWithRange &Attrs);
+
   /// Parses syntax-generic attribute arguments for attributes which are
   /// known to the implementation, and adds them to the given ParsedAttributes
   /// list with the given attribute syntax. Returns the number of arguments
@@ -3061,6 +3065,7 @@ private:
                                        const ParsedTemplateInfo &TemplateInfo,
                                        SourceLocation UsingLoc,
                                        SourceLocation &DeclEnd,
+                                       ParsedAttributesWithRange &Attrs,
                                        AccessSpecifier AS = AS_none);
   Decl *ParseAliasDeclarationAfterDeclarator(
       const ParsedTemplateInfo &TemplateInfo, SourceLocation UsingLoc,

clang/lib/Parse/ParseDecl.cpp

@@ -1650,6 +1650,13 @@ void Parser::ProhibitCXX11Attributes(ParsedAttributesWithRange &Attrs,
   }
 }
 
+void Parser::DiagnoseCXX11AttributeExtension(ParsedAttributesWithRange &Attrs) {
+  for (const ParsedAttr &PA : Attrs) {
+    if (PA.isCXX11Attribute() || PA.isC2xAttribute())
+      Diag(PA.getLoc(), diag::ext_cxx11_attr_placement) << PA << PA.getRange();
+  }
+}
+
 // Usually, `__attribute__((attrib)) class Foo {} var` means that attribute
 // applies to var, not the type Foo.
 // As an exception to the rule, __declspec(align(...)) before the

clang/lib/Parse/ParseDeclCXX.cpp

@@ -497,11 +497,7 @@ Parser::ParseUsingDirectiveOrDeclaration(DeclaratorContext Context,
   }
 
   // Otherwise, it must be a using-declaration or an alias-declaration.
-
+  return ParseUsingDeclaration(Context, TemplateInfo, UsingLoc, DeclEnd, attrs,
-  // Using declarations can't have attributes.
-  ProhibitAttributes(attrs);
-
-  return ParseUsingDeclaration(Context, TemplateInfo, UsingLoc, DeclEnd,
                                AS_none);
 }
 
@@ -627,7 +623,8 @@ bool Parser::ParseUsingDeclarator(DeclaratorContext Context,
   if (getLangOpts().CPlusPlus11 && Context == DeclaratorContext::Member &&
       Tok.is(tok::identifier) &&
       (NextToken().is(tok::semi) || NextToken().is(tok::comma) ||
-       NextToken().is(tok::ellipsis)) &&
+       NextToken().is(tok::ellipsis) || NextToken().is(tok::l_square) ||
+       NextToken().is(tok::kw___attribute)) &&
       D.SS.isNotEmpty() && LastII == Tok.getIdentifierInfo() &&
       !D.SS.getScopeRep()->getAsNamespace() &&
       !D.SS.getScopeRep()->getAsNamespaceAlias()) {
@@ -670,11 +667,10 @@ bool Parser::ParseUsingDeclarator(DeclaratorContext Context,
 ///     alias-declaration: C++11 [dcl.dcl]p1
 ///       'using' identifier attribute-specifier-seq[opt] = type-id ;
 ///
-Parser::DeclGroupPtrTy
+Parser::DeclGroupPtrTy Parser::ParseUsingDeclaration(
-Parser::ParseUsingDeclaration(DeclaratorContext Context,
+    DeclaratorContext Context, const ParsedTemplateInfo &TemplateInfo,
-                              const ParsedTemplateInfo &TemplateInfo,
+    SourceLocation UsingLoc, SourceLocation &DeclEnd,
-                              SourceLocation UsingLoc, SourceLocation &DeclEnd,
+    ParsedAttributesWithRange &PrefixAttrs, AccessSpecifier AS) {
-                              AccessSpecifier AS) {
   // Check for misplaced attributes before the identifier in an
   // alias-declaration.
   ParsedAttributesWithRange MisplacedAttrs(AttrFactory);
@@ -686,6 +682,17 @@ Parser::ParseUsingDeclaration(DeclaratorContext Context,
   ParsedAttributesWithRange Attrs(AttrFactory);
   MaybeParseAttributes(PAKM_GNU | PAKM_CXX11, Attrs);
 
+  // If we had any misplaced attributes from earlier, this is where they
+  // should have been written.
+  if (MisplacedAttrs.Range.isValid()) {
+    Diag(MisplacedAttrs.Range.getBegin(), diag::err_attributes_not_allowed)
+        << FixItHint::CreateInsertionFromRange(
+               Tok.getLocation(),
+               CharSourceRange::getTokenRange(MisplacedAttrs.Range))
+        << FixItHint::CreateRemoval(MisplacedAttrs.Range);
+    Attrs.takeAllFrom(MisplacedAttrs);
+  }
+
   // Maybe this is an alias-declaration.
   if (Tok.is(tok::equal)) {
     if (InvalidDeclarator) {
@@ -693,16 +700,7 @@ Parser::ParseUsingDeclaration(DeclaratorContext Context,
       return nullptr;
     }
 
