Generic selection expressions that accept a type operand

[aaron.ballman]

Summary

Extends _Generic selection expressions to accept a type operand in addition to an expression operand. The type operand form does not perform lvalue conversions on the operand, which allows the expression to select *qualified* types, making it more useful for generic programming in C.

C has a few operators that take either a type or an expression, such as sizeof. It is natural to extend that idea to _Generic so that it can also accept a type for the first operand. This type does not undergo any conversions, which allows it to match qualified types, incomplete types, and function types. C2x has the typeof operator as a way to get the type of an expression before lvalue conversion takes place, and so it keeps the qualification. This makes typeof a straightforward approach to determining a type operand for _Generic that considers qualifiers. This allows writing a helper macro like:

#define EXPR_HAS_TYPE(Expr, Type) _Generic(typeof(Expr), Type : 1, default : 0)

which can be called with an expression of any value category (no need to be an lvalue) and will test against (almost) any type. This is a conforming extension to C (it's defining the behavior of invalid syntax), and I believe this is a natural way to extend this functionality.

_Generic with a type operand will relax the requirements of what can be a valid association. Specifically, it allows incomplete types and non-object types (but still prevents use of variably-modified types). This relaxation only happens for the type operand form; the expression operand form continues to behave as it always has.

This extension allows incomplete and non-object types because the goal is to better enable type-generic programming in C, and so we want to allow any typed construct we can statically determine the type of. There is no reason to prevent matching against void or function types, but this does explain why we continue to prohibit variably-modified types.

Please see the RFC at https://discourse.llvm.org/t/rfc-generic-selection-expression-with-a-type-operand/70388 for more details.

[erichkeane]

clang/include/clang/AST/Expr.h

5686

I realize this is a rarely used type, but should these be bitfielded somehow? Particularly now that you're adding a 'bool' field?

5691

What is the value of these functions, if they only return 0 or 1?

5712

Should these have asserts also? Wouldn't saying this is index '0' be incorrect here if t his is the typePredicate?

clang/include/clang/Parse/Parser.h

2551

slight preference to put this declaration in the 'if' condition below, the split in location confused me for a bit trying to figure out where it was being used.

clang/lib/Sema/SemaExpr.cpp

19851

I find myself wondering if a getControllingOperand here is valuable?

[cor3ntin]

I observe there is no documentation or release notes yet :)

clang/include/clang/AST/Expr.h

5930

nitpick: wouldn't PredicateIsExpr() make more sense?

5970

Pre-existing but why not use cast here?

clang/lib/Sema/SemaExpr.cpp

1704–1711

Does that mean you can match on array of unknown bound? I struggle to imagine how this could be used.

[Fznamznon]

clang/lib/AST/ASTImporter.cpp

7033–7036

Just a NIT: would something like

ToControllingExpr = importChecked(Err, E->isExprPredicate()? E->getControllingExpr() : E->getControllingType());

look a bit more elegant (when also formatted properly)?

clang/lib/Parse/ParseExpr.cpp

3308

No else after return?

clang/lib/Parse/ParseTentative.cpp

645

[aaron.ballman]

clang/include/clang/AST/Expr.h

5686

The language which specifies that an implementation need only support 511 identifiers with block scope declared in one block and 127 nesting levels of blocks.... is silent on how many generic selection expression associations we're required to support. :-D

I think we could probably assume there's < 2^15 associations in real world code so we can cram all of this into one 32-bit value.

5691

I was on the fence about these -- it was a nice place to hang an assertion and get a "named constant", basically. I can drop them if you'd like.

5712

Nope, these should not assert -- the selection is required to have at least one association (and an association is a pair of type and expr), so this is getting you the offset to the first association (either the expr or the type).

5930

Hmmm, I think it would be: isPredicateAnExpr() and isPredicateAType() but I'm also not unhappy with the current names (I read them as "is (expression predicate)" and "is (type predicate)"). Your call!

5970

This is casting a pointer-to-an-AST-node-pointer, so cast<> wouldn't work (that only works on AST node pointers).

clang/include/clang/Parse/Parser.h

2551

Sure -- I was just trying to avoid curly braces as done in the next function. :-D I'll fix up the one in isTypeIdUnambiguously() as a drive-by so they remain consistent.

clang/lib/AST/ASTImporter.cpp

7033–7036

Alas, we can't -- that makes a ternary operator with no common type (bool ? Expr * : TypeSourceInfo *)

clang/lib/Parse/ParseExpr.cpp

3308

Good catch!

clang/lib/Parse/ParseTentative.cpp

645

Great catch!

clang/lib/Sema/SemaExpr.cpp

1704–1711

Yup, you can match on an array of unknown bound if you so desire. It's actually useful in an interesting way:

static_assert(_Generic(int[12], int[] : 0, int * : 1) == 0); // OK
static_assert(_Generic(int[12], int[] : 0, int [100] : 1) == 0); // error: type 'int[100]' in generic association compatible with previously specified type 'int[]'

so you can use an incomplete array type as an association and it will match any constant-size array type as a controlling type predicate.

19851

This pattern only comes up once, so I don't think it's worth it, but it's certainly easy enough to add if you disagree.

[erichkeane]

clang/include/clang/AST/Expr.h

5712

OH! I see, there are 2 separate lists here, 1 for types, 1 for expressions, is that it? And you're storing the controlling expression/type in the 1st one (if it exists)?

clang/lib/Sema/SemaExpr.cpp

19851

Yep, I think I agree, after seeing the rest of the review, I don't see as much value.

On this re-read, I kind of which we instead had 2 separate 'create' functions here, rather than going through void*, so that this becomes:

if (IsExpr)
  return AnyChanged ? S.CreateGenericSelectionExpr(...., GSE->getControllingExpr(), ...) : ExprEmpty();

  return AnyChanged ? S.CreateGenericSelectionExpr(...., GSE->getControllingType(), ...) : ExprEmpty();

The void* version gives me the jeebies (and is perhaps a 'worse' interface? Particularly for libclang folks?).

[aaron.ballman]

clang/include/clang/AST/Expr.h

5712

Correct! Two *almost* parallel lists where the first entry in either (but not both) list is potentially the controlling operand.

clang/lib/Sema/SemaExpr.cpp

19851

That makes this one use better but at the expense of making that create function rather awkward.

[aaron.ballman]

Updated based on review feedback.

[aaron.ballman]

In D149904#4321144, @cor3ntin wrote:

I observe there is no documentation or release notes yet :)

Those are coming up, I wanted to see if the RFC made any substantive changes to the design before doing the documentation for the extension.

clang/lib/Sema/SemaExpr.cpp

19851

I took another run at this and decided to put a FIXME comment in the header file. I agree that the interface is a bit awkward, but it's also pretty hard to make work in ActOnGenericSelectionExpr due to it taking an Expr * or a ParsedType (currently it takes a void * of the parsed type's opaque value).

[aaron.ballman]

Added a feature test macro, documentation, and a release note.

[cor3ntin]

clang/include/clang/AST/Expr.h

5691

I personally think it's cleaner and more consistent that way, I'd keep them

clang/test/Sema/generic-selection-type-extension.c

131

Can you add a type for something like that?

cpp
template <typename... T>
void f() {
    _Generic(T..., int : 1);
}

[aaron.ballman]

Added a test case for packs based on review feedback.

clang/include/clang/AST/ASTNodeTraverser.h

public:
   void VisitBlockExpr(const BlockExpr *Node) { Visit(Node->getBlockDecl()); }
 
   void VisitOpaqueValueExpr(const OpaqueValueExpr *Node) {
     if (Expr *Source = Node->getSourceExpr())
       Visit(Source);
   }
 
   void VisitGenericSelectionExpr(const GenericSelectionExpr *E) {
+    if (E->isExprPredicate()) {
       Visit(E->getControllingExpr());
       Visit(E->getControllingExpr()->getType()); // FIXME: remove
+    } else
+      Visit(E->getControllingType()->getType());
 
     for (const auto Assoc : E->associations()) {
       Visit(Assoc);
     }
   }
 
   void VisitRequiresExpr(const RequiresExpr *E) {
     for (auto *D : E->getLocalParameters())

clang/include/clang/AST/Expr.h

 ///
 /// As an extension, generic selections are allowed in C++, where the following
 /// additional semantics apply:
 ///
 /// Any generic selection whose controlling expression is type-dependent or
 /// which names a dependent type in its association list is result-dependent,
 /// which means that the choice of result expression is dependent.
 /// Result-dependent generic associations are both type- and value-dependent.
+///
+/// We also allow an extended form in both C and C++ where the controlling
+/// predicate for the selection expression is a type rather than an expression.
+/// This type argument form does not perform any conversions for the
+/// controlling type, which makes it suitable for use with qualified type
+/// associations, which is not possible with the expression form.
 class GenericSelectionExpr final
     : public Expr,
       private llvm::TrailingObjects<GenericSelectionExpr, Stmt *,
                                     TypeSourceInfo *> {
   friend class ASTStmtReader;
   friend class ASTStmtWriter;
   friend TrailingObjects;
 
   /// The number of association expressions and the index of the result
   /// expression in the case where the generic selection expression is not
   /// result-dependent. The result index is equal to ResultDependentIndex
   /// if and only if the generic selection expression is result-dependent.
-  unsigned NumAssocs, ResultIndex;
+  unsigned NumAssocs, ResultIndex, IsExprPredicate;
   enum : unsigned {
     ResultDependentIndex = std::numeric_limits<unsigned>::max(),
-    ControllingIndex = 0,
-    AssocExprStartIndex = 1
   };
 
+  unsigned getIndexOfControllingExpression() const {
+    // If controlled by an expression, the first offset into the Stmt *
+    // trailing array is the controlling expression, the associated expressions
+    // follow this.
+    assert(isExprPredicate() && "Asking for the controlling expression of a "
+                                "selection expr predicated by a type");
+    return 0;
+  }
+
+  unsigned getIndexOfControllingType() const {
+    // If controlled by a type, the first offset into the TypeSourceInfo *
+    // trailing array is the controlling type, the associated types follow this.
+    assert(isTypePredicate() && "Asking for the controlling type of a "
+                                 "selection expr predicated by an expression");
+    return 0;
+  }
+
+  unsigned getIndexOfStartOfAssociatedExprs() const {
+    // If the predicate is a type, then the associated expressions are the only
+    // Stmt * in the trailing array, otherwise we need to offset past the
+    // predicate expression.
+    return (int)isExprPredicate();
+  }
+
+  unsigned getIndexOfStartOfAssociatedTypes() const {
+    // If the predicate is a type, then the associated types follow it in the
+    // trailing array. Otherwise, the associated types are the only
+    // TypeSourceInfo * in the trailing array.
+    return (int)isTypePredicate();
+  }
+
+
   /// The location of the "default" and of the right parenthesis.
   SourceLocation DefaultLoc, RParenLoc;
 
