diff --git a/include/clad/Differentiator/VisitorBase.h b/include/clad/Differentiator/VisitorBase.h
index 917088c42..dba1540a2 100644
--- a/include/clad/Differentiator/VisitorBase.h
+++ b/include/clad/Differentiator/VisitorBase.h
@@ -362,8 +362,17 @@ namespace clad {
     /// \param[in] D The declaration to build a DeclRefExpr for.
     /// \param[in] SS The scope specifier for the declaration.
     /// \returns the DeclRefExpr for the given declaration.
-    clang::DeclRefExpr* BuildDeclRef(clang::DeclaratorDecl* D,
-                                     const clang::CXXScopeSpec* SS = nullptr);
+    clang::DeclRefExpr*
+    BuildDeclRef(clang::DeclaratorDecl* D,
+                 const clang::CXXScopeSpec* SS = nullptr,
+                 clang::ExprValueKind VK = clang::VK_LValue);
+    /// Builds a DeclRefExpr to a given Decl, adding proper nested name
+    /// qualifiers.
+    /// \param[in] D The declaration to build a DeclRefExpr for.
+    /// \param[in] NNS The nested name specifier to use.
+    clang::DeclRefExpr*
+    BuildDeclRef(clang::DeclaratorDecl* D, clang::NestedNameSpecifier* NNS,
+                 clang::ExprValueKind VK = clang::VK_LValue);
 
     /// Stores the result of an expression in a temporary variable (of the same
     /// type as is the result of the expression) and returns a reference to it.
diff --git a/lib/Differentiator/BaseForwardModeVisitor.cpp b/lib/Differentiator/BaseForwardModeVisitor.cpp
index fe14227b7..8015b8fdb 100644
--- a/lib/Differentiator/BaseForwardModeVisitor.cpp
+++ b/lib/Differentiator/BaseForwardModeVisitor.cpp
@@ -1036,8 +1036,9 @@ StmtDiff BaseForwardModeVisitor::VisitDeclRefExpr(const DeclRefExpr* DRE) {
     // Sema::BuildDeclRefExpr is responsible for adding captured fields
     // to the underlying struct of a lambda.
     if (clonedDRE->getDecl()->getDeclContext() != m_Sema.CurContext) {
-      auto referencedDecl = cast<VarDecl>(clonedDRE->getDecl());
-      clonedDRE = cast<DeclRefExpr>(BuildDeclRef(referencedDecl));
+      NestedNameSpecifier* NNS = DRE->getQualifier();
+      auto* referencedDecl = cast<VarDecl>(clonedDRE->getDecl());
+      clonedDRE = BuildDeclRef(referencedDecl, NNS);
     }
   } else
     clonedDRE = cast<DeclRefExpr>(Clone(DRE));
@@ -1052,7 +1053,7 @@ StmtDiff BaseForwardModeVisitor::VisitDeclRefExpr(const DeclRefExpr* DRE) {
       if (auto dVarDRE = dyn_cast<DeclRefExpr>(dExpr)) {
         auto dVar = cast<VarDecl>(dVarDRE->getDecl());
         if (dVar->getDeclContext() != m_Sema.CurContext)
-          dExpr = BuildDeclRef(dVar);
+          dExpr = BuildDeclRef(dVar, DRE->getQualifier());
       }
       return StmtDiff(clonedDRE, dExpr);
     }
diff --git a/lib/Differentiator/ReverseModeVisitor.cpp b/lib/Differentiator/ReverseModeVisitor.cpp
index e90036593..2a7a16aa4 100644
--- a/lib/Differentiator/ReverseModeVisitor.cpp
+++ b/lib/Differentiator/ReverseModeVisitor.cpp
@@ -1544,8 +1544,12 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
       // with Sema::BuildDeclRefExpr. This is required in some cases, e.g.
       // Sema::BuildDeclRefExpr is responsible for adding captured fields
       // to the underlying struct of a lambda.
-      if (VD->getDeclContext() != m_Sema.CurContext)
-        clonedDRE = cast<DeclRefExpr>(BuildDeclRef(VD));
+      if (VD->getDeclContext() != m_Sema.CurContext) {
+        auto* ccDRE = dyn_cast<DeclRefExpr>(clonedDRE);
+        NestedNameSpecifier* NNS = DRE->getQualifier();
+        auto* referencedDecl = cast<VarDecl>(ccDRE->getDecl());
+        clonedDRE = BuildDeclRef(referencedDecl, NNS, DRE->getValueKind());
+      }
       // This case happens when ref-type variables have to become function
       // global. Ref-type declarations cannot be moved to the function global
       // scope because they can't be separated from their inits.
@@ -1852,9 +1856,9 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
     }
 
     Expr* OverloadedDerivedFn = nullptr;
-    // If the function has a single arg and does not returns a reference or take
+    // If the function has a single arg and does not return a reference or take
     // arg by reference, we look for a derivative w.r.t. to this arg using the
-    // forward mode(it is unlikely that we need gradient of a one-dimensional'
+    // forward mode(it is unlikely that we need gradient of a one-dimensional
     // function).
     bool asGrad = true;
 
