[Error] Make llvm::cantFail include the original error messages

Summary:
The current implementation eats the current errors and just outputs
the message parameter passed to llvm::cantFail.  This change appends
the original error message(s), so the user can see exactly why
cantFail failed.  New logic is conditional on NDEBUG.

Reviewed By: lhames

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69057

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375176 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Error.h

@@ -704,6 +704,12 @@ inline void cantFail(Error Err, const char *Msg = nullptr) {
   if (Err) {
     if (!Msg)
       Msg = "Failure value returned from cantFail wrapped call";
+#ifndef NDEBUG
+    std::string Str;
+    raw_string_ostream OS(Str);
+    OS << Msg << "\n" << Err;
+    Msg = OS.str().c_str();
+#endif
     llvm_unreachable(Msg);
   }
 }
@@ -728,6 +734,13 @@ T cantFail(Expected<T> ValOrErr, const char *Msg = nullptr) {
   else {
     if (!Msg)
       Msg = "Failure value returned from cantFail wrapped call";
+#ifndef NDEBUG
+    std::string Str;
+    raw_string_ostream OS(Str);
+    auto E = ValOrErr.takeError();
+    OS << Msg << "\n" << E;
+    Msg = OS.str().c_str();
+#endif
     llvm_unreachable(Msg);
   }
 }
@@ -752,6 +765,13 @@ T& cantFail(Expected<T&> ValOrErr, const char *Msg = nullptr) {
   else {
     if (!Msg)
       Msg = "Failure value returned from cantFail wrapped call";
+#ifndef NDEBUG
+    std::string Str;
+    raw_string_ostream OS(Str);
+    auto E = ValOrErr.takeError();
+    OS << Msg << "\n" << E;
+    Msg = OS.str().c_str();
+#endif
     llvm_unreachable(Msg);
   }
 }

unittests/Support/ErrorTest.cpp

@@ -390,7 +390,8 @@ TEST(Error, FailureToHandle) {
   };
 
   EXPECT_DEATH(FailToHandle(),
-               "Failure value returned from cantFail wrapped call")
+               "Failure value returned from cantFail wrapped call\n"
+               "CustomError \\{7\\}")
       << "Unhandled Error in handleAllErrors call did not cause an "
          "abort()";
 }
@@ -409,7 +410,8 @@ TEST(Error, FailureFromHandler) {
   };
 
   EXPECT_DEATH(ReturnErrorFromHandler(),
-               "Failure value returned from cantFail wrapped call")
+               "Failure value returned from cantFail wrapped call\n"
+               "CustomError \\{7\\}")
       << " Error returned from handler in handleAllErrors call did not "
          "cause abort()";
 }
@@ -510,11 +512,12 @@ TEST(Error, CantFailSuccess) {
 // Test that cantFail results in a crash if you pass it a failure value.
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS && !defined(NDEBUG)
 TEST(Error, CantFailDeath) {
-  EXPECT_DEATH(
-      cantFail(make_error<StringError>("foo", inconvertibleErrorCode()),
-               "Cantfail call failed"),
-      "Cantfail call failed")
-    << "cantFail(Error) did not cause an abort for failure value";
+  EXPECT_DEATH(cantFail(make_error<StringError>("Original error message",
+                                                inconvertibleErrorCode()),
+                        "Cantfail call failed"),
+               "Cantfail call failed\n"
+               "Original error message")
+      << "cantFail(Error) did not cause an abort for failure value";
 
   EXPECT_DEATH(
       {