FTL: Allow implicit conversions with NonNull<T>

C++ allows certain implicit conversions for pointers, such as the
implicit conversion of "T*" to "const T*".

NonNull did not allow them, but that is easily corrected.

Bug: 185536303
Flag: EXEMPT Relaxes a compile-time constraint
Test: atest ftl_test
Change-Id: I0c14805c8fdcca979c67d2c86d57e5e678aa1b32

2 files changed, 44 insertions, 0 deletions

diff --git a/include/ftl/non_null.h b/include/ftl/non_null.h
index 4a5d8bffd0..e68428810b 100644
--- a/include/ftl/non_null.h
+++ b/include/ftl/non_null.h
@@ -68,6 +68,15 @@ class NonNull final {
   constexpr NonNull(const NonNull&) = default;
   constexpr NonNull& operator=(const NonNull&) = default;
 
+  template <typename U, typename = std::enable_if_t<std::is_convertible_v<U, Pointer>>>
+  constexpr NonNull(const NonNull<U>& other) : pointer_(other.get()) {}
+
+  template <typename U, typename = std::enable_if_t<std::is_convertible_v<U, Pointer>>>
+  constexpr NonNull& operator=(const NonNull<U>& other) {
+    pointer_ = other.get();
+    return *this;
+  }
+
   [[nodiscard]] constexpr const Pointer& get() const { return pointer_; }
   [[nodiscard]] constexpr explicit operator const Pointer&() const { return get(); }
 
diff --git a/libs/ftl/non_null_test.cpp b/libs/ftl/non_null_test.cpp
index 367b398915..457237c35e 100644
--- a/libs/ftl/non_null_test.cpp
+++ b/libs/ftl/non_null_test.cpp
@@ -81,6 +81,31 @@ static_assert(static_cast<bool>(kApplePtr));
 static_assert(std::is_same_v<decltype(ftl::as_non_null(std::declval<const int* const>())),
                              ftl::NonNull<const int*>>);
 
+class Base {};
+class Derived : public Base {};
+
+static_assert(std::is_constructible_v<ftl::NonNull<void*>, ftl::NonNull<int*>>);
+static_assert(!std::is_constructible_v<ftl::NonNull<int*>, ftl::NonNull<void*>>);
+static_assert(std::is_constructible_v<ftl::NonNull<const int*>, ftl::NonNull<int*>>);
+static_assert(!std::is_constructible_v<ftl::NonNull<int*>, ftl::NonNull<const int*>>);
+static_assert(std::is_constructible_v<ftl::NonNull<Base*>, ftl::NonNull<Derived*>>);
+static_assert(!std::is_constructible_v<ftl::NonNull<Derived*>, ftl::NonNull<Base*>>);
+static_assert(std::is_constructible_v<ftl::NonNull<std::unique_ptr<const int>>,
+                                      ftl::NonNull<std::unique_ptr<int>>>);
+static_assert(std::is_constructible_v<ftl::NonNull<std::unique_ptr<Base>>,
+                                      ftl::NonNull<std::unique_ptr<Derived>>>);
+
+static_assert(std::is_assignable_v<ftl::NonNull<void*>, ftl::NonNull<int*>>);
+static_assert(!std::is_assignable_v<ftl::NonNull<int*>, ftl::NonNull<void*>>);
+static_assert(std::is_assignable_v<ftl::NonNull<const int*>, ftl::NonNull<int*>>);
+static_assert(!std::is_assignable_v<ftl::NonNull<int*>, ftl::NonNull<const int*>>);
+static_assert(std::is_assignable_v<ftl::NonNull<Base*>, ftl::NonNull<Derived*>>);
+static_assert(!std::is_assignable_v<ftl::NonNull<Derived*>, ftl::NonNull<Base*>>);
+static_assert(std::is_assignable_v<ftl::NonNull<std::unique_ptr<const int>>,
+                                   ftl::NonNull<std::unique_ptr<int>>>);
+static_assert(std::is_assignable_v<ftl::NonNull<std::unique_ptr<Base>>,
+                                   ftl::NonNull<std::unique_ptr<Derived>>>);
+
 }  // namespace
 
 TEST(NonNull, SwapRawPtr) {
@@ -156,4 +181,14 @@ TEST(NonNull, UnorderedSetOfSmartPtr) {
   EXPECT_TRUE(ftl::contains(spi, *spi.begin()));
 }
 
+TEST(NonNull, ImplicitConversion) {
+  int i = 123;
+  int j = 345;
+  auto ip = ftl::as_non_null(&i);
+  ftl::NonNull<void*> vp{ip};
+  EXPECT_EQ(vp.get(), &i);
+  vp = ftl::as_non_null(&j);
+  EXPECT_EQ(vp.get(), &j);
+}
+
 }  // namespace android::test