From 1cdb8d295eee5faf47f4230e0c3b6a54462a81ce Mon Sep 17 00:00:00 2001
From: Werner Henze <34543625+beinhaerter@users.noreply.github.com>
Date: Sat, 4 Jan 2025 18:50:31 +0100
Subject: [PATCH] More checks for non-compilable code, plus fix for span
 (#1180)

---
 include/gsl/span               |   7 +
 tests/algorithm_tests.cpp      |   2 +-
 tests/byte_tests.cpp           |   6 +-
 tests/notnull_tests.cpp        | 156 +++++++---
 tests/owner_tests.cpp          |  25 +-
 tests/pointers_tests.cpp       |  33 +--
 tests/span_tests.cpp           | 503 +++++++++++++--------------------
 tests/strict_notnull_tests.cpp | 119 +++++---
 8 files changed, 439 insertions(+), 412 deletions(-)

diff --git a/include/gsl/span b/include/gsl/span
index a01b687..f307e6e 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -582,6 +582,8 @@ public:
     template <std::size_t Count>
     constexpr span<element_type, Count> first() const noexcept
     {
+        static_assert(Extent == dynamic_extent || Count <= Extent,
+                      "first() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data(), Count};
     }
@@ -592,6 +594,8 @@ public:
         // clang-format on
         constexpr span<element_type, Count> last() const noexcept
     {
+        static_assert(Extent == dynamic_extent || Count <= Extent,
+                      "last() cannot extract more elements from a span than it contains.");
         Expects(Count <= size());
         return span<element_type, Count>{data() + (size() - Count), Count};
     }
@@ -603,6 +607,9 @@ public:
         constexpr auto subspan() const noexcept ->
         typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type
     {
+        static_assert(Extent == dynamic_extent || (Extent >= Offset && (Count == dynamic_extent ||
+                                                                        Count <= Extent - Offset)),
+                      "subspan() cannot extract more elements from a span than it contains.");
         Expects((size() >= Offset) && (Count == dynamic_extent || (Count <= size() - Offset)));
         using type =
             typename details::calculate_subspan_type<ElementType, Extent, Offset, Count>::type;
diff --git a/tests/algorithm_tests.cpp b/tests/algorithm_tests.cpp
index e369be0..b4a4f81 100644
--- a/tests/algorithm_tests.cpp
+++ b/tests/algorithm_tests.cpp
@@ -188,7 +188,7 @@ TEST(algorithm_tests, incompatible_type)
     span<int> src_span_dyn(src);
     span<int, 4> src_span_static(src);
     span<int*> dst_span_dyn(dst);
-    span<int*, 4> dst_span_static(dst);
+    span<int*, 4> dst_span_static(gsl::make_span(dst));
 
     // every line should produce a compilation error
     copy(src_span_dyn, dst_span_dyn);
diff --git a/tests/byte_tests.cpp b/tests/byte_tests.cpp
index ca94a9b..77a947a 100644
--- a/tests/byte_tests.cpp
+++ b/tests/byte_tests.cpp
@@ -67,6 +67,8 @@ TEST(byte_tests, construction)
     to_byte(char{});
     to_byte(3);
     to_byte(3u);
+    to_byte<-1>();
+    to_byte<256u>();
 #endif
 }
 
@@ -174,7 +176,3 @@ static constexpr bool
 static_assert(!ToIntegerCompilesFor<float>, "!ToIntegerCompilesFor<float>");
 
 } // namespace
-
-#ifdef CONFIRM_COMPILATION_ERRORS
-copy(src_span_static, dst_span_static);
-#endif
diff --git a/tests/notnull_tests.cpp b/tests/notnull_tests.cpp
index b6f8b37..56b549b 100644
--- a/tests/notnull_tests.cpp
+++ b/tests/notnull_tests.cpp
@@ -18,17 +18,25 @@
 
 #include <gsl/pointers> // for not_null, operator<, operator<=, operator>
 
-#include <algorithm> // for addressof
-#include <cstdint>   // for uint16_t
-#include <memory>    // for shared_ptr, make_shared, operator<, opera...
-#include <sstream>   // for operator<<, ostringstream, basic_ostream:...
-#include <string>    // for basic_string, operator==, string, operator<<
-#include <typeinfo>  // for type_info
-#include <variant>   // for variant, monostate, get
+#include <algorithm>   // for addressof
+#include <cstdint>     // for uint16_t
+#include <memory>      // for shared_ptr, make_shared, operator<, opera...
+#include <sstream>     // for operator<<, ostringstream, basic_ostream:...
+#include <string>      // for basic_string, operator==, string, operator<<
+#include <type_traits> // for declval
+#include <typeinfo>    // for type_info
+#include <variant>     // for variant, monostate, get
 
 #include "deathTestCommon.h"
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 struct MyBase
 {
 };
@@ -141,16 +149,39 @@ bool helper_const(not_null<const int*> p) { return *p == 12; }
 int* return_pointer() { return nullptr; }
 } // namespace
 
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_A = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_A<U, void_t<decltype(gsl::not_null<void*>{std::declval<U>()})>> = true;
+
+template <typename U, int N, typename = void>
+static constexpr bool CtorCompilesFor_B = false;
+template <typename U, int N>
+static constexpr bool CtorCompilesFor_B<U, N, void_t<decltype(gsl::not_null<U>{N})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool DefaultCtorCompilesFor = false;
+template <typename U>
+static constexpr bool DefaultCtorCompilesFor<U, void_t<decltype(gsl::not_null<U>{})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_C = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_C<U, void_t<decltype(gsl::not_null<U*>{std::declval<std::unique_ptr<U>>()})>> =
+        true;
+
 TEST(notnull_tests, TestNotNullConstructors)
 {
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        not_null<int*> p = nullptr;          // yay...does not compile!
-        not_null<std::vector<char>*> p1 = 0; // yay...does not compile!
-        not_null<int*> p2;                   // yay...does not compile!
-        std::unique_ptr<int> up = std::make_unique<int>(120);
-        not_null<int*> p3 = up;
+        static_assert(CtorCompilesFor_A<void*>, "CtorCompilesFor_A<void*>");
+        static_assert(!CtorCompilesFor_A<std::nullptr_t>, "!CtorCompilesFor_A<std::nullptr_t>");
+        static_assert(!CtorCompilesFor_B<void*, 0>, "!CtorCompilesFor_B<void*, 0>");
+        static_assert(!DefaultCtorCompilesFor<void*>, "!DefaultCtorCompilesFor<void*>");
+        static_assert(!CtorCompilesFor_C<int>, "CtorCompilesFor_C<int>");
 
