[ArrayRef] Add constructor from iterator_range<U*> (NFC). (#137796)

Add a new constructor to ArrayRef that takes an iterator_range with a
random access iterator that can be converted.

This can help to avoid creating unnecessary iterator_ranges for types
where an ArrayRef can already be constructed. To be used in
#137798.


PR: #137796

Author: fhahn

llvm/include/llvm/ADT/ArrayRef.h

@@ -149,6 +149,14 @@ namespace llvm {
                  * = 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.
+    template <typename U, typename = std::enable_if_t<
+                              std::is_convertible_v<U *const *, T *const *>>>
+    ArrayRef(const iterator_range<U *> &Range)
+        : Data(Range.begin()), Length(llvm::size(Range)) {}
+
     /// @}
     /// @name Simple Operations
     /// @{

llvm/unittests/ADT/ArrayRefTest.cpp

@@ -255,6 +255,64 @@ TEST(ArrayRefTest, ArrayRefFromStdArray) {
   }
 }
 
+struct TestRandomAccessIterator {
+  using iterator_category = std::random_access_iterator_tag;
+};
+
+static_assert(!std::is_constructible_v<
+                  ArrayRef<int>, iterator_range<TestRandomAccessIterator>>,
+              "cannot construct from iterator range with non-pointer iterator");
+static_assert(!std::is_constructible_v<ArrayRef<int>, iterator_range<int>>,
+              "cannot construct from iterator range with non-pointer iterator");
+
+class TestBase {};
+
+class TestDerived : public TestBase {};
+
+static_assert(
+    !std::is_constructible_v<ArrayRef<TestDerived>, iterator_range<TestBase *>>,
+    "cannot construct ArrayRef with derived type");
+static_assert(
+    !std::is_constructible_v<ArrayRef<TestBase>, iterator_range<TestDerived *>>,
+    "cannot construct ArrayRef base type");
+static_assert(!std::is_constructible_v<ArrayRef<TestBase *>,
+                                       iterator_range<TestDerived **>>,
+              "cannot construct ArrayRef pointer of base type");
+
+static_assert(
+    !std::is_constructible_v<ArrayRef<int>, iterator_range<const int *>>,
+    "cannot construct ArrayRef with non-const elements from const iterator "
+    "range");
+static_assert(
+    std::is_constructible_v<ArrayRef<char *>, iterator_range<char **>>,
+    "should be able to construct ArrayRef from iterator_range over pointers");
+static_assert(
+    !std::is_constructible_v<ArrayRef<char *>, iterator_range<char *const *>>,
+    "should be able to construct ArrayRef from iterator_range over pointers");
+
+TEST(ArrayRefTest, ArrayRefFromIteratorRange) {
+  std::array<int, 5> A1{{42, -5, 0, 1000000, -1000000}};
+  ArrayRef<int> A2 = make_range(A1.begin(), A1.end());
+
+  EXPECT_EQ(A1.size(), A2.size());
+  for (std::size_t i = 0; i < A1.size(); ++i)
+    EXPECT_EQ(A1[i], A2[i]);
+
+  ArrayRef<const int> A3 = make_range(A1.begin(), A1.end());
+  EXPECT_EQ(A1.size(), A3.size());
+  for (std::size_t i = 0; i < A1.size(); ++i)
+    EXPECT_EQ(A1[i], A3[i]);
+}
+
+TEST(ArrayRefTest, ArrayRefFromIteratorConstRange) {
+  std::array<const int, 5> A1{{42, -5, 0, 1000000, -1000000}};
+  ArrayRef<const int> A2 = make_range(A1.begin(), A1.end());
+
+  EXPECT_EQ(A1.size(), A2.size());
+  for (std::size_t i = 0; i < A1.size(); ++i)
+    EXPECT_EQ(A1[i], A2[i]);
+}
+
 static_assert(std::is_trivially_copyable_v<ArrayRef<int>>,
               "trivially copyable");