   // GenericSelectionExpr is followed by several trailing objects.
   // They are (in order):
   //
-  // * A single Stmt * for the controlling expression.
+  // * A single Stmt * for the controlling expression or a TypeSourceInfo * for
+  //   the controlling type, depending on the result of isTypePredicate() or
+  //   isExprPredicate().
   // * An array of getNumAssocs() Stmt * for the association expressions.
   // * An array of getNumAssocs() TypeSourceInfo *, one for each of the
   //   association expressions.
   unsigned numTrailingObjects(OverloadToken<Stmt *>) const {
     // Add one to account for the controlling expression; the remainder
     // are the associated expressions.
-    return 1 + getNumAssocs();
+    return getNumAssocs() + (int)isExprPredicate();
   }
 
   unsigned numTrailingObjects(OverloadToken<TypeSourceInfo *>) const {
-    return getNumAssocs();
+    // Add one to account for the controlling type predicate, the remainder
+    // are the associated types.
+    return getNumAssocs() + (int)isTypePredicate();
   }
 
   template <bool Const> class AssociationIteratorTy;
   /// Bundle together an association expression and its TypeSourceInfo.
   /// The Const template parameter is for the const and non-const versions
   /// of AssociationTy.
   template <bool Const> class AssociationTy {
     friend class GenericSelectionExpr;

       ++E;
       ++TSI;
       ++Offset;
       return *this;
     }
     bool operator==(AssociationIteratorTy Other) const { return E == Other.E; }
   }; // class AssociationIterator
 
-  /// Build a non-result-dependent generic selection expression.
+  /// Build a non-result-dependent generic selection expression accepting an
+  /// expression predicate.
   GenericSelectionExpr(const ASTContext &Context, SourceLocation GenericLoc,
                        Expr *ControllingExpr,
                        ArrayRef<TypeSourceInfo *> AssocTypes,
                        ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
                        SourceLocation RParenLoc,
                        bool ContainsUnexpandedParameterPack,
                        unsigned ResultIndex);
 
-  /// Build a result-dependent generic selection expression.
+  /// Build a result-dependent generic selection expression accepting an
+  /// expression predicate.
   GenericSelectionExpr(const ASTContext &Context, SourceLocation GenericLoc,
                        Expr *ControllingExpr,
                        ArrayRef<TypeSourceInfo *> AssocTypes,
                        ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
                        SourceLocation RParenLoc,
                        bool ContainsUnexpandedParameterPack);
 
+  /// Build a non-result-dependent generic selection expression accepting a
+  /// type predicate.
+  GenericSelectionExpr(const ASTContext &Context, SourceLocation GenericLoc,
+                       TypeSourceInfo *ControllingType,
+                       ArrayRef<TypeSourceInfo *> AssocTypes,
+                       ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+                       SourceLocation RParenLoc,
+                       bool ContainsUnexpandedParameterPack,
+                       unsigned ResultIndex);
+
+  /// Build a result-dependent generic selection expression accepting a type
+  /// predicate.
+  GenericSelectionExpr(const ASTContext &Context, SourceLocation GenericLoc,
+                       TypeSourceInfo *ControllingType,
+                       ArrayRef<TypeSourceInfo *> AssocTypes,
+                       ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+                       SourceLocation RParenLoc,
+                       bool ContainsUnexpandedParameterPack);
+
   /// Build an empty generic selection expression for deserialization.
   explicit GenericSelectionExpr(EmptyShell Empty, unsigned NumAssocs);
 
 public:
-  /// Create a non-result-dependent generic selection expression.
+  /// Create a non-result-dependent generic selection expression accepting an
+  /// expression predicate.
   static GenericSelectionExpr *
   Create(const ASTContext &Context, SourceLocation GenericLoc,
          Expr *ControllingExpr, ArrayRef<TypeSourceInfo *> AssocTypes,
          ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
          SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack,
          unsigned ResultIndex);
 
-  /// Create a result-dependent generic selection expression.
+  /// Create a result-dependent generic selection expression accepting an
+  /// expression predicate.
   static GenericSelectionExpr *
   Create(const ASTContext &Context, SourceLocation GenericLoc,
          Expr *ControllingExpr, ArrayRef<TypeSourceInfo *> AssocTypes,
          ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
          SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack);
 
+  /// Create a non-result-dependent generic selection expression accepting a
+  /// type predicate.
+  static GenericSelectionExpr *
+  Create(const ASTContext &Context, SourceLocation GenericLoc,
+         TypeSourceInfo *ControllingType, ArrayRef<TypeSourceInfo *> AssocTypes,
+         ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+         SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack,
+         unsigned ResultIndex);
+
+  /// Create a result-dependent generic selection expression accepting a type
+  /// predicate
+  static GenericSelectionExpr *
+  Create(const ASTContext &Context, SourceLocation GenericLoc,
+         TypeSourceInfo *ControllingType, ArrayRef<TypeSourceInfo *> AssocTypes,
+         ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+         SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack);
+
   /// Create an empty generic selection expression for deserialization.
   static GenericSelectionExpr *CreateEmpty(const ASTContext &Context,
                                            unsigned NumAssocs);
 
   using Association = AssociationTy<false>;
   using ConstAssociation = AssociationTy<true>;
   using AssociationIterator = AssociationIteratorTy<false>;
   using ConstAssociationIterator = AssociationIteratorTy<true>;

     assert(!isResultDependent() &&
            "Generic selection is result-dependent but getResultIndex called!");
     return ResultIndex;
   }
 
   /// Whether this generic selection is result-dependent.
   bool isResultDependent() const { return ResultIndex == ResultDependentIndex; }
 
+  /// Whether this generic selection uses an expression as its controlling
+  /// argument.
+  bool isExprPredicate() const { return IsExprPredicate; }
+  /// Whether this generic selection uses a type as its controlling argument.
+  bool isTypePredicate() const { return !IsExprPredicate; }
+
   /// Return the controlling expression of this generic selection expression.
+  /// Only valid to call if the selection expression used an expression as its
+  /// controlling argument.
   Expr *getControllingExpr() {
-    return cast<Expr>(getTrailingObjects<Stmt *>()[ControllingIndex]);
+    return cast<Expr>(
+        getTrailingObjects<Stmt *>()[getIndexOfControllingExpression()]);
   }
   const Expr *getControllingExpr() const {
-    return cast<Expr>(getTrailingObjects<Stmt *>()[ControllingIndex]);
+    return cast<Expr>(
+        getTrailingObjects<Stmt *>()[getIndexOfControllingExpression()]);
+  }
+
+  /// Return the controlling type of this generic selection expression. Only
+  /// valid to call if the selection expression used a type as its controlling
+  /// argument.
+  TypeSourceInfo *getControllingType() {
+    return getTrailingObjects<TypeSourceInfo *>()[getIndexOfControllingType()];
+  }
+  const TypeSourceInfo* getControllingType() const {
+    return getTrailingObjects<TypeSourceInfo *>()[getIndexOfControllingType()];
   }
 
   /// Return the result expression of this controlling expression. Defined if
   /// and only if the generic selection expression is not result-dependent.
   Expr *getResultExpr() {
     return cast<Expr>(
-        getTrailingObjects<Stmt *>()[AssocExprStartIndex + getResultIndex()]);
+        getTrailingObjects<Stmt *>()[getIndexOfStartOfAssociatedExprs() +
+                                     getResultIndex()]);
   }
   const Expr *getResultExpr() const {
     return cast<Expr>(
-        getTrailingObjects<Stmt *>()[AssocExprStartIndex + getResultIndex()]);
+        getTrailingObjects<Stmt *>()[getIndexOfStartOfAssociatedExprs() +
+                                     getResultIndex()]);
   }
 
   ArrayRef<Expr *> getAssocExprs() const {
     return {reinterpret_cast<Expr *const *>(getTrailingObjects<Stmt *>() +
-                                            AssocExprStartIndex),
+                                            getIndexOfStartOfAssociatedExprs()),
             NumAssocs};
   }
   ArrayRef<TypeSourceInfo *> getAssocTypeSourceInfos() const {
-    return {getTrailingObjects<TypeSourceInfo *>(), NumAssocs};
+    return {getTrailingObjects<TypeSourceInfo *>() +
+                getIndexOfStartOfAssociatedTypes(),
+            NumAssocs};
   }
 
   /// Return the Ith association expression with its TypeSourceInfo,
   /// bundled together in GenericSelectionExpr::(Const)Association.
   Association getAssociation(unsigned I) {
     assert(I < getNumAssocs() &&
            "Out-of-range index in GenericSelectionExpr::getAssociation!");
     return Association(
-        cast<Expr>(getTrailingObjects<Stmt *>()[AssocExprStartIndex + I]),
-        getTrailingObjects<TypeSourceInfo *>()[I],
+        cast<Expr>(
+            getTrailingObjects<Stmt *>()[getIndexOfStartOfAssociatedExprs() +
+                                         I]),
+        getTrailingObjects<
+            TypeSourceInfo *>()[getIndexOfStartOfAssociatedTypes() + I],
         !isResultDependent() && (getResultIndex() == I));
   }
   ConstAssociation getAssociation(unsigned I) const {
     assert(I < getNumAssocs() &&
            "Out-of-range index in GenericSelectionExpr::getAssociation!");
     return ConstAssociation(
-        cast<Expr>(getTrailingObjects<Stmt *>()[AssocExprStartIndex + I]),
-        getTrailingObjects<TypeSourceInfo *>()[I],
+        cast<Expr>(
+            getTrailingObjects<Stmt *>()[getIndexOfStartOfAssociatedExprs() +
+                                         I]),
+        getTrailingObjects<
+            TypeSourceInfo *>()[getIndexOfStartOfAssociatedTypes() + I],
         !isResultDependent() && (getResultIndex() == I));
   }
 
   association_range associations() {
     AssociationIterator Begin(getTrailingObjects<Stmt *>() +
-                                  AssocExprStartIndex,
-                              getTrailingObjects<TypeSourceInfo *>(),
+                                  getIndexOfStartOfAssociatedExprs(),
+                              getTrailingObjects<TypeSourceInfo *>() +
+                                  getIndexOfStartOfAssociatedTypes(),
                               /*Offset=*/0, ResultIndex);
     AssociationIterator End(Begin.E + NumAssocs, Begin.TSI + NumAssocs,
                             /*Offset=*/NumAssocs, ResultIndex);
     return llvm::make_range(Begin, End);
   }
 
   const_association_range associations() const {
     ConstAssociationIterator Begin(getTrailingObjects<Stmt *>() +
-                                       AssocExprStartIndex,
-                                   getTrailingObjects<TypeSourceInfo *>(),
+                                       getIndexOfStartOfAssociatedExprs(),
+                                   getTrailingObjects<TypeSourceInfo *>() +
+                                       getIndexOfStartOfAssociatedTypes(),
                                    /*Offset=*/0, ResultIndex);
     ConstAssociationIterator End(Begin.E + NumAssocs, Begin.TSI + NumAssocs,
                                  /*Offset=*/NumAssocs, ResultIndex);
     return llvm::make_range(Begin, End);
   }
 
