[ADT] Implement Optional::transform

[kazu]

Summary

This patch implements Optional::transform for consistency with
std::optional::transform in C++23.

Note that the new function is identical to Optional::map. My plan is
to deprecate Optional::map after migrating all of its uses to
Optional::transform.

[dblaikie]

Sounds good - in the abstract it'd bee good not to duplicate the code or testing (old testing could be minimized if old implementation were rewritten as a wrapper around the new implementation) - but it's perhaps not enough to matter (hardly likely to have much in the way of bug fixes/changes to the testing or implementation that could diverge) and will make it simpler/easier to remove the old one later - so whatever works best for you.

[kazu]

In D131829#3721330, @dblaikie wrote:

Sounds good - in the abstract it'd bee good not to duplicate the code or testing (old testing could be minimized if old implementation were rewritten as a wrapper around the new implementation) - but it's perhaps not enough to matter (hardly likely to have much in the way of bug fixes/changes to the testing or implementation that could diverge) and will make it simpler/easier to remove the old one later - so whatever works best for you.

Thanks for the review! I wasn't sure I could safely turn map into a wrapper while preserving all the move semantics, the number of &, etc, so I just copied the code. :-P

We don't have a test for map, so adding a test for transform should be a pure improvement.

llvm/include/llvm/ADT/Optional.h

public:
   template <typename U>
   LLVM_DEPRECATED("Use value_or instead.", "value_or")
   constexpr T getValueOr(U &&alt) const & {
     return has_value() ? value() : std::forward<U>(alt);
   }
 
   /// Apply a function to the value if present; otherwise return None.
   template <class Function>
+  auto transform(const Function &F) const & -> Optional<decltype(F(value()))> {
+    if (*this)
+      return F(value());
+    return None;
+  }
+  template <class Function>
   auto map(const Function &F) const & -> Optional<decltype(F(value()))> {
     if (*this)
       return F(value());
     return None;
   }
 
   T &&value() && { return std::move(Storage.value()); }
   LLVM_DEPRECATED("Use value instead.", "value") T &&getValue() && {
     return std::move(Storage.value());
   }
   T &&operator*() && { return std::move(Storage.value()); }
 
   template <typename U> T value_or(U &&alt) && {
     return has_value() ? std::move(value()) : std::forward<U>(alt);
   }
   template <typename U>
   LLVM_DEPRECATED("Use value_or instead.", "value_or")
   T getValueOr(U &&alt) && {
     return has_value() ? std::move(value()) : std::forward<U>(alt);
   }
 
   /// Apply a function to the value if present; otherwise return None.
   template <class Function>
+  auto transform(
+      const Function &F) && -> Optional<decltype(F(std::move(*this).value()))> {
+    if (*this)
+      return F(std::move(*this).value());
+    return None;
+  }
+  template <class Function>
   auto map(const Function &F)
       && -> Optional<decltype(F(std::move(*this).value()))> {
     if (*this)
       return F(std::move(*this).value());
     return None;
   }
 };
 

llvm/unittests/ADT/OptionalTest.cpp

TEST(OptionalTest, MoveValueOr) {
   A = MoveOnly(5);
   MoveOnly::ResetCounts();
   EXPECT_EQ(5, std::move(A).value_or(MoveOnly(42)).val);
   EXPECT_EQ(1u, MoveOnly::MoveConstructions);
   EXPECT_EQ(0u, MoveOnly::MoveAssignments);
   EXPECT_EQ(2u, MoveOnly::Destructions);
 }
 
+TEST(OptionalTest, Transform) {
+  Optional<int> A;
+
+  Optional<int> B = A.transform([&](int N) { return N + 1; });
+  EXPECT_FALSE(B.has_value());
+
+  A = 3;
+  Optional<int> C = A.transform([&](int N) { return N + 1; });
+  EXPECT_TRUE(C.has_value());
+  EXPECT_EQ(4, C.value());
+}
+
+TEST(OptionalTest, MoveTransform) {
+  Optional<MoveOnly> A;
+
+  MoveOnly::ResetCounts();
+  Optional<int> B =
+      std::move(A).transform([&](const MoveOnly &M) { return M.val + 2; });
+  EXPECT_FALSE(B.has_value());
+  EXPECT_EQ(0u, MoveOnly::MoveConstructions);
+  EXPECT_EQ(0u, MoveOnly::MoveAssignments);
+  EXPECT_EQ(0u, MoveOnly::Destructions);
+
+  A = MoveOnly(5);
+  MoveOnly::ResetCounts();
+  Optional<int> C =
+      std::move(A).transform([&](const MoveOnly &M) { return M.val + 2; });
+  EXPECT_TRUE(C.has_value());
+  EXPECT_EQ(7, C.value());
+  EXPECT_EQ(0u, MoveOnly::MoveConstructions);
+  EXPECT_EQ(0u, MoveOnly::MoveAssignments);
+  EXPECT_EQ(0u, MoveOnly::Destructions);
+}
+
 struct EqualTo {
   template <typename T, typename U> static bool apply(const T &X, const U &Y) {
     return X == Y;
   }
 };
 
 struct NotEqualTo {
   template <typename T, typename U> static bool apply(const T &X, const U &Y) {