+#ifdef CONFIRM_COMPILATION_ERRORS
         // Forbid non-nullptr assignable types
         not_null<std::vector<int>> f(std::vector<int>{1});
         not_null<int> z(10);
@@ -276,6 +307,27 @@ TEST(notnull_tests, TestNotNullostream)
     ostream_helper<std::string>("string");
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool AssignmentCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    AssignmentCompilesFor<U, V,
+                          void_t<decltype(std::declval<gsl::not_null<U*>&>().operator=(
+                              std::declval<gsl::not_null<V*>&>()))>> = true;
+
+template <typename U, typename V, typename = void>
+static constexpr bool SCastCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    SCastCompilesFor<U, V, void_t<decltype(static_cast<U*>(std::declval<gsl::not_null<V*>&>()))>> =
+        true;
+
+template <typename U, typename V, typename = void>
+static constexpr bool RCastCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool RCastCompilesFor<
+    U, V, void_t<decltype(reinterpret_cast<U*>(std::declval<gsl::not_null<V*>&>()))>> = true;
+
 TEST(notnull_tests, TestNotNullCasting)
 {
     MyBase base;
@@ -288,15 +340,30 @@ TEST(notnull_tests, TestNotNullCasting)
     q = p; // allowed with heterogeneous copy ctor
     EXPECT_TRUE(q == p);
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    q = u; // no viable conversion possible between MyBase* and Unrelated*
-    p = q; // not possible to implicitly convert MyBase* to MyDerived*
+    static_assert(AssignmentCompilesFor<MyBase, MyDerived>,
+                  "AssignmentCompilesFor<MyBase, MyDerived>");
+    static_assert(!AssignmentCompilesFor<MyBase, Unrelated>,
+                  "!AssignmentCompilesFor<MyBase, Unrelated>");
+    static_assert(!AssignmentCompilesFor<Unrelated, MyDerived>,
+                  "!AssignmentCompilesFor<Unrelated, MyDerived>");
+    static_assert(!AssignmentCompilesFor<MyDerived, MyBase>,
+                  "!AssignmentCompilesFor<MyDerived, MyBase>");
+
+    static_assert(SCastCompilesFor<MyDerived, MyDerived>, "SCastCompilesFor<MyDerived, MyDerived>");
+    static_assert(SCastCompilesFor<MyBase, MyDerived>, "SCastCompilesFor<MyBase, MyDerived>");
+    static_assert(!SCastCompilesFor<MyDerived, MyBase>, "!SCastCompilesFor<MyDerived, MyBase>");
+    static_assert(!SCastCompilesFor<Unrelated, MyDerived>,
+                  "!SCastCompilesFor<Unrelated, MyDerived>");
+    static_assert(!RCastCompilesFor<MyDerived, MyDerived>,
+                  "!SCastCompilesFor<MyDerived, MyDerived>");
+    static_assert(!RCastCompilesFor<Unrelated, MyDerived>,
+                  "!SCastCompilesFor<Unrelated, MyDerived>");
 
-    not_null<Unrelated*> r = p;
-    not_null<Unrelated*> s = reinterpret_cast<Unrelated*>(p);
-#endif
     not_null<Unrelated*> t(reinterpret_cast<Unrelated*>(p.get()));
     EXPECT_TRUE(reinterpret_cast<void*>(p.get()) == reinterpret_cast<void*>(t.get()));
+
+    (void) static_cast<MyDerived*>(p);
+    (void) static_cast<MyBase*>(p);
 }
 
 TEST(notnull_tests, TestNotNullAssignment)
@@ -438,6 +505,18 @@ TEST(notnull_tests, TestNotNullCustomPtrComparison)
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
+template <typename U, typename = void>
+static constexpr bool TypeDeductionCtorCompilesFor = false;
+template <typename U>
+static constexpr bool
+    TypeDeductionCtorCompilesFor<U, void_t<decltype(not_null{std::declval<U>()})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool TypeDeductionHelperCompilesFor = false;
+template <typename U>
+static constexpr bool
+    TypeDeductionHelperCompilesFor<U, void_t<decltype(helper(not_null{std::declval<U>()}))>> = true;
+
 TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
 {
     {
@@ -454,9 +533,9 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         const int i = 42;
 
         not_null x{&i};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(not_null{&i});
-#endif
+        static_assert(TypeDeductionHelperCompilesFor<int*>, "TypeDeductionHelperCompilesFor<int*>");
+        static_assert(!TypeDeductionHelperCompilesFor<const int*>,
+                      "!TypeDeductionHelperCompilesFor<const int*>");
         helper_const(not_null{&i});
 
         EXPECT_TRUE(*x == 42);
@@ -478,9 +557,6 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         const int* p = &i;
 
         not_null x{p};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(not_null{p});
-#endif
         helper_const(not_null{p});
 
         EXPECT_TRUE(*x == 42);
@@ -515,12 +591,15 @@ TEST(notnull_tests, TestNotNullConstructorTypeDeduction)
         EXPECT_DEATH(helper_const(not_null{p}), expected);
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        not_null x{nullptr};
-        helper(not_null{nullptr});
-        helper_const(not_null{nullptr});
-    }
+    static_assert(TypeDeductionCtorCompilesFor<void*>, "TypeDeductionCtorCompilesFor<void*>");
+#if defined(_MSC_VER) && !defined(__clang__)
+    // Fails on gcc, clang, xcode, VS clang with
+    // "error : no type named 'type' in 'std::enable_if<false>'; 'enable_if' cannot be used to
+    // disable this declaration"
+    static_assert(!TypeDeductionCtorCompilesFor<std::nullptr_t>,
+                  "!TypeDeductionCtorCompilesFor<std::nullptr_t>");
+    static_assert(!TypeDeductionHelperCompilesFor<std::nullptr_t>,
+                  "!TypeDeductionHelperCompilesFor<std::nullptr_t>");
 #endif
 }
 
@@ -536,6 +615,11 @@ TEST(notnull_tests, TestVariantEmplace)
 }
 #endif // #if defined(__cplusplus) && (__cplusplus >= 201703L)
 
+template <typename U, typename = void>
+static constexpr bool HelperCompilesFor = false;
+template <typename U>
+static constexpr bool HelperCompilesFor<U, void_t<decltype(helper(std::declval<U>()))>> = true;
+
 TEST(notnull_tests, TestMakeNotNull)
 {
     {
@@ -552,9 +636,8 @@ TEST(notnull_tests, TestMakeNotNull)
         const int i = 42;
 
         const auto x = make_not_null(&i);
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(make_not_null(&i));
-#endif
+        static_assert(HelperCompilesFor<gsl::not_null<int*>>,
+                      "HelperCompilesFor<gsl::not_null<int*>>");
         helper_const(make_not_null(&i));
 
