diff --git a/stl/inc/valarray b/stl/inc/valarray
index d17080150c7..4c50bff4795 100644
--- a/stl/inc/valarray
+++ b/stl/inc/valarray
@@ -938,9 +938,16 @@ public:
 
     slice_array() = delete;
 
-    slice_array(const slice_array&); // not defined
+    slice_array(const slice_array&) = default;
 
-    slice_array& operator=(const slice_array&); // not defined
+    const slice_array& operator=(const slice_array& _Right) const {
+        size_t _Dst_off = _Start;
+        size_t _Src_off = _Right._Start;
+        for (size_t _Idx = 0; _Idx < _Len; ++_Idx, _Dst_off += _Stride, _Src_off += _Right._Stride) {
+            _Myptr[_Dst_off] = _Right._Myptr[_Src_off];
+        }
+        return *this;
+    }
 
 private:
     friend valarray<_Ty>;
@@ -992,7 +999,7 @@ public:
         return _Ans;
     }
 
-    size_t _Totlen() const {
+    _NODISCARD size_t _Totlen() const {
         if (_Len.size() == 0) {
             return 0;
         }
@@ -1073,15 +1080,23 @@ public:
         _GSLOP(>>= _Right[_Idx]);
     }
 
-    _Ty& _Data(size_t _Idx) const {
+    _NODISCARD _Ty& _Data(size_t _Idx) const {
         return _Myptr[_Idx];
     }
 
     gslice_array() = delete;
 
-    gslice_array(const gslice_array&); // not defined
+    gslice_array(const gslice_array&) = default;
 
-    gslice_array& operator=(const gslice_array&); // not defined
+    const gslice_array& operator=(const gslice_array& _Right) const {
+        _Sizarray _Dst_indexarray(size_t{0}, _Nslice());
+        _Sizarray _Src_indexarray(size_t{0}, _Right._Nslice());
+        const size_t _Size = _Totlen();
+        for (size_t _Idx = 0; _Idx < _Size; ++_Idx) {
+            _Myptr[_Off(_Dst_indexarray)] = _Right._Myptr[_Right._Off(_Src_indexarray)];
+        }
+        return *this;
+    }
 
 private:
     friend valarray<_Ty>;
@@ -1155,15 +1170,32 @@ public:
         _MOP(>>= _Right[_Idx]);
     }
 
-    _Ty& _Data(size_t _Idx) const {
+    _NODISCARD _Ty& _Data(size_t _Idx) const {
         return _Myptr[_Idx];
     }
 
-    bool _Mask(size_t _Idx) const {
+    _NODISCARD bool _Mask(size_t _Idx) const {
         return _Mybool[_Idx];
     }
 
-    size_t _Totlen() const {
+    _NODISCARD size_t _Start_off() const {
+        size_t _Off        = 0;
+        const size_t _Size = _Mybool.size();
+        while (_Off < _Size && !_Mybool[_Off]) {
+            ++_Off;
+        }
+        return _Off;
+    }
+
+    _NODISCARD size_t _Next_off(size_t _Off) const {
+        const size_t _Size = _Mybool.size();
+        do {
+            ++_Off;
+        } while (_Off < _Size && !_Mybool[_Off]);
+        return _Off;
+    }
+
+    _NODISCARD size_t _Totlen() const {
         size_t _Count = 0;
         for (size_t _Idx = 0; _Idx < _Mybool.size(); ++_Idx) {
             if (_Mybool[_Idx]) {
@@ -1176,9 +1208,17 @@ public:
 
     mask_array() = delete;
 
-    mask_array(const mask_array&); // not defined
+    mask_array(const mask_array&) = default;
 
