Index: lib/AST/ItaniumMangle.cpp
===================================================================
--- lib/AST/ItaniumMangle.cpp
+++ lib/AST/ItaniumMangle.cpp
@@ -2532,6 +2532,18 @@
   // <expression> ::= dt <expression> <unresolved-name>
   //              ::= pt <expression> <unresolved-name>
   if (base) {
+
+    // Ignore member expressions involving anonymous unions.
+    while (const auto *RT = base->getType()->getAs<RecordType>()) {
+      if (!RT->getDecl()->isAnonymousStructOrUnion())
+        break;
+      const auto *ME = dyn_cast<MemberExpr>(base);
+      if (!ME)
+        break;
+      base = ME->getBase();
+      isArrow = ME->isArrow();
+    }
+
     if (base->isImplicitCXXThis()) {
       // Note: GCC mangles member expressions to the implicit 'this' as
       // *this., whereas we represent them as this->. The Itanium C++ ABI
Index: test/CodeGenCXX/mangle-exprs.cpp
===================================================================
--- test/CodeGenCXX/mangle-exprs.cpp
+++ test/CodeGenCXX/mangle-exprs.cpp
@@ -217,3 +217,79 @@
   template void a<int>(decltype(noexcept(int())));
   // CHECK: void @_ZN5test51aIiEEvDTnxcvT__EE(
 }
+
+namespace test6 {
+  struct X {
+    int i;
+  };
+
+  struct Y {
+    union {
+      int i;
+    };
+  };
+
+  struct Z {
+    union {
+      X ua;
+      Y ub;
+    };
+
+    struct {
+      X s;
+    };
+
+    union {
+      union {
+        struct {
+          struct {
+            X uuss;
+          };
+        };
+      };
+    };
+  };
+
+  Z z, *zp;
+
+  template<typename T>
+  void f1(decltype(T(z.ua.i))) {}
+  template void f1<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f1IiEEvDTcvT_dtdtL_ZNS_1zEE2ua1iE
+
+  template<typename T>
+  void f2(decltype(T(z.ub.i))) {}
+  template void f2<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f2IiEEvDTcvT_dtdtL_ZNS_1zEE2ub1iE
+
+  template<typename T>
+  void f3(decltype(T(z.s.i))) {}
+  template void f3<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f3IiEEvDTcvT_dtdtL_ZNS_1zEE1s1iE
+
+  template<typename T>
+  void f4(decltype(T(z.uuss.i))) {}
+  template void f4<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f4IiEEvDTcvT_dtdtL_ZNS_1zEE4uuss1iE
+
+  template<typename T>
+  void f5(decltype(T(zp->ua.i))) {}
+  template void f5<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f5IiEEvDTcvT_dtptL_ZNS_2zpEE2ua1iE
+
+  template<typename T>
+  void f6(decltype(T(zp->ub.i))) {}
+  template void f6<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f6IiEEvDTcvT_dtptL_ZNS_2zpEE2ub1iE
+
+  template<typename T>
+  void f7(decltype(T(zp->s.i))) {}
+  template void f7<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f7IiEEvDTcvT_dtptL_ZNS_2zpEE1s1iE
+
+  template<typename T>
+  void f8(decltype(T(zp->uuss.i))) {}
+  template void f8<int>(int);
+  // CHECK-LABEL: define weak_odr void @_ZN5test62f8IiEEvDTcvT_dtptL_ZNS_2zpEE4uuss1iE
+}
+