Support: Pass wrapped Error's error code through FileError

Change FileError to pass through the error code from the Error it wraps.
This allows APIs that return ECError to transition to FileError without
changing returned std::error_code.

This was extracted from https://reviews.llvm.org/D109345.

Differential Revision: https://reviews.llvm.org/D113225

Author: dexonsmith

llvm/lib/Support/Error.cpp

@@ -80,8 +80,11 @@ std::error_code inconvertibleErrorCode() {
 }
 
 std::error_code FileError::convertToErrorCode() const {
-  return std::error_code(static_cast<int>(ErrorErrorCode::FileError),
-                         *ErrorErrorCat);
+  std::error_code NestedEC = Err->convertToErrorCode();
+  if (NestedEC == inconvertibleErrorCode())
+    return std::error_code(static_cast<int>(ErrorErrorCode::FileError),
+                           *ErrorErrorCat);
+  return NestedEC;
 }
 
 Error errorCodeToError(std::error_code EC) {

llvm/unittests/Support/ErrorTest.cpp

@@ -963,6 +963,33 @@ TEST(Error, FileErrorTest) {
   });
 }
 
+TEST(Error, FileErrorErrorCode) {
+  for (std::error_code EC : {
+           make_error_code(std::errc::not_supported),
+           make_error_code(std::errc::invalid_argument),
+           make_error_code(std::errc::no_such_file_or_directory),
+       }) {
+    EXPECT_EQ(EC, errorToErrorCode(
+                      createFileError("file.bin", EC)));
+    EXPECT_EQ(EC, errorToErrorCode(
+                      createFileError("file.bin", /*Line=*/5, EC)));
+    EXPECT_EQ(EC, errorToErrorCode(
+                      createFileError("file.bin", errorCodeToError(EC))));
+    EXPECT_EQ(EC, errorToErrorCode(
+                      createFileError("file.bin", /*Line=*/5, errorCodeToError(EC))));
+  }
+
+  // inconvertibleErrorCode() should be wrapped to avoid a fatal error.
+  EXPECT_EQ(
+      "A file error occurred.",
+      errorToErrorCode(createFileError("file.bin", inconvertibleErrorCode()))
+          .message());
+  EXPECT_EQ(
+      "A file error occurred.",
+      errorToErrorCode(createFileError("file.bin", /*Line=*/5, inconvertibleErrorCode()))
+          .message());
+}
+
 enum class test_error_code {
   unspecified = 1,
   error_1,