         EXPECT_TRUE(*x == 42);
@@ -576,9 +659,8 @@ TEST(notnull_tests, TestMakeNotNull)
         const int* p = &i;
 
         const auto x = make_not_null(p);
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(make_not_null(p));
-#endif
+        static_assert(!HelperCompilesFor<gsl::not_null<const int*>>,
+                      "!HelperCompilesFor<gsl::not_null<const int*>>");
         helper_const(make_not_null(p));
 
         EXPECT_TRUE(*x == 42);
diff --git a/tests/owner_tests.cpp b/tests/owner_tests.cpp
index 87c7cec..3b7a38b 100644
--- a/tests/owner_tests.cpp
+++ b/tests/owner_tests.cpp
@@ -17,6 +17,7 @@
 #include <gtest/gtest.h>
 
 #include <gsl/pointers> // for owner
+#include <type_traits>  // for declval
 
 using namespace gsl;
 
@@ -32,12 +33,18 @@ TEST(owner_tests, basic_test)
     delete p;
 }
 
-TEST(owner_tests, check_pointer_constraint)
-{
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        owner<int> integerTest = 10;
-        owner<std::shared_ptr<int>> sharedPtrTest(new int(10));
-    }
-#endif
-}
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
+template <typename U, typename = void>
+static constexpr bool OwnerCompilesFor = false;
+template <typename U>
+static constexpr bool OwnerCompilesFor<U, void_t<decltype(gsl::owner<U>{})>> =
+        true;
+static_assert(OwnerCompilesFor<int*>, "OwnerCompilesFor<int*>");
+static_assert(!OwnerCompilesFor<int>, "!OwnerCompilesFor<int>");
+static_assert(!OwnerCompilesFor<std::shared_ptr<int>>, "!OwnerCompilesFor<std::shared_ptr<int>>");
diff --git a/tests/pointers_tests.cpp b/tests/pointers_tests.cpp
index 7757c27..1c15712 100644
--- a/tests/pointers_tests.cpp
+++ b/tests/pointers_tests.cpp
@@ -6,6 +6,13 @@
 #include <type_traits>
 #include <utility>
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 namespace
 {
 // Custom pointer type that can be used for gsl::not_null, but for which these cannot be swapped.
@@ -22,6 +29,13 @@ struct NotMoveAssignableCustomPtr
     int dummy{}; // Without this clang warns, that NotMoveAssignableCustomPtr() is unneeded
 };
 
+template <typename U, typename = void>
+static constexpr bool SwapCompilesFor = false;
+template <typename U>
+static constexpr bool
+    SwapCompilesFor<U, void_t<decltype(gsl::swap<U>(std::declval<gsl::not_null<U>&>(),
+                                                    std::declval<gsl::not_null<U>&>()))>> = true;
+
 TEST(pointers_test, swap)
 {
     // taken from gh-1129:
@@ -69,22 +83,9 @@ TEST(pointers_test, swap)
         EXPECT_TRUE(*a == 1);
         EXPECT_TRUE(*b == 0);
     }
+
+    static_assert(!SwapCompilesFor<NotMoveAssignableCustomPtr>,
+                  "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
 }
 
-#if __cplusplus >= 201703l
-using std::void_t;
-#else  // __cplusplus >= 201703l
-template <class...>
-using void_t = void;
-#endif // __cplusplus < 201703l
-
-template <typename U, typename = void>
-static constexpr bool SwapCompilesFor = false;
-template <typename U>
-static constexpr bool
-    SwapCompilesFor<U, void_t<decltype(gsl::swap<U>(std::declval<gsl::not_null<U>&>(),
-                                                    std::declval<gsl::not_null<U>&>()))>> = true;
-static_assert(!SwapCompilesFor<NotMoveAssignableCustomPtr>,
-              "!SwapCompilesFor<NotMoveAssignableCustomPtr>");
-
 } // namespace
diff --git a/tests/span_tests.cpp b/tests/span_tests.cpp
index d86458b..01fe26d 100644
--- a/tests/span_tests.cpp
+++ b/tests/span_tests.cpp
@@ -48,6 +48,13 @@
 
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 namespace
 {
 
@@ -62,8 +69,7 @@ struct AddressOverloaded
 #if (__cplusplus > 201402L)
     [[maybe_unused]]
 #endif
-    AddressOverloaded
-    operator&() const
+    AddressOverloaded operator&() const
     {
         return {};
     }
@@ -216,6 +222,12 @@ TEST(span_test, from_pointer_length_constructor)
 
 TEST(span_test, from_pointer_pointer_construction)
 {
+    // const auto terminateHandler = std::set_terminate([] {
+    //     std::cerr << "Expected Death. from_pointer_pointer_construction";
+    //     std::abort();
+    // });
+    // const auto expected = GetExpectedDeathString(terminateHandler);
+
     int arr[4] = {1, 2, 3, 4};
 
     {
@@ -245,19 +257,11 @@ TEST(span_test, from_pointer_pointer_construction)
         EXPECT_TRUE(s.data() == &arr[0]);
     }
 
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
+    //{ // this test succeeds on all platforms, gsl::span is more relaxed than std::span where this would be UB
     //    auto workaround_macro = [&]() { span<int> s{&arr[1], &arr[0]}; };
     //    EXPECT_DEATH(workaround_macro(), expected);
     //}
 
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
-    //    int* p = nullptr;
-    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
-    //    EXPECT_DEATH(workaround_macro(), expected);
-    //}
-
     {
         int* p = nullptr;
         span<int> s{p, p};
@@ -271,19 +275,21 @@ TEST(span_test, from_pointer_pointer_construction)
         EXPECT_TRUE(s.size() == 0);
         EXPECT_TRUE(s.data() == nullptr);
     }
-
-    // this will fail the std::distance() precondition, which asserts on MSVC debug builds
-    //{
-    //    int* p = nullptr;
-    //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
-    //    EXPECT_DEATH(workaround_macro(), expected);
-    //}
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool CtorCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool CtorCompilesFor<U, V, void_t<decltype(U{std::declval<V>()})>> = true;
+
 TEST(span_test, from_array_constructor)
 {
     int arr[5] = {1, 2, 3, 4, 5};
 
