Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -449,6 +449,9 @@
     /// Index - The call index of this call.
     unsigned Index;
 
+    /// IndirectField - The indirect field being evaluated.
+    const IndirectFieldDecl *IndirectField = nullptr;
+
     // FIXME: Adding this to every 'CallStackFrame' may have a nontrivial impact
     // on the overall stack usage of deeply-recursing constexpr evaluataions.
     // (We should cache this map rather than recomputing it repeatedly.)
@@ -4370,6 +4373,7 @@
   for (const auto *I : Definition->inits()) {
     LValue Subobject = This;
     APValue *Value = &Result;
+    Frame.IndirectField = nullptr;
 
     // Determine the subobject to initialize.
     FieldDecl *FD = nullptr;
@@ -4399,6 +4403,7 @@
     } else if (IndirectFieldDecl *IFD = I->getIndirectMember()) {
       // Walk the indirect field decl's chain to find the object to initialize,
       // and make sure we've initialized every step along it.
+      Frame.IndirectField = IFD;
       for (auto *C : IFD->chain()) {
         FD = cast<FieldDecl>(C);
         CXXRecordDecl *CD = cast<CXXRecordDecl>(FD->getParent());
@@ -4437,6 +4442,7 @@
       Success = false;
     }
   }
+  Frame.IndirectField = nullptr;
 
   return Success &&
          EvaluateStmt(Ret, Info, Definition->getBody()) != ESR_Failed;
@@ -5612,6 +5618,20 @@
 
         Result.setFrom(Info.Ctx, RVal);
       }
+    } else if (Info.CurrentCall->IndirectField) {
+      // Fields of anonymous structs/unions can refer to other fields. In this
+      // case the 'this' expression corresponds to anonymous struct/union but
+      // Info.CurrentCall->This corresponds to the field's non-anonymous parent.
+      for (auto *C : Info.CurrentCall->IndirectField->chain()) {
+        const FieldDecl *FD = cast<FieldDecl>(C);
+        if (!FD->isImplicit()) {
+          assert(E->getType()->getPointeeCXXRecordDecl() == FD->getParent() &&
+                 "Should adjust LValue to refer to the same 'this'");
+          break;
+        }
+        if (!HandleLValueMember(Info, E, Result, FD))
+          return false;
+      }
     }
     return true;
   }
Index: clang/test/SemaCXX/constant-expression-cxx1y.cpp
===================================================================
--- clang/test/SemaCXX/constant-expression-cxx1y.cpp
+++ clang/test/SemaCXX/constant-expression-cxx1y.cpp
@@ -1021,3 +1021,44 @@
 }
 static_assert(evalNested(), "");
 } // namespace PR19741
+
+namespace IndirectFields {
+
+// Reference indirect field.
+struct A {
+  struct {
+    union {
+      int x = x = 3; // expected-note {{outside its lifetime}}
+    };
+  };
+  constexpr A() {}
+};
+static_assert(A().x == 3, ""); // expected-error{{not an integral constant expression}} expected-note{{in call to 'A()'}}
+
+// Reference another indirect field, with different 'this'.
+struct B {
+  struct {
+    union {
+      int x = 3;
+    };
+    int y = x;
+  };
+  constexpr B() {}
+};
+static_assert(B().y == 3, "");
+
+// Nested evaluation of indirect field initializers.
+struct C {
+  union {
+    int x = 1;
+  };
+};
+struct D {
+  struct {
+    C c;
+    int y = c.x + 1;
+  };
+};
+static_assert(D().y == 2, "");
+
+} // namespace IndirectFields