Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -45,6 +45,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include <map>
 #include <optional>
@@ -10321,21 +10322,34 @@
     }
   }
 
-  for (const auto *FD : RD.fields()) {
-    // Ill-formed if the field is an ObjectiveC pointer or of a type that is
-    // non-trivial for the purpose of calls.
+  llvm::SmallVector<const FieldDecl *> FieldsToCheck{RD.fields()};
+  while (!FieldsToCheck.empty()) {
+    const FieldDecl *FD = FieldsToCheck.pop_back_val();
+
+    // Ill-formed if the field is an ObjectiveC pointer.
     QualType FT = FD->getType();
     if (FT.getObjCLifetime() == Qualifiers::OCL_Weak) {
       PrintDiagAndRemoveAttr(4);
       return;
     }
 
-    if (const auto *RT = FT->getBaseElementTypeUnsafe()->getAs<RecordType>())
+    if (const auto *RT = FT->getBaseElementTypeUnsafe()->getAs<RecordType>()) {
+      // Check the fields of anonymous structs (and/or unions) instead of
+      // checking the type of the anonynous struct itself.
+      if (RT->getDecl()->isAnonymousStructOrUnion()) {
+        FieldsToCheck.append(RT->getDecl()->field_begin(),
+                             RT->getDecl()->field_end());
+        continue;
+      }
+
+      // Ill-formed if the field is of a type that is non-trivial for the
+      // purpose of calls.
       if (!RT->isDependentType() &&
           !cast<CXXRecordDecl>(RT->getDecl())->canPassInRegisters()) {
         PrintDiagAndRemoveAttr(5);
         return;
       }
+    }
   }
 }
 
Index: clang/test/SemaCXX/attr-trivial-abi.cpp
===================================================================
--- clang/test/SemaCXX/attr-trivial-abi.cpp
+++ clang/test/SemaCXX/attr-trivial-abi.cpp
@@ -169,3 +169,94 @@
 static_assert(__is_trivially_relocatable(S20), "");
 #endif
 } // namespace deletedCopyMoveConstructor
+
+namespace anonymousUnionsAndStructs {
+  // Test helper:
+  struct [[clang::trivial_abi]] Trivial {
+    Trivial() {}
+    Trivial(Trivial&& other) {}
+    Trivial& operator=(Trivial&& other) { return *this; }
+    ~Trivial() {}
+  };
+  static_assert(__is_trivially_relocatable(Trivial), "");
+
+  // Test helper:
+  struct Nontrivial {
+    Nontrivial() {}
+    Nontrivial(Nontrivial&& other) {}
+    Nontrivial& operator=(Nontrivial&& other) { return *this; }
+    ~Nontrivial() {}
+  };
+  static_assert(!__is_trivially_relocatable(Nontrivial), "");
+
+  // Basic smoke test, not yet related to anonymous unions or structs:
+  struct [[clang::trivial_abi]] BasicStruct {
+    BasicStruct(BasicStruct&& other) {}
+    BasicStruct& operator=(BasicStruct&& other) { return *this; }
+    ~BasicStruct() {}
+    Trivial field;
+  };
+  static_assert(__is_trivially_relocatable(BasicStruct), "");
+
+  // `StructWithAnonymousUnion` is like `BasicStruct`, but `field` is wrapped in
+  // an anonymous union, and thus trivial relocatability of `BasicStruct` and
+  // `StructWithAnonymousUnion` should be the same).
+  //
+  // It's impossible to declare a constructor for an anonymous unions so to
+  // support applying `[[clang::trivial_abi]]` to structs containing anonymous
+  // unions, and therefore when processing fields of the struct containing the
+  // anonymous union, the trivial relocatability of the *union* is ignored and
+  // instead the union's fields are recursively inspected in
+  // `checkIllFormedTrivialABIStruct`.
+  struct [[clang::trivial_abi]] StructWithAnonymousUnion {
+    StructWithAnonymousUnion(StructWithAnonymousUnion&& other) {}
+    StructWithAnonymousUnion& operator=(StructWithAnonymousUnion&& other) { return *this; }
+    ~StructWithAnonymousUnion() {}
+    union { Trivial field; };
+  };
+  static_assert(__is_trivially_relocatable(StructWithAnonymousUnion), "");
+
+  // `StructWithAnonymousStruct` is like `StructWithAnonymousUnion` but uses an
+  // anonymous `struct` rather than an anonymous `union.
+  struct [[clang::trivial_abi]] StructWithAnonymousStruct {
+    StructWithAnonymousStruct(StructWithAnonymousStruct&& other) {}
+    StructWithAnonymousStruct& operator=(StructWithAnonymousStruct&& other) { return *this; }
+    ~StructWithAnonymousStruct() {}
+    struct { Trivial field; };
+  };
+  static_assert(__is_trivially_relocatable(StructWithAnonymousStruct), "");
+
+  // `TrivialAbiAttributeAppliedToAnonymousUnion` is like
+  // `StructWithAnonymousUnion` but with `[[clang::trivial_abi]]` also applied
+  // to the anonymous union.
+  //
+  // The example below shows that it is still *not* okay to explicitly apply
+  // `[[clang::trivial_abi]]` to anonymous unions. Handling this would require
+  // relaxing the `HasNonDeletedCopyOrMoveConstructor` check when
+  // `isAnonymousStructOrUnion` in `checkIllFormedTrivialABIStruct` but when
+  // that check runs `setAnonymousStructOrUnion` hasn't been called yet (i.e. at
+  // this point it's not possible to rely on `RD->isAnonymousStructOrUnion()`).
+  struct [[clang::trivial_abi]] TrivialAbiAttributeAppliedToAnonymousUnion {
+    TrivialAbiAttributeAppliedToAnonymousUnion(TrivialAbiAttributeAppliedToAnonymousUnion&& other) {}
+    TrivialAbiAttributeAppliedToAnonymousUnion& operator=(TrivialAbiAttributeAppliedToAnonymousUnion&& other) { return *this; }
+    ~TrivialAbiAttributeAppliedToAnonymousUnion() {}
+    union [[clang::trivial_abi]] { // expected-warning {{'trivial_abi' cannot be applied to '(unnamed union}} expected-note {{copy constructors and move constructors are all deleted}}
+      Trivial field;
+    };
+  };
+  static_assert(__is_trivially_relocatable(TrivialAbiAttributeAppliedToAnonymousUnion), "");
+
+  // Like `StructWithAnonymousUnion`, but the field of the anonymous union is
+  // *not* trivial.
+  struct [[clang::trivial_abi]] StructWithAnonymousUnionWithNonTrivialField { // expected-warning {{'trivial_abi' cannot be applied to 'StructWithAnonymousUnionWithNonTrivialField'}} expected-note {{trivial_abi' is disallowed on 'StructWithAnonymousUnionWithNonTrivialField' because it has a field of a non-trivial class type}}
+    StructWithAnonymousUnionWithNonTrivialField(StructWithAnonymousUnionWithNonTrivialField&& other) {}
+    StructWithAnonymousUnionWithNonTrivialField& operator=(StructWithAnonymousUnionWithNonTrivialField&& other) { return *this; }
+    ~StructWithAnonymousUnionWithNonTrivialField() {}
+    union {
+      Nontrivial field;
+    };
+  };
+  static_assert(!__is_trivially_relocatable(StructWithAnonymousUnionWithNonTrivialField), "");
+
+}  // namespace anonymousStructsAndUnions
+