[llvm] [Support] Report EISDIR when opening a directory (PR #79880)

[James Henderson via llvm-commits]

================
@@ -1296,6 +1296,36 @@ TEST_F(FileSystemTest, UTF8ToUTF16DirectoryIteration) {
 }
 #endif
 
+#ifndef _WIN32
+TEST_F(FileSystemTest, OpenDirectoryAsFileForRead) {
+  ASSERT_NO_ERROR(fs::create_directory(Twine(TestDirectory)));
+  ASSERT_EQ(fs::create_directory(Twine(TestDirectory), false),
+            errc::file_exists);
+
+  std::string Buf(5, '?');
+  Expected<fs::file_t> FD = fs::openNativeFileForRead(TestDirectory);
+  ASSERT_NO_ERROR(errorToErrorCode(FD.takeError()));
----------------
jh7370 wrote:

This test fails on Windows here, because on Windows even attempting to opene the directory for read fails, so the `ASSERT_NO_ERROR` fires.

Side note: `ASSERT_NO_ERROR(errorToErrorCode(FD.takeError()));` can be simplified to `ASSERT_THAT_EXPECTED(FD, Succeeded());`. There are similar ones if you just want to check whether it failed with a particular message, for example (but in that case you wouldn't be able to check the actual value).

To fix this on Windows simply replace the `ASSERT_NO_ERROR` check with a check to make sure the returned error is in a failed state with the `is_directory` value (then skip the rest of the test).

https://github.com/llvm/llvm-project/pull/79880