Add a deduction guide for base::span from an iterator and a size.

Bug: 1490484
Change-Id: Ib6427f8a351f0723d9c59e31f1d0b64dcc0c1470
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4918886
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1207147}

base/containers/span.h

@@ -460,6 +460,10 @@ class GSL_POINTER span : public internal::ExtentStorage<Extent> {
 template <class T, size_t Extent>
 constexpr size_t span<T, Extent>::extent;
 
+template <typename It,
+          typename T = std::remove_reference_t<iter_reference_t<It>>>
+span(It it, StrictNumeric<size_t> size) -> span<T>;
+
 template <typename Container,
           typename T = std::remove_pointer_t<
               decltype(std::data(std::declval<Container>()))>,

base/containers/span_unittest.cc

@@ -27,6 +27,38 @@ using ::testing::Pointwise;
 
 namespace base {
 
+namespace {
+
+// Tests for span(It, StrictNumeric<size_t>) deduction guide. These tests use a
+// helper function to wrap the static_asserts, as most STL containers don't work
+// well in a constexpr context. std::array<T, N> does, but base::span has
+// specific overloads for std::array<T, n>, so that ends up being less helpful
+// than it would initially appear.
+//
+// Another alternative would be to use std::declval, but that would be fairly
+// verbose.
+[[maybe_unused]] void DeductionGuidesWithIteratorAndSize() {
+  {
+    const std::vector<int> v;
+    static_assert(
+        std::is_same_v<decltype(span(v.cbegin(), v.size())), span<const int>>);
+    static_assert(
+        std::is_same_v<decltype(span(v.begin(), v.size())), span<const int>>);
+    static_assert(
+        std::is_same_v<decltype(span(v.data(), v.size())), span<const int>>);
+  }
+
+  {
+    std::vector<int> v;
+    static_assert(
+        std::is_same_v<decltype(span(v.cbegin(), v.size())), span<const int>>);
+    static_assert(
+        std::is_same_v<decltype(span(v.begin(), v.size())), span<int>>);
+    static_assert(
+        std::is_same_v<decltype(span(v.data(), v.size())), span<int>>);
+  }
+}
+
 // Tests for span(Container&&) deduction guide.
 static_assert(std::is_same_v<decltype(span(std::declval<const std::string&>())),
                              span<const char>>);
@@ -60,6 +92,8 @@ static_assert(
     std::is_same_v<decltype(span(std::declval<std::array<float, 9>&&>())),
                    span<const float, 9>>);
 
+}  // namespace
+
 TEST(SpanTest, DefaultConstructor) {
   span<int> dynamic_span;
   EXPECT_EQ(nullptr, dynamic_span.data());