+    static_assert(!CtorCompilesFor<span<int, 6>, int[5]>, "!CtorCompilesFor<span<int, 6>, int[5]>");
+    static_assert(!CtorCompilesFor<span<int, 0>, int[5]>, "!CtorCompilesFor<span<int, 0>, int[5]>");
+    static_assert(!CtorCompilesFor<span<int>, int[2][3]>, "!CtorCompilesFor<span<int>, int[2][3]>");
+
     {
         const span<int> s{arr};
         EXPECT_TRUE(s.size() == 5);
@@ -298,70 +304,28 @@ TEST(span_test, from_array_constructor)
 
     int arr2d[2][3] = {1, 2, 3, 4, 5, 6};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 6> s{arr};
-    }
+    static_assert(!CtorCompilesFor<span<int, 0>, int[2][3]>,
+                  "!CtorCompilesFor<span<int, 0>, int[2][3]>");
+    static_assert(!CtorCompilesFor<span<int, 6>, int[2][3]>,
+                  "!CtorCompilesFor<span<int, 6>, int[2][3]>");
 
-    {
-        span<int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr[0]);
-    }
-
-    {
-        span<int> s{arr2d};
-        EXPECT_TRUE(s.size() == 6);
-        EXPECT_TRUE(s.data() == &arr2d[0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[5] == 6);
-    }
-
-    {
-        span<int, 0> s{arr2d};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr2d[0][0]);
-    }
-
-    {
-        span<int, 6> s{arr2d};
-    }
-#endif
     {
         const span<int[3]> s{std::addressof(arr2d[0]), 1};
         EXPECT_TRUE(s.size() == 1);
         EXPECT_TRUE(s.data() == std::addressof(arr2d[0]));
     }
 
-    int arr3d[2][3][2] = { { {1, 2}, {3, 4}, {5, 6} }, { {7, 8}, {9, 10}, {11, 12} } };
+    int arr3d[2][3][2] = {{{1, 2}, {3, 4}, {5, 6}}, {{7, 8}, {9, 10}, {11, 12}}};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int> s{arr3d};
-        EXPECT_TRUE(s.size() == 12);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[11] == 12);
-    }
+    static_assert(!CtorCompilesFor<span<int>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 0>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 0>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 11>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 11>, int[2][3][2]>");
+    static_assert(!CtorCompilesFor<span<int, 12>, int[2][3][2]>,
+                  "!CtorCompilesFor<span<int, 12>, int[2][3][2]>");
 
-    {
-        span<int, 0> s{arr3d};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-    }
-
-    {
-        span<int, 11> s{arr3d};
-    }
-
-    {
-        span<int, 12> s{arr3d};
-        EXPECT_TRUE(s.size() == 12);
-        EXPECT_TRUE(s.data() == &arr3d[0][0][0]);
-        EXPECT_TRUE(s[0] == 1);
-        EXPECT_TRUE(s[5] == 6);
-    }
-#endif
     {
         const span<int[3][2]> s{std::addressof(arr3d[0]), 1};
         EXPECT_TRUE(s.size() == 1);
@@ -389,6 +353,13 @@ TEST(span_test, from_dynamic_array_constructor)
     delete[] arr;
 }
 
+template <typename U, typename V, typename = void>
+static constexpr bool ConversionCompilesFor = false;
+template <typename U, typename V>
+static constexpr bool
+    ConversionCompilesFor<U, V, void_t<decltype(std::declval<void (*)(U)>()(std::declval<V>()))>> =
+        true;
+
 TEST(span_test, from_std_array_constructor)
 {
     std::array<int, 4> arr = {1, 2, 3, 4};
@@ -428,43 +399,31 @@ TEST(span_test, from_std_array_constructor)
         EXPECT_TRUE(ao_arr.data() == fs.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
+    static_assert(!CtorCompilesFor<span<int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 2>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>");
 
-        span<const int, 2> cs{arr};
-        EXPECT_TRUE(cs.size() == 2);
-        EXPECT_TRUE(cs.data() == arr.data());
-    }
+    static_assert(!CtorCompilesFor<span<int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 0>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>");
 
-    {
-        span<int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
+    static_assert(!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>");
 
-        span<const int, 0> cs{arr};
-        EXPECT_TRUE(cs.size() == 0);
-        EXPECT_TRUE(cs.data() == arr.data());
-    }
-
-    {
-        span<int, 5> s{arr};
-    }
-
-    {
-        auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
-        auto take_a_span = [](span<int> s) { static_cast<void>(s); };
-        // try to take a temporary std::array
-        take_a_span(get_an_array());
-    }
+#if !defined(_MSC_VER) || (_MSC_VER > 1942) || (__cplusplus >= 201703L)
+    // Fails on "Visual Studio 16 2019/Visual Studio 17 2022, windows-2019/2022, Debug/Release, 14".
+    static_assert(!ConversionCompilesFor<span<int>, std::array<int, 4>>,
+                  "!ConversionCompilesFor<span<int>, std::array<int, 4>>");
 #endif
 
     {
         auto get_an_array = []() -> std::array<int, 4> { return {1, 2, 3, 4}; };
-        auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
+        auto take_a_span = [](span<const int>) {};
         // try to take a temporary std::array
+        static_assert(ConversionCompilesFor<span<const int>, std::array<int, 4>>,
+                      "ConversionCompilesFor<span<const int>, std::array<int, 4>>");
         take_a_span(get_an_array());
     }
 }
@@ -493,23 +452,12 @@ TEST(span_test, from_const_std_array_constructor)
         EXPECT_TRUE(s.data() == ao_arr.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<const int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 5> s{arr};
-    }
-#endif
+    static_assert(!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, std::array<int, 4>&>");
 
     {
         auto get_an_array = []() -> const std::array<int, 4> { return {1, 2, 3, 4}; };
@@ -535,27 +483,14 @@ TEST(span_test, from_std_array_const_constructor)
         EXPECT_TRUE(s.data() == arr.data());
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<const int, 2> s{arr};
-        EXPECT_TRUE(s.size() == 2);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 0> s{arr};
-        EXPECT_TRUE(s.size() == 0);
-        EXPECT_TRUE(s.data() == arr.data());
-    }
-
-    {
-        span<const int, 5> s{arr};
-    }
-
-    {
-        span<int, 4> s{arr};
-    }
-#endif
+    static_assert(!CtorCompilesFor<span<const int, 2>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 2>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 0>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 0>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<const int, 5>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<const int, 5>, const std::array<int, 4>&>");
+    static_assert(!CtorCompilesFor<span<int, 5>, const std::array<int, 4>&>,
+                  "!CtorCompilesFor<span<int, 5>, const std::array<int, 4>&>");
 }
 
 TEST(span_test, from_container_constructor)
@@ -577,32 +512,28 @@ TEST(span_test, from_container_constructor)
     const std::string cstr = "hello";
 
