FileManager: Use llvm::Expected in new getFileRef API

`FileManager::getFileRef` is a modern API which we expect to convert to
over time.  We should modernize the error handling as well, using
`llvm::Expected` instead of `llvm::ErrorOr`, to help clients that care
about errors to ensure nothing is missed.

However, not all clients care.  I've also added another path for those
that don't:

- `FileEntryRef` is now copy- and move-assignable (using a pointer
  instead of a reference).
- `FileManager::getOptionalFileRef` returns an `llvm::Optional` instead
  of `llvm::Expected`.
- Added an `llvm::expectedToOptional` utility in case this is useful
  elsewhere.

https://reviews.llvm.org/D66705

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369943 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/Support/Error.h b/include/llvm/Support/Error.h
index c0e4d362364bee999273aaa94826b87119c52f42..f961a29b33a6a22ef6dcbfed75be776c1a02acfa 100644 (file)
--- a/include/llvm/Support/Error.h
+++ b/include/llvm/Support/Error.h
@@ -982,6 +982,20 @@ inline void consumeError(Error Err) {
   handleAllErrors(std::move(Err), [](const ErrorInfoBase &) {});
 }
 
+/// Convert an Expected to an Optional without doing anything. This method
+/// should be used only where an error can be considered a reasonable and
+/// expected return value.
+///
+/// Uses of this method are potentially indicative of problems: perhaps the
+/// error should be propagated further, or the error-producer should just
+/// return an Optional in the first place.
+template <typename T> Optional<T> expectedToOptional(Expected<T> &&E) {
+  if (E)
+    return std::move(*E);
+  consumeError(E.takeError());
+  return None;
+}
+
 /// Helper for converting an Error to a bool.
 ///
 /// This method returns true if Err is in an error state, or false if it is