[Error/unittests] Add a FailedWithMessage gtest matcher

[labath]

Summary

We already have a "Failed" matcher, which can be used to check any
property of the Error object. However, most frequently one just wants to
check the error message, and while this is possible with the "Failed"
matcher, it is also very convoluted
(Failed<ErrorInfoBase>(testing::Property(&ErrorInfoBase::message, "the
message"))).

Now, one can just write: FailedWithMessage("the message"). I expect that
most of the usages will remain this simple, but the argument of the
matcher is not limited to simple strings -- the argument of the matcher
can be any other matcher, so one can write more complicated assertions
if needed (FailedWithMessage(ContainsRegex("foo|bar"))). If one wants to
match multiple error messages, he can pass multiple arguments to the
matcher.

If one wants to match the message list as a whole (perhaps to check the
message count), I've also included a FailedWithMessageArray matcher,
which takes a single matcher receiving a vector of error message
strings.

[labath]

You can take a look at D74899 to see how this would look in action. I've converted one file with a very complex set of error assertions to ensure that this can handle a variety of use cases.

llvm/include/llvm/Testing/Support/Error.h

if (Matcher) {
       *OS << " or the error ";
       Matcher->DescribeNegationTo(OS);
     }
   }
 
 private:
   Optional<testing::Matcher<InfoT &>> Matcher;
 };
+
+class ErrorMessageMatches
+    : public testing::MatcherInterface<const ErrorHolder &> {
+public:
+  explicit ErrorMessageMatches(
+      testing::Matcher<std::vector<std::string>> Matcher)
+      : Matcher(std::move(Matcher)) {}
+
+  bool MatchAndExplain(const ErrorHolder &Holder,
+                       testing::MatchResultListener *listener) const override {
+    std::vector<std::string> Messages;
+    for (const std::shared_ptr<ErrorInfoBase> &Info: Holder.Infos)
+      Messages.push_back(Info->message());
+
+    return Matcher.MatchAndExplain(Messages, listener);
+  }
+
+  void DescribeTo(std::ostream *OS) const override {
+    *OS << "failed with Error whose message ";
+    Matcher.DescribeTo(OS);
+  }
+
+  void DescribeNegationTo(std::ostream *OS) const override {
+    *OS << "failed with an Error whose message ";
+    Matcher.DescribeNegationTo(OS);
+  }
+
+private:
+  testing::Matcher<std::vector<std::string>> Matcher;
+};
 } // namespace detail
 
 #define EXPECT_THAT_ERROR(Err, Matcher)                                        \
   EXPECT_THAT(llvm::detail::TakeError(Err), Matcher)
 #define ASSERT_THAT_ERROR(Err, Matcher)                                        \
   ASSERT_THAT(llvm::detail::TakeError(Err), Matcher)
 
 #define EXPECT_THAT_EXPECTED(Err, Matcher)                                     \

 }
 
 template <typename InfoT, typename M>
 testing::Matcher<const detail::ErrorHolder &> Failed(M Matcher) {
   return MakeMatcher(new detail::ErrorMatchesMono<InfoT>(
       testing::SafeMatcherCast<InfoT &>(Matcher)));
 }
 
+template <typename... M>
+testing::Matcher<const detail::ErrorHolder &> FailedWithMessage(M... Matcher) {
+  static_assert(sizeof...(M) > 0, "");
+  return MakeMatcher(
+      new detail::ErrorMessageMatches(testing::ElementsAre(Matcher...)));
+}
+
+template <typename M>
+testing::Matcher<const detail::ErrorHolder &> FailedWithMessageArray(M Matcher) {
+  return MakeMatcher(new detail::ErrorMessageMatches(Matcher));
+}
+
 template <typename M>
 detail::ValueMatchesPoly<M> HasValue(M Matcher) {
   return detail::ValueMatchesPoly<M>(Matcher);
 }
 
 } // namespace llvm
 
 #endif

llvm/unittests/Support/ErrorTest.cpp

EXPECT_NONFATAL_FAILURE(
       "  Actual: succeeded with value 0, (isn't > 1)");
   EXPECT_NONFATAL_FAILURE(
       EXPECT_THAT_EXPECTED(Expected<int>(make_error<CustomError>(0)),
                            HasValue(testing::Gt(1))),
       "Expected: succeeded with value (is > 1)\n"
       "  Actual: failed  (CustomError {0})");
 }
 
+TEST(Error, FailedWithMessageMatcher) {
+  EXPECT_THAT_EXPECTED(Expected<int>(make_error<CustomError>(0)),
+                       FailedWithMessage("CustomError {0}"));
+
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT_EXPECTED(Expected<int>(make_error<CustomError>(1)),
+                           FailedWithMessage("CustomError {0}")),
+      "Expected: failed with Error whose message has 1 element that is equal "
+      "to \"CustomError {0}\"\n"
+      "  Actual: failed  (CustomError {1})");
+
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT_EXPECTED(Expected<int>(0),
+                           FailedWithMessage("CustomError {0}")),
+      "Expected: failed with Error whose message has 1 element that is equal "
+      "to \"CustomError {0}\"\n"
+      "  Actual: succeeded with value 0");
+
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT_EXPECTED(Expected<int>(make_error<CustomError>(0)),
+                           FailedWithMessage("CustomError {0}", "CustomError {0}")),
+      "Expected: failed with Error whose message has 2 elements where\n"
+      "element #0 is equal to \"CustomError {0}\",\n"
+      "element #1 is equal to \"CustomError {0}\"\n"
+      "  Actual: failed  (CustomError {0}), which has 1 element");
+
+  EXPECT_NONFATAL_FAILURE(
+      EXPECT_THAT_EXPECTED(
+          Expected<int>(joinErrors(make_error<CustomError>(0),
+                                   make_error<CustomError>(0))),
+          FailedWithMessage("CustomError {0}")),
+      "Expected: failed with Error whose message has 1 element that is equal "
+      "to \"CustomError {0}\"\n"
+      "  Actual: failed  (CustomError {0}; CustomError {0}), which has 2 elements");
+
+  EXPECT_THAT_ERROR(
+      joinErrors(make_error<CustomError>(0), make_error<CustomError>(0)),
+      FailedWithMessageArray(testing::SizeIs(2)));
+}
+
 TEST(Error, C_API) {
   EXPECT_THAT_ERROR(unwrap(wrap(Error::success())), Succeeded())
       << "Failed to round-trip Error success value via C API";
   EXPECT_THAT_ERROR(unwrap(wrap(make_error<CustomError>(0))),
                     Failed<CustomError>())
       << "Failed to round-trip Error failure value via C API";
 
   auto Err =