-    // If we had any misplaced attributes from earlier, this is where they
+    ProhibitAttributes(PrefixAttrs);
-    // should have been written.
-    if (MisplacedAttrs.Range.isValid()) {
-      Diag(MisplacedAttrs.Range.getBegin(), diag::err_attributes_not_allowed)
-        << FixItHint::CreateInsertionFromRange(
-               Tok.getLocation(),
-               CharSourceRange::getTokenRange(MisplacedAttrs.Range))
-        << FixItHint::CreateRemoval(MisplacedAttrs.Range);
-      Attrs.takeAllFrom(MisplacedAttrs);
-    }
 
     Decl *DeclFromDeclSpec = nullptr;
     Decl *AD = ParseAliasDeclarationAfterDeclarator(
@@ -710,10 +708,7 @@ Parser::ParseUsingDeclaration(DeclaratorContext Context,
     return Actions.ConvertDeclToDeclGroup(AD, DeclFromDeclSpec);
   }
 
-  // C++11 attributes are not allowed on a using-declaration, but GNU ones
+  DiagnoseCXX11AttributeExtension(PrefixAttrs);
-  // are.
-  ProhibitAttributes(MisplacedAttrs);
-  ProhibitAttributes(Attrs);
 
   // Diagnose an attempt to declare a templated using-declaration.
   // In C++11, alias-declarations can be templates:
@@ -731,8 +726,10 @@ Parser::ParseUsingDeclaration(DeclaratorContext Context,
 
   SmallVector<Decl *, 8> DeclsInGroup;
   while (true) {
-    // Parse (optional) attributes (most likely GNU strong-using extension).
+    // Parse (optional) attributes.
-    MaybeParseGNUAttributes(Attrs);
+    MaybeParseAttributes(PAKM_GNU | PAKM_CXX11, Attrs);
+    DiagnoseCXX11AttributeExtension(Attrs);
+    Attrs.addAll(PrefixAttrs.begin(), PrefixAttrs.end());
 
     if (InvalidDeclarator)
       SkipUntil(tok::comma, tok::semi, StopBeforeMatch);
@@ -2639,8 +2636,6 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
   MaybeParseMicrosoftAttributes(attrs);
 
   if (Tok.is(tok::kw_using)) {
-    ProhibitAttributes(attrs);
-
     // Eat 'using'.
     SourceLocation UsingLoc = ConsumeToken();
 
@@ -2659,7 +2654,7 @@ Parser::ParseCXXClassMemberDeclaration(AccessSpecifier AS,
     SourceLocation DeclEnd;
     // Otherwise, it must be a using-declaration or an alias-declaration.
     return ParseUsingDeclaration(DeclaratorContext::Member, TemplateInfo,
-                                 UsingLoc, DeclEnd, AS);
+                                 UsingLoc, DeclEnd, attrs, AS);
   }
 
   // Hold late-parsed attributes so we can attach a Decl to them later.

clang/lib/Sema/SemaDeclAttr.cpp

@@ -2446,6 +2446,13 @@ AvailabilityAttr *Sema::mergeAvailabilityAttr(
 }
 
 static void handleAvailabilityAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  if (isa<UsingDecl, UnresolvedUsingTypenameDecl, UnresolvedUsingValueDecl>(
+          D)) {
+    S.Diag(AL.getRange().getBegin(), diag::warn_deprecated_ignored_on_using)
+        << AL;
+    return;
+  }
+
   if (!AL.checkExactlyNumArgs(S, 1))
     return;
   IdentifierLoc *Platform = AL.getArgAsIdent(0);
@@ -7257,6 +7264,11 @@ static void handleDeprecatedAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
       // namespace.
       return;
     }
+  } else if (isa<UsingDecl, UnresolvedUsingTypenameDecl,
+                 UnresolvedUsingValueDecl>(D)) {
+    S.Diag(AL.getRange().getBegin(), diag::warn_deprecated_ignored_on_using)
+        << AL;
+    return;
   }
 
   // Handle the cases where the attribute has a text message.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -12071,9 +12071,9 @@ NamedDecl *Sema::BuildUsingDeclaration(
     return nullptr;
 
   DeclContext *LookupContext = computeDeclContext(SS);
-  NamedDecl *D;
   NestedNameSpecifierLoc QualifierLoc = SS.getWithLocInContext(Context);
   if (!LookupContext || EllipsisLoc.isValid()) {
+    NamedDecl *D;
     if (HasTypenameKeyword) {
       // FIXME: not all declaration name kinds are legal here
       D = UnresolvedUsingTypenameDecl::Create(Context, CurContext,
@@ -12087,6 +12087,7 @@ NamedDecl *Sema::BuildUsingDeclaration(
     }
     D->setAccess(AS);
     CurContext->addDecl(D);
+    ProcessDeclAttributeList(S, D, AttrList);
     return D;
   }
 
