Unittest for Error forwarding, follow-up to PR #92208 (#92314)

Added a unittest demonstrating Error -> Expected<T> forwarding.

Author: mtrofin

llvm/unittests/Support/ErrorTest.cpp

@@ -13,6 +13,7 @@
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Testing/Support/Error.h"
+#include "gmock/gmock.h"
 #include "gtest/gtest-spi.h"
 #include "gtest/gtest.h"
 #include <memory>
@@ -1158,4 +1159,21 @@ TEST(Error, BadAllocFatalErrorHandlersInteraction) {
   remove_bad_alloc_error_handler();
 }
 
+TEST(Error, ForwardToExpected) {
+  auto ErrorReturningFct = [](bool Fail) {
+    return Fail ? make_error<StringError>(llvm::errc::invalid_argument,
+                                          "Some Error")
+                : Error::success();
+  };
+  auto ExpectedReturningFct = [&](bool Fail) -> Expected<int> {
+    auto Err = ErrorReturningFct(Fail);
+    if (Err)
+      return Err;
+    return 42;
+  };
+  std::optional<int> MaybeV;
+  EXPECT_THAT_ERROR(ExpectedReturningFct(true).moveInto(MaybeV), Failed());
+  EXPECT_THAT_ERROR(ExpectedReturningFct(false).moveInto(MaybeV), Succeeded());
+  EXPECT_EQ(*MaybeV, 42);
+}
 } // namespace