Don't diagnostic atomic object access as UB in an unevaluated context

We started diagnosing this situation with a more clear diagnostic
message, but it was pointed out that unevaluated contexts don't really
have the undefined behavior property as there is no runtime access
involved.

This augments the changes in https://reviews.llvm.org/D122656 to not
diagnose in an unevaluated context.

diff --git a/clang/lib/Sema/SemaExprMember.cpp b/clang/lib/Sema/SemaExprMember.cpp
index 152b39d..cb95f70 100644 (file)
--- a/clang/lib/Sema/SemaExprMember.cpp
+++ b/clang/lib/Sema/SemaExprMember.cpp
@@ -1298,7 +1298,8 @@ static ExprResult LookupMemberExpr(Sema &S, LookupResult &R,
   // 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,

diff --git a/clang/test/Sema/atomic-expr.c b/clang/test/Sema/atomic-expr.c
index 86cd32c..6c6823c 100644 (file)
--- a/clang/test/Sema/atomic-expr.c
+++ b/clang/test/Sema/atomic-expr.c
@@ -102,4 +102,10 @@ void func_16(void) {
   _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);
 }