-    mask_array& operator=(const mask_array&); // not defined
+    const mask_array& operator=(const mask_array& _Right) const {
+        const size_t _Size = _Mybool.size();
+        size_t _Dst_off    = _Start_off();
+        size_t _Src_off    = _Right._Start_off();
+        for (; _Dst_off < _Size; _Src_off = _Right._Next_off(_Src_off), _Dst_off = _Next_off(_Dst_off)) {
+            _Myptr[_Dst_off] = _Right._Myptr[_Src_off];
+        }
+        return *this;
+    }
 
 private:
     friend valarray<_Ty>;
@@ -1250,23 +1290,29 @@ public:
         _IOP(>>= _Right[_Idx]);
     }
 
-    _Ty& _Data(size_t _Idx) const {
+    _NODISCARD _Ty& _Data(size_t _Idx) const {
         return _Myptr[_Idx];
     }
 
-    size_t _Indir(size_t _Idx) const {
+    _NODISCARD size_t _Indir(size_t _Idx) const {
         return _Myindarr[_Idx];
     }
 
-    size_t _Totlen() const {
+    _NODISCARD size_t _Totlen() const {
         return _Myindarr.size();
     }
 
     indirect_array() = delete;
 
-    indirect_array(const indirect_array&); // not defined
+    indirect_array(const indirect_array&) = default;
 
-    indirect_array& operator=(const indirect_array&); // not defined
+    const indirect_array& operator=(const indirect_array& _Right) const {
+        const size_t _Size = _Totlen();
+        for (size_t _Idx = 0; _Idx < _Size; ++_Idx) {
+            _Myptr[_Indir(_Idx)] = _Right._Myptr[_Right._Indir(_Idx)];
+        }
+        return *this;
+    }
 
 private:
     friend valarray<_Ty>;
diff --git a/tests/libcxx/expected_results.txt b/tests/libcxx/expected_results.txt
index b2753dc3709..8c91cb2d01e 100644
--- a/tests/libcxx/expected_results.txt
+++ b/tests/libcxx/expected_results.txt
@@ -565,10 +565,6 @@ std/re/re.traits/transform.pass.cpp FAIL
 # STL bug: Incorrect return types.
 std/numerics/complex.number/cmplx.over/pow.pass.cpp FAIL
 
-# STL bug: Missing <valarray> assignment operators.
-std/numerics/numarray/template.mask.array/mask.array.assign/mask_array.pass.cpp FAIL
-std/numerics/numarray/template.slice.array/slice.arr.assign/slice_array.pass.cpp FAIL
-
 # STL bug: We allow fill() and swap() for array<const T, 0>.
 std/containers/sequences/array/array.fill/fill.fail.cpp FAIL
 std/containers/sequences/array/array.swap/swap.fail.cpp FAIL
diff --git a/tests/libcxx/skipped_tests.txt b/tests/libcxx/skipped_tests.txt
index 6f62c1c1b7d..61804e09fba 100644
--- a/tests/libcxx/skipped_tests.txt
+++ b/tests/libcxx/skipped_tests.txt
@@ -565,10 +565,6 @@ re\re.traits\transform.pass.cpp
 # STL bug: Incorrect return types.
 numerics\complex.number\cmplx.over\pow.pass.cpp
 
