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

[aaron.ballman]

Summary

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.

[erichkeane]

Ah! Release notes!

[aaron.ballman]

In D122681#3414900, @erichkeane wrote:

Ah! Release notes!

I didn't add one because this is augmenting https://reviews.llvm.org/D122656 which already added the release note. :-)

[erichkeane]

In D122681#3414901, @aaron.ballman wrote:

In D122681#3414900, @erichkeane wrote:

Ah! Release notes!

I didn't add one because this is augmenting https://reviews.llvm.org/D122656 which already added the release note. :-)

Ah! SGTM!

[aaron.ballman]

Thank you for the reviews, I've commit in cf4a67bcbbb109347b1e2d6cedf69bae142a182f.

clang/lib/Sema/SemaExprMember.cpp

   }
 
   // If the base type is an atomic type, this access is undefined behavior per
   // C11 6.5.2.3p5. Instead of giving a typecheck error, we'll warn the user
   // about the UB and recover by converting the atomic lvalue into a non-atomic
   // 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,
         CK_AtomicToNonAtomic, BaseExpr.get(), nullptr,
         BaseExpr.get()->getValueKind(), FPOptionsOverride());
   }
 
   // Handle field access to simple records.

clang/test/Sema/atomic-expr.c

void func_16(void) {
   xval = xp->val;   // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
   int yval = y.ival; // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
   yval = yp->ival;   // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
 
   // Using the type specifier instead of the type qualifier.
   _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);
 }