   SourceLocation getGenericLoc() const {
     return GenericSelectionExprBits.GenericLoc;

clang/include/clang/AST/RecursiveASTVisitor.h

   }
   return true;
 }
 
 // GenericSelectionExpr is a special case because the types and expressions
 // are interleaved.  We also need to watch out for null types (default
 // generic associations).
 DEF_TRAVERSE_STMT(GenericSelectionExpr, {
-  TRY_TO(TraverseStmt(S->getControllingExpr()));
+  if (S->isExprPredicate())
+    TRY_TO(TraverseStmt(S->getControllingExpr()));
+  else
+    TRY_TO(TraverseTypeLoc(S->getControllingType()->getTypeLoc()));
+
   for (const GenericSelectionExpr::Association Assoc : S->associations()) {
     if (TypeSourceInfo *TSI = Assoc.getTypeSourceInfo())
       TRY_TO(TraverseTypeLoc(TSI->getTypeLoc()));
     TRY_TO_TRAVERSE_OR_ENQUEUE_STMT(Assoc.getAssociationExpr());
   }
   ShouldVisitChildren = false;
 })
 

clang/include/clang/Basic/DiagnosticParseKinds.td

 def warn_microsoft_qualifiers_ignored : Warning<
   "qualifiers after comma in declarator list are ignored">,
   InGroup<IgnoredAttributes>;
 
 def err_duplicate_default_assoc : Error<
   "duplicate default generic association">;
 def note_previous_default_assoc : Note<
   "previous default generic association is here">;
+def ext_generic_with_type_arg : Extension<
+  "passing a type argument as the first operand to '_Generic' is a Clang "
+  "extension">, InGroup<DiagGroup<"generic-type-extension">>;
 
 def ext_c99_feature : Extension<
   "'%0' is a C99 extension">, InGroup<C99>;
 def ext_c11_feature : Extension<
   "'%0' is a C11 extension">, InGroup<C11>;
 def warn_c2x_compat_keyword : Warning<
  "'%0' is incompatible with C standards before C2x">,
  InGroup<CPre2xCompat>, DefaultIgnore;

clang/include/clang/Parse/Parser.h

       DeclSpec::FriendSpecified IsFriend = DeclSpec::FriendSpecified::No,
       const ParsedTemplateInfo *TemplateInfo = nullptr);
 
   /// Specifies the context in which type-id/expression
   /// disambiguation will occur.
   enum TentativeCXXTypeIdContext {
     TypeIdInParens,
     TypeIdUnambiguous,
-    TypeIdAsTemplateArgument
+    TypeIdAsTemplateArgument,
+    TypeIdAsGenericSelectionArgument,
   };
 
 
   /// isTypeIdInParens - Assumes that a '(' was parsed and now we want to know
   /// whether the parens contain an expression or a type-id.
   /// Returns true for a type-id and false for an expression.
   bool isTypeIdInParens(bool &isAmbiguous) {
     if (getLangOpts().CPlusPlus)
       return isCXXTypeId(TypeIdInParens, isAmbiguous);
     isAmbiguous = false;
     return isTypeSpecifierQualifier();
   }
   bool isTypeIdInParens() {
     bool isAmbiguous;
     return isTypeIdInParens(isAmbiguous);
   }
 
+  /// Checks whether the current tokens form a type-id or an expression for the
+  /// purposes of use as the initial operand to a generic selection expression.
+  /// This requires special handling in C++ because it accepts either a type or
+  /// an expression, and we need to disambiguate which is which. However, we
+  /// cannot use the same logic as we've used for sizeof expressions, because
+  /// that logic relies on the operator only accepting a single argument,
+  /// whereas _Generic accepts a list of arguments.
+  bool isTypeIdForGenericSelection() {
+    bool isAmbiguous;
+    if (getLangOpts().CPlusPlus)
+      return isCXXTypeId(TypeIdAsGenericSelectionArgument, isAmbiguous);
+    return isTypeSpecifierQualifier();
+  }
+
   /// Checks if the current tokens form type-id or expression.
   /// It is similar to isTypeIdInParens but does not suppose that type-id
   /// is in parenthesis.
   bool isTypeIdUnambiguously() {
     bool IsAmbiguous;
     if (getLangOpts().CPlusPlus)
       return isCXXTypeId(TypeIdUnambiguous, IsAmbiguous);
     return isTypeSpecifierQualifier();

clang/include/clang/Sema/Sema.h

                                 SourceLocation R,
                                 MultiExprArg Val);
 
   /// ActOnStringLiteral - The specified tokens were lexed as pasted string
   /// fragments (e.g. "foo" "bar" L"baz").
   ExprResult ActOnStringLiteral(ArrayRef<Token> StringToks,
                                 Scope *UDLScope = nullptr);
 
+  /// ControllingExprOrType is either an opaque pointer coming out of a
+  /// ParsedType or an Expr *.
   ExprResult ActOnGenericSelectionExpr(SourceLocation KeyLoc,
                                        SourceLocation DefaultLoc,
                                        SourceLocation RParenLoc,
-                                       Expr *ControllingExpr,
+                                       bool PredicateIsExpr,
+                                       void *ControllingExprOrType,
                                        ArrayRef<ParsedType> ArgTypes,
                                        ArrayRef<Expr *> ArgExprs);
+  /// ControllingExprOrType is either a TypeSourceInfo * or an Expr *.
   ExprResult CreateGenericSelectionExpr(SourceLocation KeyLoc,
                                         SourceLocation DefaultLoc,
                                         SourceLocation RParenLoc,
-                                        Expr *ControllingExpr,
+                                        bool PredicateIsExpr,
+                                        void *ControllingExprOrType,
                                         ArrayRef<TypeSourceInfo *> Types,
                                         ArrayRef<Expr *> Exprs);
 
   // Binary/Unary Operators.  'Tok' is the token for the operator.
   ExprResult CreateBuiltinUnaryOp(SourceLocation OpLoc, UnaryOperatorKind Opc,
                                   Expr *InputExpr, bool IsAfterAmp = false);
   ExprResult BuildUnaryOp(Scope *S, SourceLocation OpLoc, UnaryOperatorKind Opc,
                           Expr *Input, bool IsAfterAmp = false);

clang/lib/AST/ASTImporter.cpp

 
   return new (Importer.getToContext()) GNUNullExpr(*TypeOrErr, *BeginLocOrErr);
 }
 
 ExpectedStmt
 ASTNodeImporter::VisitGenericSelectionExpr(GenericSelectionExpr *E) {
   Error Err = Error::success();
   auto ToGenericLoc = importChecked(Err, E->getGenericLoc());
-  auto *ToControllingExpr = importChecked(Err, E->getControllingExpr());
+  Expr *ToControllingExpr = nullptr;
+  TypeSourceInfo *ToControllingType = nullptr;
+  if (E->isExprPredicate())
+    ToControllingExpr = importChecked(Err, E->getControllingExpr());
+  else
+    ToControllingType = importChecked(Err, E->getControllingType());
+  assert((ToControllingExpr || ToControllingType) &&
+         "Either the controlling expr or type must be nonnull");
   auto ToDefaultLoc = importChecked(Err, E->getDefaultLoc());
   auto ToRParenLoc = importChecked(Err, E->getRParenLoc());
   if (Err)
     return std::move(Err);
 
   ArrayRef<const TypeSourceInfo *> FromAssocTypes(E->getAssocTypeSourceInfos());
   SmallVector<TypeSourceInfo *, 1> ToAssocTypes(FromAssocTypes.size());
   if (Error Err = ImportContainerChecked(FromAssocTypes, ToAssocTypes))
     return std::move(Err);
 
   ArrayRef<const Expr *> FromAssocExprs(E->getAssocExprs());
   SmallVector<Expr *, 1> ToAssocExprs(FromAssocExprs.size());
   if (Error Err = ImportContainerChecked(FromAssocExprs, ToAssocExprs))
     return std::move(Err);
 
   const ASTContext &ToCtx = Importer.getToContext();
   if (E->isResultDependent()) {
+    if (ToControllingExpr) {
+      return GenericSelectionExpr::Create(
+          ToCtx, ToGenericLoc, ToControllingExpr, llvm::ArrayRef(ToAssocTypes),
+          llvm::ArrayRef(ToAssocExprs), ToDefaultLoc, ToRParenLoc,
+          E->containsUnexpandedParameterPack());
+    }
     return GenericSelectionExpr::Create(
-        ToCtx, ToGenericLoc, ToControllingExpr, llvm::ArrayRef(ToAssocTypes),
+        ToCtx, ToGenericLoc, ToControllingType, llvm::ArrayRef(ToAssocTypes),
         llvm::ArrayRef(ToAssocExprs), ToDefaultLoc, ToRParenLoc,
         E->containsUnexpandedParameterPack());
   }
 