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<char> s{str};
-        EXPECT_TRUE(s.size() == str.size());
-         EXPECT_TRUE(s.data() == str.data()));
-#endif
-         span<const char> cs{str};
-         EXPECT_TRUE(cs.size() == str.size());
-         EXPECT_TRUE(cs.data() == str.data());
+        static_assert(CtorCompilesFor<span<char>, std::string&> == (__cplusplus >= 201703L),
+                      "CtorCompilesFor<span<char>, std::string&> == (__cplusplus >= 201703L)");
+
+        span<const char> cs{str};
+        EXPECT_TRUE(cs.size() == str.size());
+        EXPECT_TRUE(cs.data() == str.data());
     }
 
     {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        span<char> s{cstr};
-#endif
+        static_assert(!CtorCompilesFor<span<char>, const std::string&>,
+                      "!CtorCompilesFor<span<char>, const std::string&>");
+
         span<const char> cs{cstr};
         EXPECT_TRUE(cs.size() == cstr.size());
         EXPECT_TRUE(cs.data() == cstr.data());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_vector = []() -> std::vector<int> { return {}; };
-        auto use_span = [](span<int> s) { static_cast<void>(s); };
-        use_span(get_temp_vector());
-#endif
-    }
+#if !defined(_MSC_VER) || (_MSC_VER > 1942) || (__cplusplus >= 201703L)
+    // Fails on "Visual Studio 16 2019/Visual Studio 17 2022, windows-2019/2022, Debug/Release, 14".
+    static_assert(!ConversionCompilesFor<span<int>, std::vector<int>>,
+                  "!ConversionCompilesFor<span<int>, std::vector<int>>");
+#endif // !defined(_MSC_VER) || (_MSC_VER > 1942) || (__cplusplus >= 201703L)
 
     {
         auto get_temp_vector = []() -> std::vector<int> { return {}; };
@@ -610,13 +541,8 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_vector());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_string = []() -> std::string { return {}; };
-        auto use_span = [](span<char> s) { static_cast<void>(s); };
-        use_span(get_temp_string());
-#endif
-    }
+    static_assert(!ConversionCompilesFor<span<char>, std::string>,
+                  "!ConversionCompilesFor<span<char>, std::string>");
 
     {
         auto get_temp_string = []() -> std::string { return {}; };
@@ -624,13 +550,10 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_string());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        auto get_temp_vector = []() -> const std::vector<int> { return {}; };
-        auto use_span = [](span<const char> s) { static_cast<void>(s); };
-        use_span(get_temp_vector());
-#endif
-    }
+    static_assert(!ConversionCompilesFor<span<const char>, const std::vector<int>>,
+                  "!ConversionCompilesFor<span<const char>, const std::vector<int>>");
+    static_assert(!ConversionCompilesFor<span<char>, const std::string>,
+                  "!ConversionCompilesFor<span<char>, const std::string>");
 
     {
         auto get_temp_string = []() -> const std::string { return {}; };
@@ -638,12 +561,8 @@ TEST(span_test, from_container_constructor)
         use_span(get_temp_string());
     }
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        std::map<int, int> m;
-        span<int> s{m};
-#endif
-    }
+    static_assert(!CtorCompilesFor<span<int>, std::map<int, int>&>,
+                  "!CtorCompilesFor<span<int>, std::map<int, int>&>");
 }
 
 TEST(span_test, from_convertible_span_constructor)
@@ -695,52 +614,20 @@ TEST(span_test, from_convertible_span_constructor)
         EXPECT_DEATH(T{avd}, expected);
     }
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        std::array<DerivedClass, 2> arr{};
-        span<DerivedClass> avd{arr};
-        span<const DerivedClass, 2> avcd = avd;
-        static_cast<void>(avcd);
-    }
-
-    {
-        std::array<DerivedClass, 2> arr{};
-        span<DerivedClass, 2> avd{arr};
-        span<const DerivedClass, 1> avcd = avd;
-        static_cast<void>(avcd);
-    }
-
-    {
-        std::array<DerivedClass, 2> arr{};
-        span<DerivedClass, 2> avd{arr};
-        span<const DerivedClass, 3> avcd = avd;
-        static_cast<void>(avcd);
-    }
-
-    {
-        span<DerivedClass> avd;
-        span<BaseClass> avb = avd;
-        static_cast<void>(avb);
-    }
-
-    {
-        span<int> s;
-        span<unsigned int> s2 = s;
-        static_cast<void>(s2);
-    }
-
-    {
-        span<int> s;
-        span<const unsigned int> s2 = s;
-        static_cast<void>(s2);
-    }
-
-    {
-        span<int> s;
-        span<short> s2 = s;
-        static_cast<void>(s2);
-    }
-#endif
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 2>, span<DerivedClass>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 2>, span<DerivedClass>&>");
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 1>, span<DerivedClass, 2>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 1>, span<DerivedClass, 2>&>");
+    static_assert(!ConversionCompilesFor<span<const DerivedClass, 3>, span<DerivedClass, 2>&>,
+                  "!ConversionCompilesFor<span<const DerivedClass, 3>, span<DerivedClass, 2>&>");
+    static_assert(!ConversionCompilesFor<span<BaseClass, 3>, span<DerivedClass>&>,
+                  "!ConversionCompilesFor<span<BaseClass, 3>, span<DerivedClass>&>");
+    static_assert(!ConversionCompilesFor<span<unsigned int>, span<int>&>,
+                  "!ConversionCompilesFor<span<unsigned int>, span<int>&>");
+    static_assert(!ConversionCompilesFor<span<const unsigned int>, span<int>&>,
+                  "!ConversionCompilesFor<span<const unsigned int>, span<int>&>");
+    static_assert(!ConversionCompilesFor<span<short>, span<int>&>,
+                  "!ConversionCompilesFor<span<short>, span<int>&>");
 }
 
 TEST(span_test, copy_move_and_assignment)
@@ -800,10 +687,9 @@ TEST(span_test, first)
     {
         span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
-        EXPECT_TRUE(av.first<6>().size() == 6);
-        EXPECT_TRUE(av.first<-1>().size() == -1);
+        (void) av.first<6>();
 #endif
-        EXPECT_DEATH(av.first(6).size(), expected);
+        EXPECT_DEATH(av.first(6), expected);
     }
 
     {
@@ -844,9 +730,9 @@ TEST(span_test, last)
     {
         span<int, 5> av = arr;
 #ifdef CONFIRM_COMPILATION_ERRORS
-        EXPECT_TRUE(av.last<6>().size() == 6);
+        (void) av.last<6>();
 #endif
-        EXPECT_DEATH(av.last(6).size(), expected);
+        EXPECT_DEATH(av.last(6), expected);
     }
 