@@ -2149,8 +2153,7 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
         StmtDiff argDiff = Visit(arg);
         CallArgs.push_back(argDiff.getExpr_dx());
       }
-      if (baseDiff.getExpr()) {
-        Expr* baseE = baseDiff.getExpr();
+      if (Expr* baseE = baseDiff.getExpr()) {
         call = BuildCallExprToMemFn(baseE, calleeFnForwPassFD->getName(),
                                     CallArgs, Loc);
       } else {
@@ -2167,6 +2170,28 @@ Expr* getArraySizeExpr(const ArrayType* AT, ASTContext& context,
           utils::BuildMemberExpr(m_Sema, getCurrentScope(), callRes, "adjoint");
       return StmtDiff(resValue, resAdjoint, resAdjoint);
     } // Recreate the original call expression.
+
+    if (const auto* OCE = dyn_cast<CXXOperatorCallExpr>(CE)) {
+      auto* FD = const_cast<CXXMethodDecl*>(
+          dyn_cast<CXXMethodDecl>(OCE->getCalleeDecl()));
+
+      NestedNameSpecifierLoc NNS(FD->getQualifier(),
+                                 /*Data=*/nullptr);
+      auto DAP = DeclAccessPair::make(FD, FD->getAccess());
+      auto* memberExpr = MemberExpr::Create(
+          m_Context, Clone(OCE->getArg(0)), /*isArrow=*/false, Loc, NNS, noLoc,
+          FD, DAP, FD->getNameInfo(),
+          /*TemplateArgs=*/nullptr, m_Context.BoundMemberTy,
+          CLAD_COMPAT_ExprValueKind_R_or_PR_Value,
+          ExprObjectKind::OK_Ordinary CLAD_COMPAT_CLANG9_MemberExpr_ExtraParams(
+              NOUR_None));
+      call = m_Sema
+                 .BuildCallToMemberFunction(getCurrentScope(), memberExpr, Loc,
+                                            CallArgs, Loc)
+                 .get();
+      return StmtDiff(call);
+    }
+
     call = m_Sema
                .ActOnCallExpr(getCurrentScope(), Clone(CE->getCallee()), Loc,
                               CallArgs, Loc)
diff --git a/lib/Differentiator/VisitorBase.cpp b/lib/Differentiator/VisitorBase.cpp
index f739abd2f..b6156397c 100644
--- a/lib/Differentiator/VisitorBase.cpp
+++ b/lib/Differentiator/VisitorBase.cpp
@@ -236,11 +236,38 @@ namespace clad {
   }
 
   DeclRefExpr* VisitorBase::BuildDeclRef(DeclaratorDecl* D,
-                                         const CXXScopeSpec* SS /*=nullptr*/) {
+                                         const CXXScopeSpec* SS /*=nullptr*/,
+                                         ExprValueKind VK /*=VK_LValue*/) {
     QualType T = D->getType();
     T = T.getNonReferenceType();
     return cast<DeclRefExpr>(clad_compat::GetResult<Expr*>(
-        m_Sema.BuildDeclRefExpr(D, T, VK_LValue, D->getBeginLoc(), SS)));
+        m_Sema.BuildDeclRefExpr(D, T, VK, D->getBeginLoc(), SS)));
+  }
+
+  DeclRefExpr* VisitorBase::BuildDeclRef(DeclaratorDecl* D,
+                                         NestedNameSpecifier* NNS,
+                                         ExprValueKind VK /*=VK_LValue*/) {
+    std::vector<NestedNameSpecifier*> NNChain;
+    CXXScopeSpec CSS;
+    while (NNS) {
+      NNChain.push_back(NNS);
+      NNS = NNS->getPrefix();
+    }
+
+    std::reverse(NNChain.begin(), NNChain.end());
+
+    for (size_t i = 0; i < NNChain.size(); ++i) {
+      NNS = NNChain[i];
+      // FIXME: this needs to be extended to support more NNS kinds. An
+      // inspiration can be take from getFullyQualifiedNestedNameSpecifier in
+      // llvm-project/clang/lib/AST/QualTypeNames.cpp
+      if (NNS->getKind() == NestedNameSpecifier::Namespace) {
+        NamespaceDecl* NS = NNS->getAsNamespace();
+        CSS.Extend(m_Context, NS, noLoc, noLoc);
+      }
+    }
+
+    return BuildDeclRef(D, &CSS, VK);
   }
 
   IdentifierInfo*
diff --git a/test/Gradient/Lambdas.C b/test/Gradient/Lambdas.C
index f9b06aeeb..35776e2d6 100644
--- a/test/Gradient/Lambdas.C
+++ b/test/Gradient/Lambdas.C
@@ -13,7 +13,7 @@ double f1(double i, double j) {
 }
 
 // CHECK:     inline void operator_call_pullback(double t, double _d_y, double *_d_t) const;
-// CHECK-NEXT:     void f1_grad(double i, double j, double *_d_i, double *_d_j) {
+// CHECK:     void f1_grad(double i, double j, double *_d_i, double *_d_j) {
 // CHECK-NEXT:         auto _f = []{{ ?}}(double t) {
 // CHECK-NEXT:             return t * t + 1.;
 // CHECK-NEXT:         }{{;?}}
@@ -34,12 +34,12 @@ double f2(double i, double j) {
 }
 
