diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1534,6 +1534,7 @@
     APValue::LValueBase Base;
     CharUnits Offset;
     SubobjectDesignator Designator;
+    const Expr *LExpr = nullptr;
     bool IsNullPtr : 1;
     bool InvalidBase : 1;
 
@@ -1542,6 +1543,7 @@
     const CharUnits &getLValueOffset() const { return Offset; }
     SubobjectDesignator &getLValueDesignator() { return Designator; }
     const SubobjectDesignator &getLValueDesignator() const { return Designator;}
+    const Expr *getExpr() const { return LExpr; }
     bool isNullPointer() const { return IsNullPtr;}
 
     unsigned getLValueCallIndex() const { return Base.getCallIndex(); }
@@ -1579,6 +1581,8 @@
       Offset = CharUnits::fromQuantity(0);
       InvalidBase = BInvalid;
       Designator = SubobjectDesignator(getType(B));
+      if (!LExpr)
+        LExpr = B.dyn_cast<const Expr *>();
       IsNullPtr = false;
     }
 
@@ -6077,8 +6081,11 @@
     if (!handleTrivialCopy(Info, MD->getParamDecl(0), Args[0], RHSValue,
                            MD->getParent()->isUnion()))
       return false;
+    const Expr *LHS = This->getExpr();
+    if (!LHS)
+      return false;
     if (Info.getLangOpts().CPlusPlus20 && MD->isTrivial() &&
-        !HandleUnionActiveMemberChange(Info, Args[0], *This))
+        !HandleUnionActiveMemberChange(Info, LHS, *This))
       return false;
     if (!handleAssignment(Info, Args[0], *This, MD->getThisType(),
                           RHSValue))
@@ -8039,6 +8046,11 @@
   LValueExprEvaluator(EvalInfo &Info, LValue &Result, bool InvalidBaseOK) :
     LValueExprEvaluatorBaseTy(Info, Result, InvalidBaseOK) {}
 
+  bool evaluate(const Expr *E) {
+    Result.LExpr = E;
+    return Visit(E);
+  }
+
   bool VisitVarDecl(const Expr *E, const VarDecl *VD);
   bool VisitUnaryPreIncDec(const UnaryOperator *UO);
 
@@ -8100,7 +8112,7 @@
   assert(!E->isValueDependent());
   assert(E->isGLValue() || E->getType()->isFunctionType() ||
          E->getType()->isVoidType() || isa<ObjCSelectorExpr>(E));
-  return LValueExprEvaluator(Info, Result, InvalidBaseOK).Visit(E);
+  return LValueExprEvaluator(Info, Result, InvalidBaseOK).evaluate(E);
 }
 
 bool LValueExprEvaluator::VisitDeclRefExpr(const DeclRefExpr *E) {
@@ -8571,6 +8583,8 @@
       : ExprEvaluatorBaseTy(info), Result(Result),
         InvalidBaseOK(InvalidBaseOK) {}
 
+  bool evaluate(const Expr *E) { return Visit(E); }
+
   bool Success(const APValue &V, const Expr *E) {
     Result.setFrom(Info.Ctx, V);
     return true;
@@ -8680,7 +8694,7 @@
                             bool InvalidBaseOK) {
   assert(!E->isValueDependent());
   assert(E->isPRValue() && E->getType()->hasPointerRepresentation());
-  return PointerExprEvaluator(Info, Result, InvalidBaseOK).Visit(E);
+  return PointerExprEvaluator(Info, Result, InvalidBaseOK).evaluate(E);
 }
 
 bool PointerExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
diff --git a/clang/test/SemaCXX/constant-expression-cxx2a.cpp b/clang/test/SemaCXX/constant-expression-cxx2a.cpp
--- a/clang/test/SemaCXX/constant-expression-cxx2a.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx2a.cpp
@@ -1447,3 +1447,11 @@
   constexpr bool b = [a = S(), b = S()] { return a.p == b.p; }();
   static_assert(!b);
 }
+
+namespace PR45879 {
+struct Base {
+  int m;
+};
+struct Derived : Base {};
+constexpr int k = ((Base{} = Derived{}), 0);
+} // namespace PR45879