     {
@@ -871,6 +757,9 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<2, 2>().size()) == 2);
         EXPECT_TRUE(decltype(av.subspan<2, 2>())::extent == 2);
         EXPECT_TRUE(av.subspan(2, 2).size() == 2);
+
+        EXPECT_TRUE((av.subspan<2, 3>().size()) == 3);
+        EXPECT_TRUE(decltype(av.subspan<2, 3>())::extent == 3);
         EXPECT_TRUE(av.subspan(2, 3).size() == 3);
     }
 
@@ -887,8 +776,12 @@ TEST(span_test, subspan)
         EXPECT_TRUE(decltype(av.subspan<0, 5>())::extent == 5);
         EXPECT_TRUE(av.subspan(0, 5).size() == 5);
 
-        EXPECT_DEATH(av.subspan(0, 6).size(), expected);
-        EXPECT_DEATH(av.subspan(1, 5).size(), expected);
+#ifdef CONFIRM_COMPILATION_ERRORS
+        (void) av.subspan<0, 6>();
+        (void) av.subspan<1, 5>();
+#endif
+        EXPECT_DEATH(av.subspan(0, 6), expected);
+        EXPECT_DEATH(av.subspan(1, 5), expected);
     }
 
     {
@@ -896,14 +789,22 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<4, 0>().size()) == 0);
         EXPECT_TRUE(decltype(av.subspan<4, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(4, 0).size() == 0);
+
+        EXPECT_TRUE((av.subspan<5, 0>().size()) == 0);
+        EXPECT_TRUE(decltype(av.subspan<5, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(5, 0).size() == 0);
-        EXPECT_DEATH(av.subspan(6, 0).size(), expected);
+
+#ifdef CONFIRM_COMPILATION_ERRORS
+        (void) av.subspan<6, 0>();
+#endif
+        EXPECT_DEATH(av.subspan(6, 0), expected);
     }
 
     {
         span<int, 5> av = arr;
         EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(decltype(av.subspan<1>())::extent == 4);
+        EXPECT_TRUE(av.subspan(1).size() == 4);
     }
 
     {
@@ -911,35 +812,58 @@ TEST(span_test, subspan)
         EXPECT_TRUE((av.subspan<0, 0>().size()) == 0);
         EXPECT_TRUE(decltype(av.subspan<0, 0>())::extent == 0);
         EXPECT_TRUE(av.subspan(0, 0).size() == 0);
-        EXPECT_DEATH((av.subspan<1, 0>().size()), expected);
+
+        EXPECT_DEATH((av.subspan<1, 0>()), expected);
+        EXPECT_DEATH((av.subspan(1, 0)), expected);
     }
 
     {
         span<int> av;
+        EXPECT_TRUE((av.subspan<0>().size()) == 0);
+        EXPECT_TRUE(decltype(av.subspan<0>())::extent == dynamic_extent);
         EXPECT_TRUE(av.subspan(0).size() == 0);
-        EXPECT_DEATH(av.subspan(1).size(), expected);
+
+        EXPECT_DEATH(av.subspan<1>(), expected);
+        EXPECT_TRUE(decltype(av.subspan<1>())::extent == dynamic_extent);
+        EXPECT_DEATH(av.subspan(1), expected);
     }
 
     {
         span<int> av = arr;
         EXPECT_TRUE(av.subspan(0).size() == 5);
+        EXPECT_TRUE(av.subspan<0>().size() == 5);
         EXPECT_TRUE(av.subspan(1).size() == 4);
+        EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(av.subspan(4).size() == 1);
+        EXPECT_TRUE(av.subspan<4>().size() == 1);
         EXPECT_TRUE(av.subspan(5).size() == 0);
-        EXPECT_DEATH(av.subspan(6).size(), expected);
+        EXPECT_TRUE(av.subspan<5>().size() == 0);
+        EXPECT_DEATH(av.subspan(6), expected);
+        EXPECT_DEATH(av.subspan<6>(), expected);
         const auto av2 = av.subspan(1);
         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
+        const auto av3 = av.subspan<1>();
+        for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av3[i] == static_cast<int>(i) + 2);
     }
 
     {
         span<int, 5> av = arr;
         EXPECT_TRUE(av.subspan(0).size() == 5);
+        EXPECT_TRUE(av.subspan<0>().size() == 5);
         EXPECT_TRUE(av.subspan(1).size() == 4);
+        EXPECT_TRUE(av.subspan<1>().size() == 4);
         EXPECT_TRUE(av.subspan(4).size() == 1);
+        EXPECT_TRUE(av.subspan<4>().size() == 1);
         EXPECT_TRUE(av.subspan(5).size() == 0);
-        EXPECT_DEATH(av.subspan(6).size(), expected);
+        EXPECT_TRUE(av.subspan<5>().size() == 0);
+        EXPECT_DEATH(av.subspan(6), expected);
+#ifdef CONFIRM_COMPILATION_ERRORS
+        EXPECT_DEATH(av.subspan<6>(), expected);
+#endif
         const auto av2 = av.subspan(1);
         for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av2[i] == static_cast<int>(i) + 2);
+        const auto av3 = av.subspan<1>();
+        for (std::size_t i = 0; i < 4; ++i) EXPECT_TRUE(av3[i] == static_cast<int>(i) + 2);
     }
 }
 
@@ -1114,9 +1038,21 @@ TEST(span_test, rbegin_rend)
     }
 }
 
+template <typename U, typename = void>
+static constexpr bool AsWritableBytesCompilesFor = false;
+template <typename U>
+static constexpr bool
+    AsWritableBytesCompilesFor<U, void_t<decltype(as_writable_bytes(std::declval<U>()))>> = true;
+
 TEST(span_test, as_bytes)
 {
     int a[] = {1, 2, 3, 4};
+
+    static_assert(AsWritableBytesCompilesFor<span<int>>, "AsWriteableBytesCompilesFor<span<int>>");
+    // you should not be able to get writeable bytes for const objects
+    static_assert(!AsWritableBytesCompilesFor<span<const int>>,
+                  "!AsWriteableBytesCompilesFor<span<const int>>");
+
     {
         const span<const int> s = a;
         EXPECT_TRUE(s.size() == 4);
@@ -1147,17 +1083,6 @@ TEST(span_test, as_writable_bytes)
 {
     int a[] = {1, 2, 3, 4};
 
-    {
-#ifdef CONFIRM_COMPILATION_ERRORS
-        // you should not be able to get writeable bytes for const objects
-        span<const int> s = a;
-        EXPECT_TRUE(s.size() == 4);
-        span<const byte> bs = as_writable_bytes(s);
-        EXPECT_TRUE(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
-        EXPECT_TRUE(bs.size() == s.size_bytes());
-#endif
-    }
-
     {
         span<int> s;
         const auto bs = as_writable_bytes(s);
@@ -1197,30 +1122,15 @@ TEST(span_test, fixed_size_conversions)
         static_cast<void>(s);
     }
 