+  if (ToControllingExpr) {
+    return GenericSelectionExpr::Create(
+        ToCtx, ToGenericLoc, ToControllingExpr, llvm::ArrayRef(ToAssocTypes),
+        llvm::ArrayRef(ToAssocExprs), ToDefaultLoc, ToRParenLoc,
+        E->containsUnexpandedParameterPack(), E->getResultIndex());
+  }
   return GenericSelectionExpr::Create(
-      ToCtx, ToGenericLoc, ToControllingExpr, llvm::ArrayRef(ToAssocTypes),
+      ToCtx, ToGenericLoc, ToControllingType, llvm::ArrayRef(ToAssocTypes),
       llvm::ArrayRef(ToAssocExprs), ToDefaultLoc, ToRParenLoc,
       E->containsUnexpandedParameterPack(), E->getResultIndex());
 }
 
 ExpectedStmt ASTNodeImporter::VisitPredefinedExpr(PredefinedExpr *E) {
 
   Error Err = Error::success();
   auto ToBeginLoc = importChecked(Err, E->getBeginLoc());

clang/lib/AST/ComputeDependence.cpp

 }
 
 ExprDependence clang::computeDependence(GenericSelectionExpr *E,
                                         bool ContainsUnexpandedPack) {
   auto D = ContainsUnexpandedPack ? ExprDependence::UnexpandedPack
                                   : ExprDependence::None;
   for (auto *AE : E->getAssocExprs())
     D |= AE->getDependence() & ExprDependence::Error;
+
+  if (E->isExprPredicate())
     D |= E->getControllingExpr()->getDependence() & ExprDependence::Error;
+  else
+    D |= toExprDependenceAsWritten(
+        E->getControllingType()->getType()->getDependence());
 
   if (E->isResultDependent())
     return D | ExprDependence::TypeValueInstantiation;
   return D | (E->getResultExpr()->getDependence() &
               ~ExprDependence::UnexpandedPack);
 }
 
 ExprDependence clang::computeDependence(DesignatedInitExpr *E) {

clang/lib/AST/Expr.cpp

     const ASTContext &, SourceLocation GenericLoc, Expr *ControllingExpr,
     ArrayRef<TypeSourceInfo *> AssocTypes, ArrayRef<Expr *> AssocExprs,
     SourceLocation DefaultLoc, SourceLocation RParenLoc,
     bool ContainsUnexpandedParameterPack, unsigned ResultIndex)
     : Expr(GenericSelectionExprClass, AssocExprs[ResultIndex]->getType(),
            AssocExprs[ResultIndex]->getValueKind(),
            AssocExprs[ResultIndex]->getObjectKind()),
       NumAssocs(AssocExprs.size()), ResultIndex(ResultIndex),
-      DefaultLoc(DefaultLoc), RParenLoc(RParenLoc) {
+      IsExprPredicate(true), DefaultLoc(DefaultLoc), RParenLoc(RParenLoc) {
   assert(AssocTypes.size() == AssocExprs.size() &&
          "Must have the same number of association expressions"
          " and TypeSourceInfo!");
   assert(ResultIndex < NumAssocs && "ResultIndex is out-of-bounds!");
 
   GenericSelectionExprBits.GenericLoc = GenericLoc;
-  getTrailingObjects<Stmt *>()[ControllingIndex] = ControllingExpr;
+  getTrailingObjects<Stmt *>()[getIndexOfControllingExpression()] =
+      ControllingExpr;
   std::copy(AssocExprs.begin(), AssocExprs.end(),
-            getTrailingObjects<Stmt *>() + AssocExprStartIndex);
+            getTrailingObjects<Stmt *>() + getIndexOfStartOfAssociatedExprs());
   std::copy(AssocTypes.begin(), AssocTypes.end(),
-            getTrailingObjects<TypeSourceInfo *>());
+            getTrailingObjects<TypeSourceInfo *>() +
+                getIndexOfStartOfAssociatedTypes());
+
+  setDependence(computeDependence(this, ContainsUnexpandedParameterPack));
+}
+
+GenericSelectionExpr::GenericSelectionExpr(
+    const ASTContext &, SourceLocation GenericLoc,
+    TypeSourceInfo *ControllingType, ArrayRef<TypeSourceInfo *> AssocTypes,
+    ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+    SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack,
+    unsigned ResultIndex)
+    : Expr(GenericSelectionExprClass, AssocExprs[ResultIndex]->getType(),
+           AssocExprs[ResultIndex]->getValueKind(),
+           AssocExprs[ResultIndex]->getObjectKind()),
+      NumAssocs(AssocExprs.size()), ResultIndex(ResultIndex),
+      IsExprPredicate(false), DefaultLoc(DefaultLoc), RParenLoc(RParenLoc) {
+  assert(AssocTypes.size() == AssocExprs.size() &&
+         "Must have the same number of association expressions"
+         " and TypeSourceInfo!");
+  assert(ResultIndex < NumAssocs && "ResultIndex is out-of-bounds!");
+
+  GenericSelectionExprBits.GenericLoc = GenericLoc;
+  getTrailingObjects<TypeSourceInfo *>()[getIndexOfControllingType()] =
+      ControllingType;
+  std::copy(AssocExprs.begin(), AssocExprs.end(),
+            getTrailingObjects<Stmt *>() + getIndexOfStartOfAssociatedExprs());
+  std::copy(AssocTypes.begin(), AssocTypes.end(),
+            getTrailingObjects<TypeSourceInfo *>() +
+                getIndexOfStartOfAssociatedTypes());
 
   setDependence(computeDependence(this, ContainsUnexpandedParameterPack));
 }
 
 GenericSelectionExpr::GenericSelectionExpr(
     const ASTContext &Context, SourceLocation GenericLoc, Expr *ControllingExpr,
     ArrayRef<TypeSourceInfo *> AssocTypes, ArrayRef<Expr *> AssocExprs,
     SourceLocation DefaultLoc, SourceLocation RParenLoc,
     bool ContainsUnexpandedParameterPack)
     : Expr(GenericSelectionExprClass, Context.DependentTy, VK_PRValue,
            OK_Ordinary),
       NumAssocs(AssocExprs.size()), ResultIndex(ResultDependentIndex),
-      DefaultLoc(DefaultLoc), RParenLoc(RParenLoc) {
+      IsExprPredicate(true), DefaultLoc(DefaultLoc), RParenLoc(RParenLoc) {
+  assert(AssocTypes.size() == AssocExprs.size() &&
+         "Must have the same number of association expressions"
+         " and TypeSourceInfo!");
+
+  GenericSelectionExprBits.GenericLoc = GenericLoc;
+  getTrailingObjects<Stmt *>()[getIndexOfControllingExpression()] =
+      ControllingExpr;
+  std::copy(AssocExprs.begin(), AssocExprs.end(),
+            getTrailingObjects<Stmt *>() + getIndexOfStartOfAssociatedExprs());
+  std::copy(AssocTypes.begin(), AssocTypes.end(),
+            getTrailingObjects<TypeSourceInfo *>() +
+                getIndexOfStartOfAssociatedTypes());
+
+  setDependence(computeDependence(this, ContainsUnexpandedParameterPack));
+}
+
+GenericSelectionExpr::GenericSelectionExpr(
+    const ASTContext &Context, SourceLocation GenericLoc,
+    TypeSourceInfo *ControllingType, ArrayRef<TypeSourceInfo *> AssocTypes,
+    ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+    SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack)
+    : Expr(GenericSelectionExprClass, Context.DependentTy, VK_PRValue,
+           OK_Ordinary),
+      NumAssocs(AssocExprs.size()), ResultIndex(ResultDependentIndex),
+      IsExprPredicate(false), DefaultLoc(DefaultLoc), RParenLoc(RParenLoc) {
   assert(AssocTypes.size() == AssocExprs.size() &&
          "Must have the same number of association expressions"
          " and TypeSourceInfo!");
 
   GenericSelectionExprBits.GenericLoc = GenericLoc;
-  getTrailingObjects<Stmt *>()[ControllingIndex] = ControllingExpr;
+  getTrailingObjects<TypeSourceInfo *>()[getIndexOfControllingType()] =
+      ControllingType;
   std::copy(AssocExprs.begin(), AssocExprs.end(),
-            getTrailingObjects<Stmt *>() + AssocExprStartIndex);
+            getTrailingObjects<Stmt *>() + getIndexOfStartOfAssociatedExprs());
   std::copy(AssocTypes.begin(), AssocTypes.end(),
-            getTrailingObjects<TypeSourceInfo *>());
+            getTrailingObjects<TypeSourceInfo *>() +
+                getIndexOfStartOfAssociatedTypes());
 
   setDependence(computeDependence(this, ContainsUnexpandedParameterPack));
 }
 
 GenericSelectionExpr::GenericSelectionExpr(EmptyShell Empty, unsigned NumAssocs)
     : Expr(GenericSelectionExprClass, Empty), NumAssocs(NumAssocs) {}
 
 GenericSelectionExpr *GenericSelectionExpr::Create(

   void *Mem = Context.Allocate(
       totalSizeToAlloc<Stmt *, TypeSourceInfo *>(1 + NumAssocs, NumAssocs),
       alignof(GenericSelectionExpr));
   return new (Mem) GenericSelectionExpr(
       Context, GenericLoc, ControllingExpr, AssocTypes, AssocExprs, DefaultLoc,
       RParenLoc, ContainsUnexpandedParameterPack);
 }
 
