Index: lib/AST/ExprConstant.cpp
===================================================================
--- lib/AST/ExprConstant.cpp
+++ lib/AST/ExprConstant.cpp
@@ -7022,11 +7022,9 @@
   }
 
   case Builtin::BIstrlen:
-    // A call to strlen is not a constant expression.
-    if (Info.getLangOpts().CPlusPlus11)
-      Info.CCEDiag(E, diag::note_constexpr_invalid_function)
-        << /*isConstexpr*/0 << /*isConstructor*/0 << "'strlen'";
-    else
+    // As a GCC compatibility extension, we support strlen() 
+    // as a constant expression.
+    if (!Info.getLangOpts().CPlusPlus11)
       Info.CCEDiag(E, diag::note_invalid_subexpr_in_const_expr);
     // Fall through.
   case Builtin::BI__builtin_strlen: {
Index: test/SemaCXX/constexpr-strlen.cpp
===================================================================
--- test/SemaCXX/constexpr-strlen.cpp
+++ test/SemaCXX/constexpr-strlen.cpp
@@ -8,7 +8,7 @@
 
 # 10 "SemaCXX/constexpr-strlen.cpp" 2
 constexpr int n = __builtin_strlen("hello"); // ok
-constexpr int m = strlen("hello"); // expected-error {{constant expression}} expected-note {{non-constexpr function 'strlen' cannot be used in a constant expression}}
+constexpr int m = strlen("hello"); // ok
 
 // Make sure we can evaluate a call to strlen.
 int arr[3]; // expected-note {{here}}