Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -1973,6 +1973,8 @@
   "function concept declaration must be a definition">;
 def err_var_concept_not_initialized : Error<
   "variable concept declaration must be initialized">;
+def err_concept_tag : Error<
+  "%select{class|struct|interface|union|enum}0 cannot be marked concept">;
 
 // C++11 char16_t/char32_t
 def warn_cxx98_compat_unicode_type : Warning<
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -3662,6 +3662,19 @@
     return TagD;
   }
 
+  if (DS.isConceptSpecified()) {
+    // C++ Concepts TS [dcl.spec.concept]p1: A concept definition refers to
+    // either a function concept and its definition or a variable concept and
+    // its initializer.
+    if (Tag)
+      Diag(DS.getConceptSpecLoc(), diag::err_concept_tag)
+          << GetDiagnosticTypeSpecifierID(DS.getTypeSpecType());
+    else
+      Diag(DS.getConceptSpecLoc(), diag::err_concept_decl_non_template);
+    // Don't emit warnings after this error.
+    return TagD;
+  }
+
   DiagnoseFunctionSpecifiers(DS);
 
   if (DS.isFriendSpecified()) {
Index: test/SemaCXX/cxx-concept-declaration.cpp
===================================================================
--- test/SemaCXX/cxx-concept-declaration.cpp
+++ test/SemaCXX/cxx-concept-declaration.cpp
@@ -23,3 +23,13 @@
 template<typename T>
 concept bool D6; // expected-error {{variable concept declaration must be initialized}}
 
+// Tag
+concept class CC1 {}; // expected-error {{class cannot be marked concept}}
+concept struct CS1 {}; // expected-error {{struct cannot be marked concept}}
+concept union CU1 {}; // expected-error {{union cannot be marked concept}}
+concept enum CE1 {}; // expected-error {{enum cannot be marked concept}}
+template <typename T> concept class TCC1 {}; // expected-error {{class cannot be marked concept}}
+template <typename T> concept struct TCS1 {}; // expected-error {{struct cannot be marked concept}}
+template <typename T> concept union TCU1 {}; // expected-error {{union cannot be marked concept}}
+
+extern concept bool ExtC; // expected-error {{'concept' can only appear on the definition of a function template or variable template}}