-// initialization or assignment to static span that REDUCES size is NOT ok
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 2> s = arr;
-    }
-    {
-        span<int, 2> s2 = s4;
-        static_cast<void>(s2);
-    }
-#endif
+    // initialization or assignment to static span that REDUCES size is NOT ok
+    static_assert(!ConversionCompilesFor<span<int, 2>, int[4]>,
+                  "!ConversionCompilesFor<span<int, 2>, int[4]>");
+    static_assert(!ConversionCompilesFor<span<int, 2>, span<int, 4>>,
+                  "!ConversionCompilesFor<span<int, 2>, span<int, 4>>");
 
     // even when done dynamically
-    {
-        /*
-        // this now results in a compile-time error, rather than runtime.
-        // There is no suitable conversion from dynamic span to fixed span.
-        span<int> s = arr;
-        auto f = [&]() {
-            const span<int, 2> s2 = s;
-            static_cast<void>(s2);
-        };
-        EXPECT_DEATH(f(), expected);
-        */
-    }
+    static_assert(!ConversionCompilesFor<span<int, 2>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 2>, span<int>>");
 
     // but doing so explicitly is ok
 
@@ -1234,32 +1144,24 @@ TEST(span_test, fixed_size_conversions)
         static_cast<void>(s1);
     }
 
-    /*
-     // this is not a legal operation in std::span, so we are no longer supporting it
-     // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
-     // then convert from span<int, dynamic_extent> to span<int, 1>
-     // The dynamic to fixed extents are not supported in the standard
-     // to make this work, span<int, 1> would need to be span<int>.
-     {
-
-         // NB: implicit conversion to span<int,1> from span<int>
-         span<int, 1> s1 = s4.first(1);
-         static_cast<void>(s1);
-     }
-     */
+    // this is not a legal operation in std::span, so we are no longer supporting it
+    // conversion from span<int, 4> to span<int, dynamic_extent> via call to `first`
+    // then convert from span<int, dynamic_extent> to span<int, 1>
+    // The dynamic to fixed extents are not supported in the standard
+    // to make this work, span<int, 1> would need to be span<int>.
+    static_assert(!ConversionCompilesFor<span<int, 1>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 1>, span<int>>");
 
     // initialization or assignment to static span that requires size INCREASE is not ok.
     int arr2[2] = {1, 2};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        span<int, 4> s3 = arr2;
-    }
-    {
-        span<int, 2> s2 = arr2;
-        span<int, 4> s4a = s2;
-    }
-#endif
+    static_assert(!ConversionCompilesFor<span<int, 4>, int[2]>,
+                  "!ConversionCompilesFor<span<int, 4>, int[2]>");
+    static_assert(!ConversionCompilesFor<span<int, 2>, int[2]>,
+                  "!ConversionCompilesFor<span<int, 2>, int[2]>");
+    static_assert(!ConversionCompilesFor<span<int, 4>, span<int, 2>>,
+                  "!ConversionCompilesFor<span<int, 4>, span<int, 2>>");
+
     {
         auto f = [&]() {
             const span<int, 4> _s4{arr2, 2};
@@ -1268,16 +1170,11 @@ TEST(span_test, fixed_size_conversions)
         EXPECT_DEATH(f(), expected);
     }
 
-    /*
-     // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size
-     span.
-     // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
-     span<int> av = arr2; auto f = [&]() {
-         const span<int, 4> _s4 = av;
-         static_cast<void>(_s4);
-     };
-     EXPECT_DEATH(f(), expected);
-     */
+    // This no longer compiles. There is no suitable conversion from dynamic span to a fixed size
+    // span.
+    // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
+    static_assert(!ConversionCompilesFor<span<int, 4>, span<int>>,
+                  "!ConversionCompilesFor<span<int, 4>, span<int>>");
 }
 
 TEST(span_test, interop_with_std_regex)
@@ -1376,8 +1273,6 @@ TEST(span_test, msvc_compile_error_PR1100)
     int arr[]{1, 7, 2, 9};
     gsl::span sp{arr, std::size(arr)};
     std::ranges::sort(sp);
-    for (const auto& e : sp) {
-        (void)e;
-    }
+    for (const auto& e : sp) { (void) e; }
 }
 #endif // defined(__cpp_lib_span) && defined(__cpp_lib_ranges)
diff --git a/tests/strict_notnull_tests.cpp b/tests/strict_notnull_tests.cpp
index 82f9c5c..d75f5d6 100644
--- a/tests/strict_notnull_tests.cpp
+++ b/tests/strict_notnull_tests.cpp
@@ -17,10 +17,19 @@
 #include <gsl/pointers> // for not_null, operator<, operator<=, operator>
 #include <gtest/gtest.h>
 
+#include <type_traits> // for declval
+
 #include "deathTestCommon.h"
 
 using namespace gsl;
 
+#if __cplusplus >= 201703l
+using std::void_t;
+#else  // __cplusplus >= 201703l
+template <class...>
+using void_t = void;
+#endif // __cplusplus < 201703l
+
 // stand-in for a user-defined ref-counted class
 template <typename T>
 struct RefCounted
@@ -53,21 +62,39 @@ GSL_SUPPRESS(f.4) // NO-FORMAT: attribute
 bool strict_helper_const(strict_not_null<const int*> p) { return *p == 12; }
 
 int* return_pointer() { return nullptr; }
-#ifdef CONFIRM_COMPILATION_ERRORS
-const int* return_pointer_const() { return nullptr; }
-#endif
 } // namespace
 