+GenericSelectionExpr *GenericSelectionExpr::Create(
+    const ASTContext &Context, SourceLocation GenericLoc,
+    TypeSourceInfo *ControllingType, ArrayRef<TypeSourceInfo *> AssocTypes,
+    ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+    SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack,
+    unsigned ResultIndex) {
+  unsigned NumAssocs = AssocExprs.size();
+  void *Mem = Context.Allocate(
+      totalSizeToAlloc<Stmt *, TypeSourceInfo *>(1 + NumAssocs, NumAssocs),
+      alignof(GenericSelectionExpr));
+  return new (Mem) GenericSelectionExpr(
+      Context, GenericLoc, ControllingType, AssocTypes, AssocExprs, DefaultLoc,
+      RParenLoc, ContainsUnexpandedParameterPack, ResultIndex);
+}
+
+GenericSelectionExpr *GenericSelectionExpr::Create(
+    const ASTContext &Context, SourceLocation GenericLoc,
+    TypeSourceInfo *ControllingType, ArrayRef<TypeSourceInfo *> AssocTypes,
+    ArrayRef<Expr *> AssocExprs, SourceLocation DefaultLoc,
+    SourceLocation RParenLoc, bool ContainsUnexpandedParameterPack) {
+  unsigned NumAssocs = AssocExprs.size();
+  void *Mem = Context.Allocate(
+      totalSizeToAlloc<Stmt *, TypeSourceInfo *>(1 + NumAssocs, NumAssocs),
+      alignof(GenericSelectionExpr));
+  return new (Mem) GenericSelectionExpr(
+      Context, GenericLoc, ControllingType, AssocTypes, AssocExprs, DefaultLoc,
+      RParenLoc, ContainsUnexpandedParameterPack);
+}
+
 GenericSelectionExpr *
 GenericSelectionExpr::CreateEmpty(const ASTContext &Context,
                                   unsigned NumAssocs) {
   void *Mem = Context.Allocate(
       totalSizeToAlloc<Stmt *, TypeSourceInfo *>(1 + NumAssocs, NumAssocs),
       alignof(GenericSelectionExpr));
   return new (Mem) GenericSelectionExpr(EmptyShell(), NumAssocs);
 }

clang/lib/AST/StmtPrinter.cpp

   } else {
     OS << " ";
     PrintExpr(Node->getArgumentExpr());
   }
 }
 
 void StmtPrinter::VisitGenericSelectionExpr(GenericSelectionExpr *Node) {
   OS << "_Generic(";
-  PrintExpr(Node->getControllingExpr());
-  for (const GenericSelectionExpr::Association Assoc : Node->associations()) {
+  if (Node->isExprPredicate())
+    PrintExpr(Node->getControllingExpr());
+  else
+    Node->getControllingType()->getType().print(OS, Policy);
+
+  for (const GenericSelectionExpr::Association &Assoc : Node->associations()) {
     OS << ", ";
     QualType T = Assoc.getType();
     if (T.isNull())
       OS << "default";
     else
       T.print(OS, Policy);
     OS << ": ";
     PrintExpr(Assoc.getAssociationExpr());

clang/lib/Analysis/ExprMutationAnalyzer.cpp

const ast_matchers::internal::VariadicDynCastAllOfMatcher<Stmt, CXXTypeidExpr>
     cxxTypeidExpr;
 
 AST_MATCHER(CXXTypeidExpr, isPotentiallyEvaluated) {
   return Node.isPotentiallyEvaluated();
 }
 
 AST_MATCHER_P(GenericSelectionExpr, hasControllingExpr,
               ast_matchers::internal::Matcher<Expr>, InnerMatcher) {
+  if (Node.isTypePredicate())
+    return false;
   return InnerMatcher.matches(*Node.getControllingExpr(), Finder, Builder);
 }
 
 const auto nonConstReferenceType = [] {
   return hasUnqualifiedDesugaredType(
       referenceType(pointee(unless(isConstQualified()))));
 };
 

clang/lib/Parse/ParseExpr.cpp

 ///           _Generic ( assignment-expression , generic-assoc-list )
 ///    generic-assoc-list:
 ///           generic-association
 ///           generic-assoc-list , generic-association
 ///    generic-association:
 ///           type-name : assignment-expression
 ///           default : assignment-expression
 /// \endverbatim
+///
+/// As an extension, Clang also accepts:
+/// \verbatim
+///   generic-selection:
+///          _Generic ( type-name, generic-assoc-list )
+/// \endverbatim
 ExprResult Parser::ParseGenericSelectionExpression() {
   assert(Tok.is(tok::kw__Generic) && "_Generic keyword expected");
   if (!getLangOpts().C11)
     Diag(Tok, diag::ext_c11_feature) << Tok.getName();
 
   SourceLocation KeyLoc = ConsumeToken();
   BalancedDelimiterTracker T(*this, tok::l_paren);
   if (T.expectAndConsume())
     return ExprError();
 
+  // We either have a controlling expression or we have a controlling type, and
+  // we need to figure out which it is.
+  TypeResult ControllingType;
   ExprResult ControllingExpr;
-  {
+  if (isTypeIdForGenericSelection()) {
+    ControllingType = ParseTypeName();
+    if (ControllingType.isInvalid()) {
+      SkipUntil(tok::r_paren, StopAtSemi);
+      return ExprError();
+    } else {
+      const auto *LIT = cast<LocInfoType>(ControllingType.get().get());
+      SourceLocation Loc = LIT->getTypeSourceInfo()->getTypeLoc().getBeginLoc();
+      Diag(Loc, diag::ext_generic_with_type_arg);
+    }
+  } else {
     // C11 6.5.1.1p3 "The controlling expression of a generic selection is
     // not evaluated."
     EnterExpressionEvaluationContext Unevaluated(
         Actions, Sema::ExpressionEvaluationContext::Unevaluated);
     ControllingExpr =
         Actions.CorrectDelayedTyposInExpr(ParseAssignmentExpression());
     if (ControllingExpr.isInvalid()) {
       SkipUntil(tok::r_paren, StopAtSemi);

     }
     Exprs.push_back(ER.get());
   } while (TryConsumeToken(tok::comma));
 
   T.consumeClose();
   if (T.getCloseLocation().isInvalid())
     return ExprError();
 
-  return Actions.ActOnGenericSelectionExpr(KeyLoc, DefaultLoc,
-                                           T.getCloseLocation(),
-                                           ControllingExpr.get(),
-                                           Types, Exprs);
+  void *ExprOrTy = ControllingExpr.isUsable()
+                       ? ControllingExpr.get()
+                       : ControllingType.get().getAsOpaquePtr();
+
+  return Actions.ActOnGenericSelectionExpr(
+      KeyLoc, DefaultLoc, T.getCloseLocation(), ControllingExpr.isUsable(),
+      ExprOrTy, Types, Exprs);
 }
 
 /// Parse A C++1z fold-expression after the opening paren and optional
 /// left-hand-side expression.
 ///
 /// \verbatim
 ///   fold-expression:
 ///       ( cast-expression fold-operator ... )

clang/lib/Parse/ParseTentative.cpp

if (TPR == TPResult::Error)
     TPR = TPResult::True;
 
   if (TPR == TPResult::Ambiguous) {
     // We are supposed to be inside parens, so if after the abstract declarator
     // we encounter a ')' this is a type-id, otherwise it's an expression.
     if (Context == TypeIdInParens && Tok.is(tok::r_paren)) {
       TPR = TPResult::True;
       isAmbiguous = true;
-
+    // We are supopsed to be inside the first operand to a _Generic selection
+    // expression, so if we find a comma after the declarator, we've found a
+    // type and not an expression.
+    } else if (Context == TypeIdAsGenericSelectionArgument && Tok.is(tok::comma)) {
+      TPR = TPResult::True;
+      isAmbiguous = true;
     // We are supposed to be inside a template argument, so if after
     // the abstract declarator we encounter a '>', '>>' (in C++0x), or
     // ','; or, in C++0x, an ellipsis immediately preceding such, this
     // is a type-id. Otherwise, it's an expression.
     } else if (Context == TypeIdAsTemplateArgument &&
                (Tok.isOneOf(tok::greater, tok::comma) ||
                 (getLangOpts().CPlusPlus11 &&
                  (Tok.isOneOf(tok::greatergreater,

clang/lib/Sema/SemaExpr.cpp

            (*this, LHS, RHS, LHSType, RHSType, ACK == ACK_CompAssign);
 }
 
 //===----------------------------------------------------------------------===//
 //  Semantic Analysis for various Expression Types
 //===----------------------------------------------------------------------===//
 
 
-ExprResult
-Sema::ActOnGenericSelectionExpr(SourceLocation KeyLoc,
-                                SourceLocation DefaultLoc,
-                                SourceLocation RParenLoc,
-                                Expr *ControllingExpr,
-                                ArrayRef<ParsedType> ArgTypes,
-                                ArrayRef<Expr *> ArgExprs) {
+ExprResult Sema::ActOnGenericSelectionExpr(
+    SourceLocation KeyLoc, SourceLocation DefaultLoc, SourceLocation RParenLoc,
+    bool PredicateIsExpr, void *ControllingExprOrType,
+    ArrayRef<ParsedType> ArgTypes, ArrayRef<Expr *> ArgExprs) {
   unsigned NumAssocs = ArgTypes.size();
   assert(NumAssocs == ArgExprs.size());
 
   TypeSourceInfo **Types = new TypeSourceInfo*[NumAssocs];
   for (unsigned i = 0; i < NumAssocs; ++i) {
     if (ArgTypes[i])
       (void) GetTypeFromParser(ArgTypes[i], &Types[i]);
     else
       Types[i] = nullptr;
   }
 
-  ExprResult ER =
-      CreateGenericSelectionExpr(KeyLoc, DefaultLoc, RParenLoc, ControllingExpr,
-                                 llvm::ArrayRef(Types, NumAssocs), ArgExprs);
+  // If we have a controlling type, we need to convert it from a parsed type
+  // into a semantic type and then pass that along.
+  if (!PredicateIsExpr) {
+    TypeSourceInfo *ControllingType;
+    (void)GetTypeFromParser(ParsedType::getFromOpaquePtr(ControllingExprOrType),
+                            &ControllingType);
+    assert(ControllingType && "couldn't get the type out of the parser");
+    ControllingExprOrType = ControllingType;
+  }
+
+  ExprResult ER = CreateGenericSelectionExpr(
+      KeyLoc, DefaultLoc, RParenLoc, PredicateIsExpr, ControllingExprOrType,
+      llvm::ArrayRef(Types, NumAssocs), ArgExprs);
   delete [] Types;
   return ER;
 }
 