-# STL bug: Missing <valarray> assignment operators.
-numerics\numarray\template.mask.array\mask.array.assign\mask_array.pass.cpp
-numerics\numarray\template.slice.array\slice.arr.assign\slice_array.pass.cpp
-
 # STL bug: We allow fill() and swap() for array<const T, 0>.
 containers\sequences\array\array.fill\fill.fail.cpp
 containers\sequences\array\array.swap\swap.fail.cpp
diff --git a/tests/std/test.lst b/tests/std/test.lst
index dc0c7187509..9ce41e6b7c2 100644
--- a/tests/std/test.lst
+++ b/tests/std/test.lst
@@ -160,6 +160,7 @@ tests\GH_000545_include_compare
 tests\GH_000685_condition_variable_any
 tests\GH_000690_overaligned_function
 tests\GH_000890_pow_template
+tests\GH_000940_missing_valarray_copy
 tests\GH_001010_filesystem_error_encoding
 tests\GH_001017_discrete_distribution_out_of_range
 tests\LWG2597_complex_branch_cut
diff --git a/tests/std/tests/GH_000940_missing_valarray_copy/env.lst b/tests/std/tests/GH_000940_missing_valarray_copy/env.lst
new file mode 100644
index 00000000000..19f025bd0e6
--- /dev/null
+++ b/tests/std/tests/GH_000940_missing_valarray_copy/env.lst
@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_matrix.lst
diff --git a/tests/std/tests/GH_000940_missing_valarray_copy/test.cpp b/tests/std/tests/GH_000940_missing_valarray_copy/test.cpp
new file mode 100644
index 00000000000..91106130487
--- /dev/null
+++ b/tests/std/tests/GH_000940_missing_valarray_copy/test.cpp
@@ -0,0 +1,87 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <iostream>
+#include <valarray>
+
+template <class T>
+bool eq(const std::valarray<T>& v, std::initializer_list<T> il) {
+    return std::equal(begin(v), end(v), il.begin(), il.end());
+}
+
+void test_slice() {
+    std::valarray<int> v{0, 1, 2, 3, 4};
+
+    std::slice_array<int> slice_array      = v[std::slice(2, 2, 2)];
+    std::slice_array<int> slice_array_copy = slice_array;
+    (void) slice_array_copy;
+
+    assert(eq(v, {0, 1, 2, 3, 4}));
+
+    std::slice_array<int> other_slice_array = v[std::slice(0, 2, 1)];
+    other_slice_array                       = slice_array;
+
+    assert(eq(v, {2, 4, 2, 3, 4}));
+}
+
+void test_gslice() {
+    std::valarray<int> v{0, 1, 2, 3, 4};
+
+    std::gslice gslice(2, std::valarray<std::size_t>{2}, std::valarray<std::size_t>{2});
+    std::gslice_array<int> gslice_array      = v[gslice];
+    std::gslice_array<int> gslice_array_copy = gslice_array;
+    (void) gslice_array_copy;
+
+    assert(eq(v, {0, 1, 2, 3, 4}));
+
+    std::gslice other_gslice(0, std::valarray<std::size_t>{2}, std::valarray<std::size_t>{1});
+    std::gslice_array<int> other_gslice_array = v[other_gslice];
+    other_gslice_array                        = gslice_array;
+
+    assert(eq(v, {2, 4, 2, 3, 4}));
+}
+
+void test_mask() {
+    std::valarray<int> v{0, 1, 2, 3, 4};
+
+    std::valarray<bool> mask{true, false, false, false, true};
+    std::mask_array<int> mask_array      = v[mask];
+    std::mask_array<int> mask_array_copy = mask_array;
+    (void) mask_array_copy;
+
+    assert(eq(v, {0, 1, 2, 3, 4}));
+
+    std::valarray<bool> other_mask{false, true, true, false, false};
+    std::mask_array<int> other_mask_array = v[other_mask];
+    other_mask_array                      = mask_array;
+
+    assert(eq(v, {0, 0, 4, 3, 4}));
+}
+
+void test_indirect() {
+    std::valarray<int> v{0, 1, 2, 3, 4};
+
+    std::valarray<std::size_t> indices{2, 3};
+    std::indirect_array<int> indirect_array      = v[indices];
+    std::indirect_array<int> indirect_array_copy = indirect_array;
+    (void) indirect_array_copy;
+
+    assert(eq(v, {0, 1, 2, 3, 4}));
+
+    std::valarray<std::size_t> other_indices{4, 0};
+    std::indirect_array<int> other_indirect_array = v[other_indices];
+    other_indirect_array                          = indirect_array;
+
+    assert(eq(v, {3, 1, 2, 3, 2}));
+}
+
+int main() {
+    test_slice();
+    test_gslice();
+    test_mask();
+    test_indirect();
+    return 0;
+}