@@ -12096,6 +12097,7 @@ NamedDecl *Sema::BuildUsingDeclaration(
                           UsingName, HasTypenameKeyword);
     UD->setAccess(AS);
     CurContext->addDecl(UD);
+    ProcessDeclAttributeList(S, UD, AttrList);
     UD->setInvalidDecl(Invalid);
     return UD;
   };

clang/test/Parser/cxx0x-attributes.cpp

@@ -131,12 +131,12 @@ extern "C++" [[]] { } // expected-error {{an attribute list cannot appear here}}
 [[]] static_assert(true, ""); //expected-error {{an attribute list cannot appear here}}
 [[]] asm(""); // expected-error {{an attribute list cannot appear here}}
 
-[[]] using ns::i; // expected-error {{an attribute list cannot appear here}}
+[[]] using ns::i;
 [[unknown]] using namespace ns; // expected-warning {{unknown attribute 'unknown' ignored}}
 [[noreturn]] using namespace ns; // expected-error {{'noreturn' attribute only applies to functions}}
 namespace [[]] ns2 {} // expected-warning {{attributes on a namespace declaration are a C++17 extension}}
 
-using [[]] alignas(4) [[]] ns::i; // expected-error {{an attribute list cannot appear here}}
+using[[]] alignas(4)[[]] ns::i;          // expected-error {{an attribute list cannot appear here}} expected-error {{'alignas' attribute only applies to variables, data members and tag types}} expected-warning {{ISO C++}}
 using [[]] alignas(4) [[]] foobar = int; // expected-error {{an attribute list cannot appear here}} expected-error {{'alignas' attribute only applies to}}
 
 void bad_attributes_in_do_while() {
@@ -157,7 +157,16 @@ void bad_attributes_in_do_while() {
 [[]] using T = int; // expected-error {{an attribute list cannot appear here}}
 using T [[]] = int; // ok
 template<typename T> using U [[]] = T;
-using ns::i [[]]; // expected-error {{an attribute list cannot appear here}}
+using ns::i [[]];
+using ns::i [[]], ns::i [[]]; // expected-warning {{use of multiple declarators in a single using declaration is a C++17 extension}}
+struct using_in_struct_base {
+  typedef int i, j, k, l;
+};
+struct using_in_struct : using_in_struct_base {
+  [[]] using using_in_struct_base::i;
+  using using_in_struct_base::j [[]];
+  [[]] using using_in_struct_base::k [[]], using_in_struct_base::l [[]]; // expected-warning {{use of multiple declarators in a single using declaration is a C++17 extension}}
+};
 using [[]] ns::i; // expected-error {{an attribute list cannot appear here}}
 using T [[unknown]] = int; // expected-warning {{unknown attribute 'unknown' ignored}}
 using T [[noreturn]] = int; // expected-error {{'noreturn' attribute only applies to functions}}

clang/test/SemaCXX/cxx11-attributes-on-using-declaration.cpp

@@ -0,0 +1,42 @@
+// RUN: %clang_cc1 -pedantic -triple x86_64-apple-macos11 -std=c++20 -fsyntax-only -verify %s
+
+static_assert(__has_extension(cxx_attributes_on_using_declarations), "");
+
+namespace NS { typedef int x; }
+
+[[clang::annotate("foo")]] using NS::x; // expected-warning{{ISO C++ does not allow an attribute list to appear here}}
+
+
+[[deprecated]] using NS::x;                                    // expected-warning {{'deprecated' currently has no effect on a using declaration}} expected-warning{{ISO C++ does not allow}}
+using NS::x [[deprecated]];                                    // expected-warning {{'deprecated' currently has no effect on a using declaration}} expected-warning{{ISO C++ does not allow}}
+using NS::x __attribute__((deprecated));                       // expected-warning {{'deprecated' currently has no effect on a using declaration}}
+using NS::x __attribute__((availability(macos,introduced=1))); // expected-warning {{'availability' currently has no effect on a using declaration}}
+
+[[clang::availability(macos,introduced=1)]] using NS::x; // expected-warning {{'availability' currently has no effect on a using declaration}} expected-warning{{ISO C++ does not allow}}
+
+// expected-warning@+1 3 {{ISO C++ does not allow an attribute list to appear here}}
+[[clang::annotate("A")]] using NS::x [[clang::annotate("Y")]], NS::x [[clang::annotate("Z")]];
+
+template <class T>
+struct S : T {
+  [[deprecated]] using typename T::x; // expected-warning{{ISO C++ does not allow}} expected-warning {{'deprecated' currently has no effect on a using declaration}}
+  [[deprecated]] using T::y;          // expected-warning{{ISO C++ does not allow}} expected-warning {{'deprecated' currently has no effect on a using declaration}}
+
+  using typename T::z [[deprecated]]; // expected-warning{{ISO C++ does not allow}} expected-warning {{'deprecated' currently has no effect on a using declaration}}
+  using T::a [[deprecated]];          // expected-warning{{ISO C++ does not allow}} expected-warning {{'deprecated' currently has no effect on a using declaration}}
+};
+
+struct Base {};
+
+template <class B>
+struct DepBase1 : B {
+  using B::B [[]];
+
+};
+template <class B>
+struct DepBase2 : B {
+  using B::B __attribute__(());
+};
+
+DepBase1<Base> db1;
+DepBase2<Base> db2;