Index: clang/include/clang/Basic/Attr.td
===================================================================
--- clang/include/clang/Basic/Attr.td
+++ clang/include/clang/Basic/Attr.td
@@ -267,8 +267,10 @@
   string Namespace = namespace;
   int Version = version;
 }
-class C2x<string namespace, string name> : Spelling<name, "C2x"> {
+class C2x<string namespace, string name, int version = 1>
+    : Spelling<name, "C2x"> {
   string Namespace = namespace;
+  int Version = version;
 }
 
 class Keyword<string name> : Spelling<name, "Keyword">;
@@ -1221,7 +1223,8 @@
 
 def Deprecated : InheritableAttr {
   let Spellings = [GCC<"deprecated">, Declspec<"deprecated">,
-                   CXX11<"","deprecated", 201309>, C2x<"", "deprecated">];
+                   CXX11<"","deprecated", 201309>,
+                   C2x<"", "deprecated", 201904>];
   let Args = [StringArgument<"Message", 1>,
               // An optional string argument that enables us to provide a
               // Fix-It.
@@ -1278,7 +1281,8 @@
 }
 
 def FallThrough : StmtAttr {
-  let Spellings = [CXX11<"", "fallthrough", 201603>, C2x<"", "fallthrough">,
+  let Spellings = [CXX11<"", "fallthrough", 201603>,
+                   C2x<"", "fallthrough", 201904>,
                    CXX11<"clang", "fallthrough">, GCC<"fallthrough">];
 //  let Subjects = [NullStmt];
   let Documentation = [FallthroughDocs];
@@ -2442,7 +2446,7 @@
 
 def Unused : InheritableAttr {
   let Spellings = [CXX11<"", "maybe_unused", 201603>, GCC<"unused">,
-                   C2x<"", "maybe_unused">];
+                   C2x<"", "maybe_unused", 201904>];
   let Subjects = SubjectList<[Var, ObjCIvar, Type, Enum, EnumConstant, Label,
                               Field, ObjCMethod, FunctionLike]>;
   let Documentation = [WarnMaybeUnusedDocs];
@@ -2528,7 +2532,8 @@
 }
 
 def WarnUnusedResult : InheritableAttr {
-  let Spellings = [CXX11<"", "nodiscard", 201907>, C2x<"", "nodiscard">,
+  let Spellings = [CXX11<"", "nodiscard", 201907>,
+                   C2x<"", "nodiscard", 201904>,
                    CXX11<"clang", "warn_unused_result">,
                    GCC<"warn_unused_result">];
   let Subjects = SubjectList<[ObjCMethod, Enum, Record, FunctionLike]>;
Index: clang/test/Preprocessor/has_c_attribute.c
===================================================================
--- clang/test/Preprocessor/has_c_attribute.c
+++ clang/test/Preprocessor/has_c_attribute.c
@@ -1,22 +1,44 @@
-// RUN: %clang_cc1 -fdouble-square-bracket-attributes -std=c11 -E %s -o - | FileCheck %s
-// RUN: %clang_cc1 -std=c2x -E %s -o - | FileCheck %s
-
-// CHECK: has_fallthrough
-#if __has_c_attribute(fallthrough)
-  int has_fallthrough();
-#endif
-
-// CHECK: does_not_have_selectany
-#if !__has_c_attribute(selectany)
-  int does_not_have_selectany();
-#endif
-
-// CHECK: has_nodiscard_underscore
-#if __has_c_attribute(__nodiscard__)
-  int has_nodiscard_underscore();
-#endif
-
-// CHECK: has_clang_annotate
-#if __has_c_attribute(clang::annotate)
-  int has_clang_annotate();
-#endif
+// RUN: %clang_cc1 -fdouble-square-bracket-attributes -std=c11 -E -P %s -o - | FileCheck %s
+// RUN: %clang_cc1 -std=c2x -E -P %s -o - | FileCheck %s
+
+#define C2x(x) x: __has_c_attribute(x)
+
+// CHECK: fallthrough: 201904L
+C2x(fallthrough)
+
+// CHECK: __nodiscard__: 201904L
+C2x(__nodiscard__)
+
+// CHECK: selectany: 0
+C2x(selectany); // Known attribute not supported in C mode
+
+// CHECK: frobble: 0
+C2x(frobble) // Unknown attribute
+
+// CHECK: frobble::frobble: 0
+C2x(frobble::frobble) // Unknown vendor namespace
+
+// CHECK: clang::annotate: 1
+C2x(clang::annotate)
+
+// CHECK: deprecated: 201904L
+C2x(deprecated)
+
+// CHECK: maybe_unused: 201904L
+C2x(maybe_unused)
+
+// CHECK: __gnu__::warn_unused_result: 201904L
+C2x(__gnu__::warn_unused_result)
+
+// CHECK: gnu::__warn_unused_result__: 201904L
+C2x(gnu::__warn_unused_result__)
+
+// We do somewhat support the __clang__ vendor namespace, but it is a
+// predefined macro and thus we encourage users to use _Clang instead.
+// Because of this, we do not support __has_c_attribute for that
+// vendor namespace.
+//
+// Note, we can't use C2x here because it will expand __clang__ to 1
+// too early.
+// CHECK: 1::fallthrough: 0
+__clang__::fallthrough: __has_c_attribute(__clang__::fallthrough)
Index: clang/utils/TableGen/ClangAttrEmitter.cpp
===================================================================
--- clang/utils/TableGen/ClangAttrEmitter.cpp
+++ clang/utils/TableGen/ClangAttrEmitter.cpp
@@ -3064,18 +3064,22 @@
     // attribute version information should be taken from the SD-6 standing
     // document, which can be found at:
     // https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations
+    //
+    // C2x-style attributes have the same kind of version information
+    // associated with them. The unscoped attribute version information should
+    // be taken from the specification of the attribute in the C Standard.
     int Version = 1;
 
-    if (Variety == "CXX11") {
-        std::vector<Record *> Spellings = Attr->getValueAsListOfDefs("Spellings");
-        for (const auto &Spelling : Spellings) {
-          if (Spelling->getValueAsString("Variety") == "CXX11") {
-            Version = static_cast<int>(Spelling->getValueAsInt("Version"));
-            if (Scope.empty() && Version == 1)
-              PrintError(Spelling->getLoc(), "C++ standard attributes must "
-              "have valid version information.");
-            break;
-          }
+    if (Variety == "CXX11" || Variety == "C2x") {
+      std::vector<Record *> Spellings = Attr->getValueAsListOfDefs("Spellings");
+      for (const auto &Spelling : Spellings) {
+        if (Spelling->getValueAsString("Variety") == Variety) {
+          Version = static_cast<int>(Spelling->getValueAsInt("Version"));
+          if (Scope.empty() && Version == 1)
+            PrintError(Spelling->getLoc(), "Standard attributes must have "
+                                           "valid version information.");
+          break;
+        }
       }
     }