Make Span complain if constructed with a parameter that won't outlive it, except…

Make Span complain if constructed with a parameter that won't outlive it, except if that parameter is also a span or appears to be a view type.

PiperOrigin-RevId: 461612357
Change-Id: Ibba36f44465176db47dd21e1866134549143fa64

absl/types/internal/span.h

@@ -28,6 +28,9 @@
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
+template <typename T>
+class Span;
+
 namespace span_internal {
 // A constexpr min function
 constexpr size_t Min(size_t a, size_t b) noexcept { return a < b ? a : b; }
@@ -121,6 +124,36 @@ struct IsConvertible : IsConvertibleHelper<From, To>::type {};
 template <typename From, typename To>
 using EnableIfConvertibleTo =
     typename std::enable_if<IsConvertible<From, To>::value>::type;
+
+// IsView is true for types where the return type of .data() is the same for
+// mutable and const instances. This isn't foolproof, but it's only used to
+// enable a compiler warning.
+template <typename T, typename = void, typename = void>
+struct IsView {
+  static constexpr bool value = false;
+};
+
+template <typename T>
+struct IsView<
+    T, absl::void_t<decltype(span_internal::GetData(std::declval<const T&>()))>,
+    absl::void_t<decltype(span_internal::GetData(std::declval<T&>()))>> {
+ private:
+  using Container = std::remove_const_t<T>;
+  using ConstData =
+      decltype(span_internal::GetData(std::declval<const Container&>()));
+  using MutData = decltype(span_internal::GetData(std::declval<Container&>()));
+ public:
+  static constexpr bool value = std::is_same<ConstData, MutData>::value;
+};
+
+// These enablers result in 'int' so they can be used as typenames or defaults
+// in template paramters lists.
+template <typename T>
+using EnableIfIsView = std::enable_if_t<IsView<T>::value, int>;
+
+template <typename T>
+using EnableIfNotIsView = std::enable_if_t<!IsView<T>::value, int>;
+
 }  // namespace span_internal
 ABSL_NAMESPACE_END
 }  // namespace absl

absl/types/span.h

@@ -60,6 +60,7 @@
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/attributes.h"
 #include "absl/base/internal/throw_delegate.h"
 #include "absl/base/macros.h"
 #include "absl/base/optimization.h"
@@ -160,12 +161,12 @@ class Span {
 
   // Used to SFINAE-enable a function when the slice elements are const.
   template <typename U>
-  using EnableIfConstView =
+  using EnableIfValueIsConst =
       typename std::enable_if<std::is_const<T>::value, U>::type;
 
   // Used to SFINAE-enable a function when the slice elements are mutable.
   template <typename U>
-  using EnableIfMutableView =
+  using EnableIfValueIsMutable =
       typename std::enable_if<!std::is_const<T>::value, U>::type;
 
  public:
@@ -196,13 +197,34 @@ class Span {
   // Explicit reference constructor for a mutable `Span<T>` type. Can be
   // replaced with MakeSpan() to infer the type parameter.
   template <typename V, typename = EnableIfConvertibleFrom<V>,
-            typename = EnableIfMutableView<V>>
-  explicit Span(V& v) noexcept  // NOLINT(runtime/references)
+            typename = EnableIfValueIsMutable<V>,
+            typename = span_internal::EnableIfNotIsView<V>>
+  explicit Span(
+      V& v
+          ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/references)
       : Span(span_internal::GetData(v), v.size()) {}
 
   // Implicit reference constructor for a read-only `Span<const T>` type
   template <typename V, typename = EnableIfConvertibleFrom<V>,
-            typename = EnableIfConstView<V>>
+            typename = EnableIfValueIsConst<V>,
+            typename = span_internal::EnableIfNotIsView<V>>
+  constexpr Span(
+      const V& v
+          ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/explicit)
+      : Span(span_internal::GetData(v), v.size()) {}
+
+  // Overloads of the above two functions that are only enabled for view types.
+  // This is so we can drop the ABSL_ATTRIBUTE_LIFETIME_BOUND annotation. These
+  // overloads must be made unique by using a different template parameter list
+  // (hence the = 0 for the IsView enabler).
+  template <typename V, typename = EnableIfConvertibleFrom<V>,
+            typename = EnableIfValueIsMutable<V>,
+            span_internal::EnableIfIsView<V> = 0>
+  explicit Span(V& v) noexcept  // NOLINT(runtime/references)
+      : Span(span_internal::GetData(v), v.size()) {}
+  template <typename V, typename = EnableIfConvertibleFrom<V>,
+            typename = EnableIfValueIsConst<V>,
+            span_internal::EnableIfIsView<V> = 0>
   constexpr Span(const V& v) noexcept  // NOLINT(runtime/explicit)
       : Span(span_internal::GetData(v), v.size()) {}
 
@@ -242,7 +264,7 @@ class Span {
   //   Process(ints);
   //
   template <typename LazyT = T,
-            typename = EnableIfConstView<LazyT>>
+            typename = EnableIfValueIsConst<LazyT>>
   Span(std::initializer_list<value_type> v
            ABSL_ATTRIBUTE_LIFETIME_BOUND) noexcept  // NOLINT(runtime/explicit)
       : Span(v.begin(), v.size()) {}