Index: clang/lib/Sema/SemaExprMember.cpp
===================================================================
--- clang/lib/Sema/SemaExprMember.cpp
+++ clang/lib/Sema/SemaExprMember.cpp
@@ -1298,7 +1298,8 @@
   // lvalue. Because this is inherently unsafe as an atomic operation, the
   // warning defaults to an error.
   if (const auto *ATy = BaseType->getAs<AtomicType>()) {
-    S.Diag(OpLoc, diag::warn_atomic_member_access);
+    S.DiagRuntimeBehavior(OpLoc, nullptr,
+                          S.PDiag(diag::warn_atomic_member_access));
     BaseType = ATy->getValueType().getUnqualifiedType();
     BaseExpr = ImplicitCastExpr::Create(
         S.Context, IsArrow ? S.Context.getPointerType(BaseType) : BaseType,
Index: clang/test/Sema/atomic-expr.c
===================================================================
--- clang/test/Sema/atomic-expr.c
+++ clang/test/Sema/atomic-expr.c
@@ -102,4 +102,10 @@
   _Atomic(struct { int val; }) z;
   z.val = 12;       // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
   int zval = z.val; // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+
+  // Don't diagnose in an unevaluated context, however.
+  (void)sizeof(x.val);
+  (void)sizeof(xp->val);
+  (void)sizeof(y.ival);
+  (void)sizeof(yp->ival);
 }