-ExprResult
-Sema::CreateGenericSelectionExpr(SourceLocation KeyLoc,
-                                 SourceLocation DefaultLoc,
-                                 SourceLocation RParenLoc,
-                                 Expr *ControllingExpr,
-                                 ArrayRef<TypeSourceInfo *> Types,
-                                 ArrayRef<Expr *> Exprs) {
+ExprResult Sema::CreateGenericSelectionExpr(
+    SourceLocation KeyLoc, SourceLocation DefaultLoc, SourceLocation RParenLoc,
+    bool PredicateIsExpr, void *ControllingExprOrType,
+    ArrayRef<TypeSourceInfo *> Types, ArrayRef<Expr *> Exprs) {
   unsigned NumAssocs = Types.size();
   assert(NumAssocs == Exprs.size());
-
-  // Decay and strip qualifiers for the controlling expression type, and handle
-  // placeholder type replacement. See committee discussion from WG14 DR423.
-  {
+  assert(ControllingExprOrType &&
+         "Must have either a controlling expression or a controlling type");
+
+  Expr *ControllingExpr = nullptr;
+  TypeSourceInfo *ControllingType = nullptr;
+  if (PredicateIsExpr) {
+    // Decay and strip qualifiers for the controlling expression type, and
+    // handle placeholder type replacement. See committee discussion from WG14
+    // DR423.
     EnterExpressionEvaluationContext Unevaluated(
         *this, Sema::ExpressionEvaluationContext::Unevaluated);
-    ExprResult R = DefaultFunctionArrayLvalueConversion(ControllingExpr);
+    ExprResult R = DefaultFunctionArrayLvalueConversion(
+        reinterpret_cast<Expr *>(ControllingExprOrType));
     if (R.isInvalid())
       return ExprError();
     ControllingExpr = R.get();
+  } else {
+    // The extension form uses the type directly rather than converting it.
+    ControllingType = reinterpret_cast<TypeSourceInfo *>(ControllingExprOrType);
+    if (!ControllingType)
+      return ExprError();
   }
 
   bool TypeErrorFound = false,
-       IsResultDependent = ControllingExpr->isTypeDependent(),
-       ContainsUnexpandedParameterPack
-         = ControllingExpr->containsUnexpandedParameterPack();
+       IsResultDependent = ControllingExpr
+                               ? ControllingExpr->isTypeDependent()
+                               : ControllingType->getType()->isDependentType(),
+       ContainsUnexpandedParameterPack =
+           ControllingExpr
+               ? ControllingExpr->containsUnexpandedParameterPack()
+               : ControllingType->getType()->containsUnexpandedParameterPack();
 
   // The controlling expression is an unevaluated operand, so side effects are
   // likely unintended.
-  if (!inTemplateInstantiation() && !IsResultDependent &&
+  if (!inTemplateInstantiation() && !IsResultDependent && ControllingExpr &&
       ControllingExpr->HasSideEffects(Context, false))
     Diag(ControllingExpr->getExprLoc(),
          diag::warn_side_effects_unevaluated_context);
 
   for (unsigned i = 0; i < NumAssocs; ++i) {
     if (Exprs[i]->containsUnexpandedParameterPack())
       ContainsUnexpandedParameterPack = true;
 
     if (Types[i]) {
       if (Types[i]->getType()->containsUnexpandedParameterPack())
         ContainsUnexpandedParameterPack = true;
 
       if (Types[i]->getType()->isDependentType()) {
         IsResultDependent = true;
       } else {
+        // We relax the restriction on use of incomplete types and non-object
+        // types with the type-based extension of _Generic. Allowing incomplete
+        // objects means those can be used as "tags" for a type-safe way to map
+        // to a value. Similarly, matching on function types rather than
+        // function pointer types can be useful. However, the restriction on VM
+        // types makes sense to retain as there are open questions about how
+        // the selection can be made at compile time.
+        //
         // C11 6.5.1.1p2 "The type name in a generic association shall specify a
         // complete object type other than a variably modified type."
         unsigned D = 0;
-        if (Types[i]->getType()->isIncompleteType())
+        if (ControllingExpr && Types[i]->getType()->isIncompleteType())
           D = diag::err_assoc_type_incomplete;
-        else if (!Types[i]->getType()->isObjectType())
+        else if (ControllingExpr && !Types[i]->getType()->isObjectType())
           D = diag::err_assoc_type_nonobject;
         else if (Types[i]->getType()->isVariablyModifiedType())
           D = diag::err_assoc_type_variably_modified;
-        else {
+        else if (ControllingExpr) {
           // Because the controlling expression undergoes lvalue conversion,
           // array conversion, and function conversion, an association which is
           // of array type, function type, or is qualified can never be
           // reached. We will warn about this so users are less surprised by
           // the unreachable association. However, we don't have to handle
           // function types; that's not an object type, so it's handled above.
           //
           // The logic is somewhat different for C++ because C++ has different
           // lvalue to rvalue conversion rules than C. [conv.lvalue]p1 says,
           // If T is a non-class type, the type of the prvalue is the cv-
           // unqualified version of T. Otherwise, the type of the prvalue is T.
           // The result of these rules is that all qualified types in an
           // association in C are unreachable, and in C++, only qualified non-
           // class types are unreachable.
+          //
+          // NB: this does not apply when the first operand is a type rather
+          // than an expression, because the type form does not undergo
+          // conversion.
           unsigned Reason = 0;
           QualType QT = Types[i]->getType();
           if (QT->isArrayType())
             Reason = 1;
           else if (QT.hasQualifiers() &&
                    (!LangOpts.CPlusPlus || !QT->isRecordType()))
             Reason = 2;
 

       }
     }
   }
   if (TypeErrorFound)
     return ExprError();
 
   // If we determined that the generic selection is result-dependent, don't
   // try to compute the result expression.
-  if (IsResultDependent)
-    return GenericSelectionExpr::Create(Context, KeyLoc, ControllingExpr, Types,
+  if (IsResultDependent) {
+    if (ControllingExpr)
+      return GenericSelectionExpr::Create(Context, KeyLoc, ControllingExpr,
+                                          Types, Exprs, DefaultLoc, RParenLoc,
+                                          ContainsUnexpandedParameterPack);
+    return GenericSelectionExpr::Create(Context, KeyLoc, ControllingType, Types,
                                         Exprs, DefaultLoc, RParenLoc,
                                         ContainsUnexpandedParameterPack);
+  }
 
   SmallVector<unsigned, 1> CompatIndices;
   unsigned DefaultIndex = -1U;
   // Look at the canonical type of the controlling expression in case it was a
   // deduced type like __auto_type. However, when issuing diagnostics, use the
   // type the user wrote in source rather than the canonical one.
   for (unsigned i = 0; i < NumAssocs; ++i) {
     if (!Types[i])
       DefaultIndex = i;
-    else if (Context.typesAreCompatible(
+    else if (ControllingExpr &&
+             Context.typesAreCompatible(
                  ControllingExpr->getType().getCanonicalType(),
-                                        Types[i]->getType()))
+      CompatIndices.push_back(i);
+    else if (ControllingType &&
+             Context.typesAreCompatible(
+                 ControllingType->getType().getCanonicalType(),
+                 Types[i]->getType()))
       CompatIndices.push_back(i);
   }
 
+  auto GetControllingRangeAndType = [](Expr *ControllingExpr,
+                                       TypeSourceInfo *ControllingType) {
+    // We strip parens here because the controlling expression is typically
+    // parenthesized in macro definitions.
+    if (ControllingExpr)
+      ControllingExpr = ControllingExpr->IgnoreParens();
+
+    SourceRange SR = ControllingExpr
+                         ? ControllingExpr->getSourceRange()
+                         : ControllingType->getTypeLoc().getSourceRange();
+    QualType QT = ControllingExpr ? ControllingExpr->getType()
+                                  : ControllingType->getType();
+
+    return std::make_pair(SR, QT);
+  };
+
   // C11 6.5.1.1p2 "The controlling expression of a generic selection shall have
   // type compatible with at most one of the types named in its generic
   // association list."
   if (CompatIndices.size() > 1) {
-    // We strip parens here because the controlling expression is typically
-    // parenthesized in macro definitions.
-    ControllingExpr = ControllingExpr->IgnoreParens();
-    Diag(ControllingExpr->getBeginLoc(), diag::err_generic_sel_multi_match)
-        << ControllingExpr->getSourceRange() << ControllingExpr->getType()
-        << (unsigned)CompatIndices.size();
+    auto P = GetControllingRangeAndType(ControllingExpr, ControllingType);
+    SourceRange SR = P.first;
+    Diag(SR.getBegin(), diag::err_generic_sel_multi_match)
+        << SR << P.second << (unsigned)CompatIndices.size();
     for (unsigned I : CompatIndices) {
       Diag(Types[I]->getTypeLoc().getBeginLoc(),
            diag::note_compat_assoc)
         << Types[I]->getTypeLoc().getSourceRange()
         << Types[I]->getType();
     }
     return ExprError();
   }
 
   // C11 6.5.1.1p2 "If a generic selection has no default generic association,
   // its controlling expression shall have type compatible with exactly one of
   // the types named in its generic association list."
   if (DefaultIndex == -1U && CompatIndices.size() == 0) {
-    // We strip parens here because the controlling expression is typically
-    // parenthesized in macro definitions.
-    ControllingExpr = ControllingExpr->IgnoreParens();
-    Diag(ControllingExpr->getBeginLoc(), diag::err_generic_sel_no_match)
-        << ControllingExpr->getSourceRange() << ControllingExpr->getType();
+    auto P = GetControllingRangeAndType(ControllingExpr, ControllingType);
+    SourceRange SR = P.first;
+    Diag(SR.getBegin(), diag::err_generic_sel_no_match) << SR << P.second;
     return ExprError();
   }
 
   // C11 6.5.1.1p3 "If a generic selection has a generic association with a
   // type name that is compatible with the type of the controlling expression,
   // then the result expression of the generic selection is the expression
   // in that generic association. Otherwise, the result expression of the
   // generic selection is the expression in the default generic association."
   unsigned ResultIndex =
     CompatIndices.size() ? CompatIndices[0] : DefaultIndex;
 
+  if (ControllingExpr) {
+    return GenericSelectionExpr::Create(
+        Context, KeyLoc, ControllingExpr, Types, Exprs, DefaultLoc, RParenLoc,
+        ContainsUnexpandedParameterPack, ResultIndex);
+  }
   return GenericSelectionExpr::Create(
-      Context, KeyLoc, ControllingExpr, Types, Exprs, DefaultLoc, RParenLoc,
+      Context, KeyLoc, ControllingType, Types, Exprs, DefaultLoc, RParenLoc,
       ContainsUnexpandedParameterPack, ResultIndex);
 }
 
