[ArrayRef] Provide constructors from any type with a data() member (#145735)

The assumption here is that if data() returns a pointer and size()
exists it's something representing contiguous storage.

Modeled after the behavior of C++20 std::span and enables two-way
conversion between std::span (or really any span-like type) and
ArrayRef. Add a unit test that verifies this if std::span is around.

This also means we can get rid of specific conversions for std::vector
and SmallVector.

Author: d0k

llvm/include/llvm/ADT/ArrayRef.h

@@ -84,18 +84,19 @@ namespace llvm {
       assert(begin <= end);
     }
 
-    /// Construct an ArrayRef from a SmallVector. This is templated in order to
-    /// avoid instantiating SmallVectorTemplateCommon<T> whenever we
-    /// copy-construct an ArrayRef.
-    template<typename U>
-    /*implicit*/ ArrayRef(const SmallVectorTemplateCommon<T, U> &Vec)
-      : Data(Vec.data()), Length(Vec.size()) {
-    }
-
-    /// Construct an ArrayRef from a std::vector.
-    template<typename A>
-    /*implicit*/ ArrayRef(const std::vector<T, A> &Vec)
-      : Data(Vec.data()), Length(Vec.size()) {}
+    /// Construct an ArrayRef from a type that has a data() method that returns
+    /// a pointer convertible to const T *.
+    template <
+        typename C,
+        typename = std::enable_if_t<
+            std::conjunction_v<
+                std::is_convertible<
+                    decltype(std::declval<const C &>().data()) *,
+                    const T *const *>,
+                std::is_integral<decltype(std::declval<const C &>().size())>>,
+            void>>
+    /*implicit*/ constexpr ArrayRef(const C &V)
+        : Data(V.data()), Length(V.size()) {}
 
     /// Construct an ArrayRef from a std::array
     template <size_t N>
@@ -123,32 +124,6 @@ namespace llvm {
 #pragma GCC diagnostic pop
 #endif
 
-    /// Construct an ArrayRef<const T*> from ArrayRef<T*>. This uses SFINAE to
-    /// ensure that only ArrayRefs of pointers can be converted.
-    template <typename U>
-    ArrayRef(const ArrayRef<U *> &A,
-             std::enable_if_t<std::is_convertible<U *const *, T const *>::value>
-                 * = nullptr)
-        : Data(A.data()), Length(A.size()) {}
-
-    /// Construct an ArrayRef<const T*> from a SmallVector<T*>. This is
-    /// templated in order to avoid instantiating SmallVectorTemplateCommon<T>
-    /// whenever we copy-construct an ArrayRef.
-    template <typename U, typename DummyT>
-    /*implicit*/ ArrayRef(
-        const SmallVectorTemplateCommon<U *, DummyT> &Vec,
-        std::enable_if_t<std::is_convertible<U *const *, T const *>::value> * =
-            nullptr)
-        : Data(Vec.data()), Length(Vec.size()) {}
-
-    /// Construct an ArrayRef<const T*> from std::vector<T*>. This uses SFINAE
-    /// to ensure that only vectors of pointers can be converted.
-    template <typename U, typename A>
-    ArrayRef(const std::vector<U *, A> &Vec,
-             std::enable_if_t<std::is_convertible<U *const *, T const *>::value>
-                 * = nullptr)
-        : Data(Vec.data()), Length(Vec.size()) {}
-
     /// Construct an ArrayRef<T> from iterator_range<U*>. This uses SFINAE
     /// to ensure that this is only used for iterator ranges over plain pointer
     /// iterators.

llvm/unittests/ADT/ArrayRefTest.cpp

@@ -8,10 +8,16 @@
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Allocator.h"
-#include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
 #include <limits>
 #include <vector>
+#if __has_include(<version>)
+#include <version>
+#endif
+#ifdef __cpp_lib_span
+#include <span>
+#endif
+
 using namespace llvm;
 
 // Check that the ArrayRef-of-pointer converting constructor only allows adding
@@ -406,4 +412,15 @@ TEST(ArrayRefTest, MutableArrayRefDeductionGuides) {
   }
 }
 
+#ifdef __cpp_lib_span
+static_assert(std::is_constructible_v<ArrayRef<int>, std::span<const int>>,
+              "should be able to construct ArrayRef from const std::span");
+static_assert(std::is_constructible_v<std::span<const int>, ArrayRef<int>>,
+              "should be able to construct const std::span from ArrayRef");
+static_assert(std::is_constructible_v<ArrayRef<int>, std::span<int>>,
+              "should be able to construct ArrayRef from mutable std::span");
+static_assert(!std::is_constructible_v<std::span<int>, ArrayRef<int>>,
+              "cannot construct mutable std::span from ArrayRef");
+#endif
+
 } // end anonymous namespace