Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7941,6 +7941,8 @@
   " 1,2,4 or 8 byte type (%0 invalid)">;
 def err_atomic_builtin_ext_int_size : Error<
   "Atomic memory operand must have a power-of-two size">;
+def err_atomic_builtin_ext_int_prohibit : Error<
+  "argument to atomic builtin of type '_ExtInt' is not supported">;
 def err_atomic_op_needs_atomic : Error<
   "address argument to atomic operation must be a pointer to _Atomic "
   "type (%0 invalid)">;
Index: clang/lib/Sema/SemaChecking.cpp
===================================================================
--- clang/lib/Sema/SemaChecking.cpp
+++ clang/lib/Sema/SemaChecking.cpp
@@ -4956,6 +4956,11 @@
                 ? 0
                 : 1);
 
+  if (ValType->isExtIntType()) {
+    Diag(Ptr->getExprLoc(), diag::err_atomic_builtin_ext_int_prohibit);
+    return ExprError();
+  }
+
   return AE;
 }
 
Index: clang/test/Sema/builtins.c
===================================================================
--- clang/test/Sema/builtins.c
+++ clang/test/Sema/builtins.c
@@ -285,12 +285,16 @@
   __sync_fetch_and_add(ptr, value); // expected-error {{Atomic memory operand must have a power-of-two size}}
   // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
   __sync_nand_and_fetch(ptr, value); // expected-error {{Atomic memory operand must have a power-of-two size}}
+
+  __atomic_fetch_add(ptr, 1, 0); // expected-error {{argument to atomic builtin of type '_ExtInt' is not supported}}
 }
 
 void test_ei_i64i(_ExtInt(64) *ptr, int value) {
   __sync_fetch_and_add(ptr, value); // expect success
   // expected-warning@+1 {{the semantics of this intrinsic changed with GCC version 4.4 - the newer semantics are provided here}}
   __sync_nand_and_fetch(ptr, value); // expect success
+
+  __atomic_fetch_add(ptr, 1, 0); // expected-error {{argument to atomic builtin of type '_ExtInt' is not supported}}
 }
 
 void test_ei_ii42(int *ptr, _ExtInt(42) value) {