Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -85,6 +85,10 @@
 - Assignment expressions in C11 and later mode now properly strip the _Atomic
   qualifier when determining the type of the assignment expression. Fixes
   `Issue 48742 <https://github.com/llvm/llvm-project/issues/48742>`_.
+- Improved the diagnostic when accessing a member of an atomic structure or
+  union object in C; was previously an unhelpful error, but now issues a
+  `-Watomic-access` warning which defaults to an error. Fixes
+  `Issue 54563 <https://github.com/llvm/llvm-project/issues/54563>`_.
 - Unevaluated lambdas in dependant contexts no longer result in clang crashing.
   This fixes Issues `50376 <https://github.com/llvm/llvm-project/issues/50376>`_,
   `51414 <https://github.com/llvm/llvm-project/issues/51414>`_,
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -6439,6 +6439,9 @@
             "%select{incomplete |array |function |reference |atomic |qualified "
             "|sizeless ||integer }0type "
             "%1 %select{|||||||which is not trivially copyable|}0">;
+def warn_atomic_member_access : Warning<
+  "accessing a member of an atomic structure or union is undefined behavior">,
+  InGroup<DiagGroup<"atomic-access">>, DefaultError;
 
 // Expressions.
 def ext_sizeof_alignof_function_type : Extension<
Index: clang/lib/Sema/SemaExprMember.cpp
===================================================================
--- clang/lib/Sema/SemaExprMember.cpp
+++ clang/lib/Sema/SemaExprMember.cpp
@@ -1292,6 +1292,20 @@
     }
   }
 
+  // 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);
+    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.
   if (const RecordType *RTy = BaseType->getAs<RecordType>()) {
     TypoExpr *TE = nullptr;
Index: clang/test/Sema/atomic-expr.c
===================================================================
--- clang/test/Sema/atomic-expr.c
+++ clang/test/Sema/atomic-expr.c
@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 %s -verify -fsyntax-only
-// expected-no-diagnostics
+// RUN: %clang_cc1 %s -verify=off -fsyntax-only -Wno-atomic-access
+// off-no-diagnostics
 
 _Atomic(unsigned int) data1;
 int _Atomic data2;
@@ -75,3 +76,30 @@
   _Static_assert(__builtin_types_compatible_p(__typeof__(x = 2), int), "incorrect");
   _Static_assert(__builtin_types_compatible_p(__typeof__(x += 2), int), "incorrect");
 }
+
+// Ensure that member access of an atomic structure or union type is properly
+// diagnosed as being undefined behavior; Issue 54563.
+void func_16(void) {
+  // LHS member access.
+  _Atomic struct { int val; } x, *xp;
+  x.val = 12;   // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+  xp->val = 12; // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+
+  _Atomic union {
+    int ival;
+    float fval;
+  } y, *yp;
+  y.ival = 12;     // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+  yp->fval = 1.2f; // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+
+  // RHS member access.
+  int xval = x.val; // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+  xval = xp->val;   // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+  int yval = y.val; // expected-error {{accessing a member of an atomic structure or union is undefined behavior}}
+  yval = yp->val;   // 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}}
+}