 // CHECK:     inline void operator_call_pullback(double t, double k, double _d_y, double *_d_t, double *_d_k) const;
-// CHECK-NEXT:     void f2_grad(double i, double j, double *_d_i, double *_d_j) {
+// CHECK:     void f2_grad(double i, double j, double *_d_i, double *_d_j) {
 // CHECK-NEXT:             auto _f = []{{ ?}}(double t, double k) {
 // CHECK-NEXT:                 return t + k;
 // CHECK-NEXT:             }{{;?}}
 // CHECK:        double _d_x = 0.;
-// CHECK-NEXT:             double x = operator()(i + j, i);
+// CHECK-NEXT:             double x = _f.operator()(i + j, i);
 // CHECK-NEXT:             _d_x += 1;
 // CHECK-NEXT:             {
 // CHECK-NEXT:                 double _r0 = 0.;
diff --git a/test/ValidCodeGen/ValidCodeGen.C b/test/ValidCodeGen/ValidCodeGen.C
new file mode 100644
index 000000000..11f1f6fc7
--- /dev/null
+++ b/test/ValidCodeGen/ValidCodeGen.C
@@ -0,0 +1,76 @@
+// RUN: %cladclang -std=c++14 %s -I%S/../../include -oValidCodeGen.out 2>&1 | %filecheck %s
+// RUN: ./ValidCodeGen.out | %filecheck_exec %s
+// RUN: %cladclang -std=c++14 -Xclang -plugin-arg-clad -Xclang -enable-tbr %s -I%S/../../include -oValidCodeGenWithTBR.out
+// RUN: ./ValidCodeGenWithTBR.out | %filecheck_exec %s
+// CHECK-NOT: {{.*error|warning|note:.*}}
+
+#include "clad/Differentiator/Differentiator.h"
+#include "clad/Differentiator/STLBuiltins.h"
+#include "../TestUtils.h"
+#include "../PrintOverloads.h"
+
+namespace TN {
+    int coefficient = 3;
+
+    template <typename T>
+    struct Test2 {
+        T operator[](T x) {
+            return 4*x;
+        }
+    };
+}
+
+namespace clad {
+namespace custom_derivatives {
+namespace class_functions {
+    template <typename T>
+    void operator_subscript_pullback(::TN::Test2<T>* obj, T x, T d_u, ::TN::Test2<T>* d_obj, T* d_x) {
+        (*d_x) += 4*d_u;
+    }
+}}}
+
+double fn(double x) {
+    // fwd and rvs mode test
+    return x*TN::coefficient; // in this test, it's important that this nested name is copied into the generated code properly in both modes
+}
+
+double fn2(double x, double y) {
+    // rvs mode test
+    TN::Test2<double> t; // this type needs to be copied into the derived code properly
+    auto q = t[x]; // in this test, it's important that this operator call is copied into the generated code properly and that the pullback function is called with all the needed namespace prefixes
+    return q;
+}
+
+int main() {
+    double dx, dy;
+    INIT_DIFFERENTIATE(fn, "x");
+    INIT_GRADIENT(fn);
+    INIT_GRADIENT(fn2);
+
+    TEST_GRADIENT(fn, /*numOfDerivativeArgs=*/1, 3, &dx);  // CHECK-EXEC: {3.00}
+    TEST_GRADIENT(fn2, /*numOfDerivativeArgs=*/2, 3, 4, &dx, &dy);  // CHECK-EXEC: {4.00, 0.00}
+    TEST_DIFFERENTIATE(fn, 3) // CHECK-EXEC: {3.00}
+}
+
+//CHECK:     double fn_darg0(double x) {
+//CHECK-NEXT:         double _d_x = 1;
+//CHECK-NEXT:         return _d_x * TN::coefficient + x * 0;
+//CHECK-NEXT:     }
+
+//CHECK:     void fn_grad(double x, double *_d_x) {
+//CHECK-NEXT:         *_d_x += 1 * TN::coefficient;
+//CHECK-NEXT:     }
+
+//CHECK:     void fn2_grad(double x, double y, double *_d_x, double *_d_y) {
+//CHECK-NEXT:         TN::Test2<double> _d_t({});
+//CHECK-NEXT:         TN::Test2<double> t;
+//CHECK-NEXT:         TN::Test2<double> _t0 = t;
+//CHECK-NEXT:         double _d_q = 0.;
+//CHECK-NEXT:         double q = t.operator[](x);
+//CHECK-NEXT:         _d_q += 1;
+//CHECK-NEXT:         {
+//CHECK-NEXT:             double _r0 = 0.;
+//CHECK-NEXT:             clad::custom_derivatives::class_functions::operator_subscript_pullback(&_t0, x, _d_q, &_d_t, &_r0);
+//CHECK-NEXT:             *_d_x += _r0;
+//CHECK-NEXT:         }
+//CHECK-NEXT:     }