Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp	(revision 205775)
+++ lib/Sema/SemaDeclCXX.cpp	(working copy)
@@ -1177,12 +1177,19 @@
       // C++1y doesn't require constexpr functions to contain a 'return'
       // statement. We still do, unless the return type is void, because
       // otherwise if there's no return statement, the function cannot
-      // be used in a core constant expression.
+      // be used in a core constant expression. Also, deduced return type
+      // with no return statements are perfectly legal. For example:
+      // template<typename T> struct X { constexpr auto f() {} };
+
       bool OK = getLangOpts().CPlusPlus1y && Dcl->getReturnType()->isVoidType();
+
+      if (!getLangOpts().CPlusPlus1y ||
+          !Dcl->getReturnType()->getContainedAutoType()) {
       Diag(Dcl->getLocation(),
            OK ? diag::warn_cxx11_compat_constexpr_body_no_return
               : diag::err_constexpr_body_no_return);
       return OK;
+      }
     }
     if (ReturnStmts.size() > 1) {
       Diag(ReturnStmts.back(),
Index: test/SemaCXX/cxx1y-deduced-return-type.cpp
===================================================================
--- test/SemaCXX/cxx1y-deduced-return-type.cpp	(revision 205775)
+++ test/SemaCXX/cxx1y-deduced-return-type.cpp	(working copy)
@@ -261,6 +261,7 @@
 
 namespace Constexpr {
   constexpr auto f1(int n) { return n; }
+  template<typename T> struct X { constexpr auto f() {} }; // PR18746
   struct NonLiteral { ~NonLiteral(); } nl; // expected-note {{user-provided destructor}}
   constexpr auto f2(int n) { return nl; } // expected-error {{return type 'Constexpr::NonLiteral' is not a literal type}}
 }