 /// getUDSuffixLoc - Create a SourceLocation for a ud-suffix, given the
 /// location of the token and the offset of the ud-suffix within it.
 static SourceLocation getUDSuffixLoc(Sema &S, SourceLocation TokLoc,
                                      unsigned Offset) {
   return Lexer::AdvanceToTokenCharacter(TokLoc, Offset, S.getSourceManager(),

       if (AssocExpr.isUsable()) {
         AssocExprs.push_back(AssocExpr.get());
         AnyChanged = true;
       } else {
         AssocExprs.push_back(OrigAssocExpr);
       }
     }
 
+    void *ExOrTy = nullptr;
+    bool IsExpr = GSE->isExprPredicate();
+    if (IsExpr)
+      ExOrTy = GSE->getControllingExpr();
+    else
+      ExOrTy = GSE->getControllingType();
     return AnyChanged ? S.CreateGenericSelectionExpr(
                             GSE->getGenericLoc(), GSE->getDefaultLoc(),
-                            GSE->getRParenLoc(), GSE->getControllingExpr(),
+                            GSE->getRParenLoc(), IsExpr, ExOrTy,
                             GSE->getAssocTypeSourceInfos(), AssocExprs)
                       : ExprEmpty();
   }
 
   // [Clang extension]
   //   -- If e has the form __builtin_choose_expr(...), the set of potential
   //      results is the union of the sets of potential results of the
   //      second and third subexpressions.

clang/lib/Sema/SemaExprObjC.cpp

     assert(uo->getOpcode() == UO_Extension);
     Expr *sub = stripARCUnbridgedCast(uo->getSubExpr());
     return UnaryOperator::Create(Context, sub, UO_Extension, sub->getType(),
                                  sub->getValueKind(), sub->getObjectKind(),
                                  uo->getOperatorLoc(), false,
                                  CurFPFeatureOverrides());
   } else if (GenericSelectionExpr *gse = dyn_cast<GenericSelectionExpr>(e)) {
     assert(!gse->isResultDependent());
+    assert(!gse->isTypePredicate());
 
     unsigned n = gse->getNumAssocs();
     SmallVector<Expr *, 4> subExprs;
     SmallVector<TypeSourceInfo *, 4> subTypes;
     subExprs.reserve(n);
     subTypes.reserve(n);
     for (const GenericSelectionExpr::Association assoc : gse->associations()) {
       subTypes.push_back(assoc.getTypeSourceInfo());

clang/lib/Sema/SemaOverload.cpp

 
       // Replace the resulting type information before rebuilding the generic
       // selection expression.
       ArrayRef<Expr *> A = GSE->getAssocExprs();
       SmallVector<Expr *, 4> AssocExprs(A.begin(), A.end());
       unsigned ResultIdx = GSE->getResultIndex();
       AssocExprs[ResultIdx] = SubExpr;
 
+      if (GSE->isExprPredicate())
+        return GenericSelectionExpr::Create(
+            Context, GSE->getGenericLoc(), GSE->getControllingExpr(),
+            GSE->getAssocTypeSourceInfos(), AssocExprs, GSE->getDefaultLoc(),
+            GSE->getRParenLoc(), GSE->containsUnexpandedParameterPack(),
+            ResultIdx);
       return GenericSelectionExpr::Create(
-          Context, GSE->getGenericLoc(), GSE->getControllingExpr(),
+          Context, GSE->getGenericLoc(), GSE->getControllingType(),
           GSE->getAssocTypeSourceInfos(), AssocExprs, GSE->getDefaultLoc(),
           GSE->getRParenLoc(), GSE->containsUnexpandedParameterPack(),
           ResultIdx);
     }
     // Rather than fall through to the unreachable, return the original generic
     // selection expression.
     return GSE;
   }

clang/lib/Sema/SemaPseudoObject.cpp

Expr *rebuild(Expr *e) {
              gse->associations()) {
           Expr *assocExpr = assoc.getAssociationExpr();
           if (assoc.isSelected())
             assocExpr = rebuild(assocExpr);
           assocExprs.push_back(assocExpr);
           assocTypes.push_back(assoc.getTypeSourceInfo());
         }
 
+        if (gse->isExprPredicate())
           return GenericSelectionExpr::Create(
               S.Context, gse->getGenericLoc(), gse->getControllingExpr(),
               assocTypes, assocExprs, gse->getDefaultLoc(), gse->getRParenLoc(),
               gse->containsUnexpandedParameterPack(), resultIndex);
+        return GenericSelectionExpr::Create(
+            S.Context, gse->getGenericLoc(), gse->getControllingType(),
+            assocTypes, assocExprs, gse->getDefaultLoc(), gse->getRParenLoc(),
+            gse->containsUnexpandedParameterPack(), resultIndex);
       }
 
       if (ChooseExpr *ce = dyn_cast<ChooseExpr>(e)) {
         assert(!ce->isConditionDependent());
 
         Expr *LHS = ce->getLHS(), *RHS = ce->getRHS();
         Expr *&rebuiltExpr = ce->isConditionTrue() ? LHS : RHS;
         rebuiltExpr = rebuild(rebuiltExpr);

clang/lib/Sema/TreeTransform.h

   ExprResult RebuildChooseExpr(SourceLocation BuiltinLoc,
                                      Expr *Cond, Expr *LHS, Expr *RHS,
                                      SourceLocation RParenLoc) {
     return SemaRef.ActOnChooseExpr(BuiltinLoc,
                                    Cond, LHS, RHS,
                                    RParenLoc);
   }
 
-  /// Build a new generic selection expression.
+  /// Build a new generic selection expression with an expression predicate.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
   ExprResult RebuildGenericSelectionExpr(SourceLocation KeyLoc,
                                          SourceLocation DefaultLoc,
                                          SourceLocation RParenLoc,
                                          Expr *ControllingExpr,
                                          ArrayRef<TypeSourceInfo *> Types,
                                          ArrayRef<Expr *> Exprs) {
     return getSema().CreateGenericSelectionExpr(KeyLoc, DefaultLoc, RParenLoc,
+                                                /*PredicateIsExpr=*/true,
                                                 ControllingExpr, Types, Exprs);
   }
 
+  /// Build a new generic selection expression with a type predicate.
+  ///
+  /// By default, performs semantic analysis to build the new expression.
+  /// Subclasses may override this routine to provide different behavior.
+  ExprResult RebuildGenericSelectionExpr(SourceLocation KeyLoc,
+                                         SourceLocation DefaultLoc,
+                                         SourceLocation RParenLoc,
+                                         TypeSourceInfo *ControllingType,
+                                         ArrayRef<TypeSourceInfo *> Types,
+                                         ArrayRef<Expr *> Exprs) {
+    return getSema().CreateGenericSelectionExpr(KeyLoc, DefaultLoc, RParenLoc,
+                                                /*PredicateIsExpr=*/false,
+                                                ControllingType, Types, Exprs);
+  }
+
   /// Build a new overloaded operator call expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// The semantic analysis provides the behavior of template instantiation,
   /// copying with transformations that turn what looks like an overloaded
   /// operator call into a use of a builtin operator, performing
   /// argument-dependent lookup, etc. Subclasses may override this routine to
   /// provide different behavior.

 ExprResult
 TreeTransform<Derived>::TransformUserDefinedLiteral(UserDefinedLiteral *E) {
   return getDerived().TransformCallExpr(E);
 }
 
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformGenericSelectionExpr(GenericSelectionExpr *E) {
-  ExprResult ControllingExpr =
-    getDerived().TransformExpr(E->getControllingExpr());
-  if (ControllingExpr.isInvalid())
+  ExprResult ControllingExpr;
+  TypeSourceInfo *ControllingType = nullptr;
+  if (E->isExprPredicate())
+    ControllingExpr = getDerived().TransformExpr(E->getControllingExpr());
+  else
+    ControllingType = getDerived().TransformType(E->getControllingType());
+
+  if (ControllingExpr.isInvalid() && !ControllingType)
     return ExprError();
 
   SmallVector<Expr *, 4> AssocExprs;
   SmallVector<TypeSourceInfo *, 4> AssocTypes;
   for (const GenericSelectionExpr::Association Assoc : E->associations()) {
     TypeSourceInfo *TSI = Assoc.getTypeSourceInfo();
     if (TSI) {
       TypeSourceInfo *AssocType = getDerived().TransformType(TSI);
       if (!AssocType)
         return ExprError();
       AssocTypes.push_back(AssocType);
     } else {
       AssocTypes.push_back(nullptr);
     }
 
     ExprResult AssocExpr =
         getDerived().TransformExpr(Assoc.getAssociationExpr());
     if (AssocExpr.isInvalid())
       return ExprError();
     AssocExprs.push_back(AssocExpr.get());
   }
 
+  if (!ControllingType)
   return getDerived().RebuildGenericSelectionExpr(E->getGenericLoc(),
                                                   E->getDefaultLoc(),
                                                   E->getRParenLoc(),
                                                   ControllingExpr.get(),
                                                   AssocTypes,
                                                   AssocExprs);
+  return getDerived().RebuildGenericSelectionExpr(
+      E->getGenericLoc(), E->getDefaultLoc(), E->getRParenLoc(),
+      ControllingType, AssocTypes, AssocExprs);
 }
 