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_A = false;
+template <typename U>
+static constexpr bool
+    CtorCompilesFor_A<U, void_t<decltype(gsl::strict_not_null<void*>{std::declval<U>()})>> = true;
+
+template <typename U, int N, typename = void>
+static constexpr bool CtorCompilesFor_B = false;
+template <typename U, int N>
+static constexpr bool CtorCompilesFor_B<U, N, void_t<decltype(gsl::strict_not_null<U>{N})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool DefaultCtorCompilesFor = false;
+template <typename U>
+static constexpr bool DefaultCtorCompilesFor<U, void_t<decltype(gsl::strict_not_null<U>{})>> = true;
+
+template <typename U, typename = void>
+static constexpr bool CtorCompilesFor_C = false;
+template <typename U>
+static constexpr bool CtorCompilesFor_C<
+    U, void_t<decltype(gsl::strict_not_null<U*>{std::declval<std::unique_ptr<U>>()})>> = true;
+
 TEST(strict_notnull_tests, TestStrictNotNullConstructors)
 {
     {
+        static_assert(CtorCompilesFor_A<void*>, "CtorCompilesFor_A<void*>");
+        static_assert(!CtorCompilesFor_A<std::nullptr_t>, "!CtorCompilesFor_A<std::nullptr_t>");
+        static_assert(!CtorCompilesFor_B<void*, 0>, "!CtorCompilesFor_B<void*, 0>");
+        static_assert(!DefaultCtorCompilesFor<void*>, "!DefaultCtorCompilesFor<void*>");
+        static_assert(!CtorCompilesFor_C<int>, "CtorCompilesFor_C<int>");
 #ifdef CONFIRM_COMPILATION_ERRORS
-        strict_not_null<int*> p = nullptr;          // yay...does not compile!
-        strict_not_null<std::vector<char>*> p1 = 0; // yay...does not compile!
-        strict_not_null<int*> p2;                   // yay...does not compile!
-        std::unique_ptr<int> up = std::make_unique<int>(120);
-        strict_not_null<int*> p3 = up;
-
         // Forbid non-nullptr assignable types
         strict_not_null<std::vector<int>> f(std::vector<int>{1});
         strict_not_null<int> z(10);
@@ -162,6 +189,26 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructors)
     }
 }
 
+template <typename U, typename = void>
+static constexpr bool StrictHelperCompilesFor = false;
+template <typename U>
+static constexpr bool
+    StrictHelperCompilesFor<U, void_t<decltype(strict_helper(std::declval<U>()))>> = true;
+
+
+template <typename U, typename = void>
+static constexpr bool StrictHelperConstCompilesFor = false;
+template <typename U>
+static constexpr bool
+    StrictHelperConstCompilesFor<U, void_t<decltype(strict_helper_const(std::declval<U>()))>> =
+        true;
+
+
+template <typename U, typename = void>
+static constexpr bool HelperCompilesFor = false;
+template <typename U>
+static constexpr bool HelperCompilesFor<U, void_t<decltype(helper(std::declval<U>()))>> = true;
+
 TEST(strict_notnull_tests, TestStrictNotNull)
 {
     {
@@ -170,14 +217,15 @@ TEST(strict_notnull_tests, TestStrictNotNull)
 
 #ifdef CONFIRM_COMPILATION_ERRORS
         strict_not_null<int*> snn = &x;
-        strict_helper(&x);
-        strict_helper_const(&x);
-        strict_helper(return_pointer());
-        strict_helper_const(return_pointer_const());
 #endif
+        static_assert(!StrictHelperCompilesFor<int*>, "!StrictHelperCompilesFor<int*>");
+        static_assert(!StrictHelperConstCompilesFor<int*>,
+                      "!StrictHelperCompilesFor<int*>");
 
         const strict_not_null<int*> snn1{&x};
 
+        static_assert(StrictHelperCompilesFor<const strict_not_null<int*>>,
+                          "StrictHelperCompilesFor<const strict_not_null<int*>>");
         helper(snn1);
         helper_const(snn1);
 
@@ -190,17 +238,17 @@ TEST(strict_notnull_tests, TestStrictNotNull)
 
 #ifdef CONFIRM_COMPILATION_ERRORS
         strict_not_null<int*> snn = &x;
-        strict_helper(&x);
-        strict_helper_const(&x);
-        strict_helper(return_pointer());
-        strict_helper_const(return_pointer_const());
 #endif
+        static_assert(!StrictHelperCompilesFor<const int*>, "!StrictHelperFor<const int*>");
+        static_assert(!StrictHelperConstCompilesFor<const int*>,
+                      "!StrictHelperCompilesFor<const int*>");
 
         const strict_not_null<const int*> snn1{&x};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(snn1);
-#endif
+        static_assert(!HelperCompilesFor<const strict_not_null<const int*>>,
+                      "!HelperCompilesFor<const strict_not_null<const int*>>");
+        static_assert(StrictHelperConstCompilesFor<const strict_not_null<const int*>>,
+                      "StrictHelperCompilesFor<const strict_not_null<const int*>>");
         helper_const(snn1);
 
         EXPECT_TRUE(*snn1 == 42);
@@ -227,9 +275,8 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         strict_not_null<const int*> snn1{&x};
         const strict_not_null<const int*> snn2{&x};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        strict_helper(snn1);
-#endif
+        static_assert(!StrictHelperCompilesFor<strict_not_null<const int*>>,
+                      "!StrictHelperCompilesFor<strict_not_null<const int*>>");
         strict_helper_const(snn1);
         strict_helper_const(snn2);
 
@@ -261,9 +308,8 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         const not_null<const int*> nn1 = snn;
         const not_null<const int*> nn2{snn};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(snn);
-#endif
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
         helper_const(snn);
 
         EXPECT_TRUE(snn == nn1);
@@ -303,9 +349,8 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         const strict_not_null<const int*> snn1{nn};
         const strict_not_null<const int*> snn2{nn};
 
-#ifdef CONFIRM_COMPILATION_ERRORS
-        strict_helper(nn);
-#endif
+        static_assert(!StrictHelperCompilesFor<not_null<const int*>>,
+                      "!StrictHelperCompilesFor<not_null<const int*>>");
         strict_helper_const(nn);
 
         EXPECT_TRUE(snn1 == nn);
@@ -319,12 +364,6 @@ TEST(strict_notnull_tests, TestStrictNotNull)
         EXPECT_TRUE(hash_nn(snn1) == hash_nn(snn2));
         EXPECT_TRUE(hash_snn(snn1) == hash_snn(nn));
     }
-
-#ifdef CONFIRM_COMPILATION_ERRORS
-    {
-        strict_not_null<int*> p{nullptr};
-    }
-#endif
 }
 
 #if defined(__cplusplus) && (__cplusplus >= 201703L)
@@ -351,9 +390,8 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)
         const int i = 42;
 
         strict_not_null x{&i};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(strict_not_null{&i});
-#endif
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
         helper_const(strict_not_null{&i});
 
         EXPECT_TRUE(*x == 42);
@@ -375,9 +413,8 @@ TEST(strict_notnull_tests, TestStrictNotNullConstructorTypeDeduction)
         const int* p = &i;
 
         strict_not_null x{p};
-#ifdef CONFIRM_COMPILATION_ERRORS
-        helper(strict_not_null{p});
-#endif
+        static_assert(!HelperCompilesFor<strict_not_null<const int*>>,
+                      "!HelperCompilesFor<strict_not_null<const int*>>");
         helper_const(strict_not_null{p});
 
         EXPECT_TRUE(*x == 42);