[orc-rt] Add support for constructing Expected<Error> values. (#161656)

These will be used in upcoming RPC support patches where the outer
Expected value captures any RPC-infrastructure errors, and the inner
Error is returned from the romet call (i.e. the remote handler's return
type is Error).

orc-rt/include/orc-rt/Error.h

@@ -288,6 +288,10 @@ private:
   Error *Err;
 };
 
+/// Tag to force construction of an Expected value in the success state. See
+/// Expected constructor for details.
+struct ForceExpectedSuccessValue {};
+
 template <typename T> class ORC_RT_NODISCARD Expected {
 
   template <class OtherT> friend class Expected;
@@ -310,6 +314,13 @@ public:
     new (getErrorStorage()) error_type(Err.takePayload());
   }
 
+  template <typename OtherT>
+  Expected(OtherT &&Val, ForceExpectedSuccessValue _,
+           std::enable_if_t<std::is_convertible_v<OtherT, T>> * = nullptr)
+      : HasError(false), Unchecked(true) {
+    new (getStorage()) storage_type(std::forward<OtherT>(Val));
+  }
+
   /// Create an Expected from a T value.
   template <typename OtherT>
   Expected(OtherT &&Val,

orc-rt/unittests/ErrorTest.cpp

@@ -386,6 +386,29 @@ TEST(ErrorTest, ExpectedCovariance) {
   (void)!!A2;
 }
 
+// Test that Expected<Error> works as expected with .
+TEST(ErrorTest, ExpectedError) {
+  {
+    // Test success-success case.
+    Expected<Error> E(Error::success(), ForceExpectedSuccessValue());
+    EXPECT_TRUE(!!E);
+    cantFail(E.takeError());
+    auto Err = std::move(*E);
+    EXPECT_FALSE(!!Err);
+  }
+
+  {
+    // Test "failure" success case.
+    Expected<Error> E(make_error<StringError>("foo"),
+                      ForceExpectedSuccessValue());
+    EXPECT_TRUE(!!E);
+    cantFail(E.takeError());
+    auto Err = std::move(*E);
+    EXPECT_TRUE(!!Err);
+    EXPECT_EQ(toString(std::move(Err)), "foo");
+  }
+}
+
 // Test that the ExitOnError utility works as expected.
 TEST(ErrorTest, CantFailSuccess) {
   cantFail(Error::success());