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformParenExpr(ParenExpr *E) {
   ExprResult SubExpr = getDerived().TransformExpr(E->getSubExpr());
   if (SubExpr.isInvalid())
     return ExprError();

clang/test/Parser/generic-selection-type-extension-pedantic.c

+// RUN: %clang_cc1 -std=c2x -fsyntax-only -verify -pedantic %s
+
+// Test that we get the extension warning when appropriate and that it shows up
+// in the right location.
+void test(void) {
+  (void)_Generic(
+    int,  // expected-warning {{passing a type argument as the first operand to '_Generic' is a Clang extension}}
+    int : 0);
+  (void)_Generic(
+    12,
+    int : 0);
+}
+

clang/test/Parser/generic-selection-type-extension.c

+// RUN: %clang_cc1 -std=c2x -fsyntax-only -verify -Wno-unused %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cpp -Wno-unused -x c++ %s
+
+// Test various parsing situations for the Clang extension to _Generic which
+// accepts a type name instead of an expression as the first operand.
+
+int foo();
+
+void test() {
+  // We can parse a simple type name.
+  _Generic(int, int : 0);
+
+  // We can also parse tag types.
+  struct S { int i; };
+  enum E { A };
+  union U { int i; };
+  _Generic(struct S, default : 0);
+  _Generic(enum E, default : 0);
+  _Generic(union U, default : 0);
+  
+  // We can also parse array types.
+  _Generic(int[12], default : 0);
+  
+  // And pointer to array types, too.
+  _Generic(int(*)[12], default : 0);
+  
+  // We do not accept a parenthesized type name.
+  _Generic((int), int : 0); // expected-error {{expected expression}}
+  
+  // We can parse more complex types as well. Note, this is a valid spelling of
+  // a function  pointer type in C but is not a valid spelling of a function
+  // pointer type in C++. Surprise!
+  _Generic(__typeof__(foo())(*)(__typeof__(&foo)), int (*)(int (*)()) : 0); // cpp-error {{expected expression}} \
+                                                                               cpp-error {{expected '(' for function-style cast or type construction}}
+
+  // C being the magical language that it is, lets you define a type anywhere
+  // you can spell a type.
+  _Generic(struct T { int a; }, default : 0); // cpp-error {{'T' cannot be defined in a type specifier}}
+}
+
+#ifdef __cplusplus
+template <typename Ty>
+struct S {
+  template <template <typename> typename Uy>
+  struct T {
+    typedef typename Uy<Ty>::type foo;
+  };
+};
+
+template <typename Ty>
+struct inst {
+  typedef Ty type;
+};
+
+void cpp_test() {
+  // Ensure we can parse more complex C++ typenames as well.
+  _Generic(S<int>::T<inst>::foo, int : 1);
+  
+  // And that the type name doesn't confuse us when given an initialization
+  // expression.
+  _Generic(S<int>::T<inst>::foo{}, int : 1);
+}
+
+template <typename Ty, int N = _Generic(Ty, int : 0, default : 1)>
+constexpr Ty bar() { return N; }
+
+static_assert(bar<int>() == 0);
+static_assert(bar<float>() == 1);
+#endif // __cplusplus

clang/test/Sema/generic-selection-type-extension.c

+// RUN: %clang_cc1 -std=c2x -fsyntax-only -verify -Wno-unused %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-unused -x c++ %s
+
+// Test that the semantic behavior of the extension allowing the user to pass a
+// type as the first argument to _Generic.
+
+// Test that we match on basic types.
+static_assert(_Generic(int, int : 1, default : 0) == 1);
+static_assert(_Generic(_BitInt(12), int : 1, _BitInt(10) : 2, _BitInt(12) : 3) == 3);
+
+// Test that we correctly fall back to the default association appropriately.
+static_assert(_Generic(int, long : 1, default : 0) == 0);
+
+// Ensure we correctly match constant arrays by their extent.
+static_assert(_Generic(int[12], int[0] : 0, int * : 0, int[12] : 1, default : 0) == 1);
+
+// Ensure we correctly match function types by their signature.
+static_assert(_Generic(int(int), void(void) : 0, int(void) : 0, void(int) : 0, int(int) : 1, default : 0) == 1);
+
+// Test that we still diagnose when no associations match and that the
+// diagnostic includes qualifiers.
+static_assert(_Generic(const int, long : 1)); // expected-error {{controlling expression type 'const int' not compatible with any generic association type}}
+
+// Test that qualifiers work as expected and do not issue a diagnostic when
+// using the type form.
+static_assert(_Generic(const int, int : 0, const int : 1) == 1);
+static_assert(_Generic(int volatile _Atomic const, int : 0, const int : 0, volatile int : 0, _Atomic int : 0, _Atomic const volatile int : 1) == 1);
+
+// Test that inferred qualifiers also work as expected.
+const int ci = 0;
+static_assert(_Generic(__typeof__(ci), int : 0, const int : 1) == 1);
+// And that the expression form still complains about qualified associations
+// and matches the correct association.
+static_assert(_Generic(ci, int : 1, const int : 0) == 1); // expected-warning {{due to lvalue conversion of the controlling expression, association of type 'const int' will never be selected because it is qualified}}
+
+// The type operand form of _Generic allows incomplete and non-object types,
+// but the expression operand form still rejects them.
+static_assert(_Generic(struct incomplete, struct incomplete : 1, default : 0) == 1);
+static_assert(_Generic(struct another_incomplete, struct incomplete : 1, default : 0) == 0);
+static_assert(_Generic(1, struct also_incomplete : 1, default : 0) == 0); // expected-error {{type 'struct also_incomplete' in generic association incomplete}}
+
+void foo(int);
+static_assert(_Generic(__typeof__(foo), void(int) : 1, default : 0) == 1);
+static_assert(_Generic(foo, void(int) : 1, default : 0) == 0); // expected-error {{type 'void (int)' in generic association not an object type}}
+
+// Ensure we still get a diagnostic for duplicated associations for the type
+// form, even when using qualified type, and that the diagnostic includes
+// qualifiers.
+static_assert(_Generic(const int,
+                         const int : 1, // expected-note {{compatible type 'const int' specified here}}
+                         int : 2,
+                         const int : 3  // expected-error {{type 'const int' in generic association compatible with previously specified type 'const int'}}
+                      ) == 1);
+
+// Verify that we are matching using the canonical type of the type operand...
+typedef int Int;
+typedef const Int CInt;
+typedef CInt OtherCInt;
+static_assert(_Generic(volatile CInt, const volatile int : 1, default : 0) == 1);
+static_assert(_Generic(const int, CInt : 1, default : 0) == 1);
+
+// ...and that duplicate associations are doing so as well.
+static_assert(_Generic(const int,
+                         CInt : 1,     // expected-note {{compatible type 'CInt' (aka 'const int') specified here}}
+                         const volatile int : 2,
+                         OtherCInt : 3 // expected-error {{type 'OtherCInt' (aka 'const int') in generic association compatible with previously specified type 'CInt' (aka 'const int')}}
+                      ) == 1);
+
+// Also test that duplicate array or function types are caught.
+static_assert(_Generic(const int,
+                         int[12] : 0,  // expected-note {{compatible type 'int[12]' specified here}}
+                         int[12] : 0,  // expected-error {{type 'int[12]' in generic association compatible with previously specified type 'int[12]'}}
+                         int(int) : 0, // expected-note {{compatible type 'int (int)' specified here}}
+                         int(int) : 0, // expected-error {{type 'int (int)' in generic association compatible with previously specified type 'int (int)'}}
+                         default : 1
+                      ) == 1);
+
+
+// Tests that only make sense for C++:
+#ifdef __cplusplus
+// Ensure that _Generic works within a template argument list.
+template <typename Ty, int N = _Generic(Ty, int : 0, default : 1)>
+constexpr Ty bar() { return N; }
+
+static_assert(bar<int>() == 0);
+static_assert(bar<float>() == 1);
+
+// Or that it can be used as a non-type template argument.
+static_assert(bar<int, _Generic(int, int : 1, default : 0)>() == 1);
+
+// Ensure that a dependent type works as expected.
+template <typename Ty>
+struct Dependent {
+  // If we checked the type early, this would fail to compile without any
+  // instantiation. Instead, it only fails with the bad instantiation.
+  static_assert(_Generic(Ty, int : 1)); // expected-error {{controlling expression type 'double' not compatible with any generic association type}} \
+                                           expected-note@#BadInstantiation {{in instantiation of template class 'Dependent<double>' requested here}}
+};
+
+template struct Dependent<int>; // Good instantiation
+template struct Dependent<double>; // #BadInstantiation
+
+// Another template instantiation test, this time for a variable template with
+// a type-dependent initializer.
+template <typename Ty>
+constexpr auto Val = _Generic(Ty, Ty : Ty{});
+
+static_assert(Val<int> == 0);
+static_assert(__is_same(decltype(Val<Dependent<int>>), const Dependent<int>));
+
+// Ensure that pack types also work as expected.
+template <unsigned Arg, unsigned... Args> struct Or {
+  enum { result = Arg | Or<Args...>::result };
+};
+
+template <unsigned Arg> struct Or<Arg> {
+  enum { result = Arg };
+};
+
+template <class... Args> struct TypeMask {
+  enum {
+   result = Or<_Generic(Args, int: 1, long: 2, short: 4, float: 8)...>::result
+  };
+};
+
+static_assert(TypeMask<int, long, short>::result == 7, "fail");
+static_assert(TypeMask<float, short>::result == 12, "fail");
+static_assert(TypeMask<int, float, float>::result == 9, "fail");
+#endif // __cplusplus
+

clang/test/SemaCXX/generic-selection.cpp

 // RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
 
 template <typename T, typename U = void*>
 struct A {
   enum {
-    id = _Generic(T(), // expected-error {{controlling expression type 'char' not compatible with any generic association type}}
+    id = _Generic(T{}, // expected-error {{controlling expression type 'char' not compatible with any generic association type}}
         int: 1, // expected-note {{compatible type 'int' specified here}}
         float: 2,
         U: 3) // expected-error {{type 'int' in generic association compatible with previously specified type 'int'}}
   };
 };
 
 static_assert(A<int>::id == 1, "fail");
 static_assert(A<float>::id == 2, "fail");
 static_assert(A<double, double>::id == 3, "fail");
 
 A<char> a1; // expected-note {{in instantiation of template class 'A<char>' requested here}}
 A<short, int> a2; // expected-note {{in instantiation of template class 'A<short, int>' requested here}}
 
 template <typename T, typename U>
 struct B {
   enum {
-    id = _Generic(T(),
+    id = _Generic(T{},
         int: 1, // expected-note {{compatible type 'int' specified here}}
         int: 2, // expected-error {{type 'int' in generic association compatible with previously specified type 'int'}}
         U: 3)
   };
 };
 
 template <unsigned Arg, unsigned... Args> struct Or {
   enum { result = Arg | Or<Args...>::result };
 };
 
 template <unsigned Arg> struct Or<Arg> {
   enum { result = Arg };
 };
 
 template <class... Args> struct TypeMask {
   enum {
-   result = Or<_Generic(Args(), int: 1, long: 2, short: 4, float: 8)...>::result
+   result = Or<_Generic(Args{}, int: 1, long: 2, short: 4, float: 8)...>::result
   };
 };
 
 static_assert(TypeMask<int, long, short>::result == 7, "fail");
 static_assert(TypeMask<float, short>::result == 12, "fail");
 static_assert(TypeMask<int, float, float>::result == 9, "fail");