Exit on pr merge with -d and merge queue

When running `gh pr merge -d` on a repo with a merge queue policy, exit before attempting to merge.

pkg/cmd/pr/merge/merge.go

@@ -241,7 +241,15 @@ func (m *mergeContext) warnIfDiverged() {
 // Check if the current state of the pull request allows for merging
 func (m *mergeContext) canMerge() error {
 	if m.mergeQueueRequired {
-		// a pull request can always be added to the merge queue
+		// Requesting branch deletion on a PR with a merge queue
+		// policy is not allowed. Doing so can unexpectedly
+		// delete branches before merging, close the PR, and remove
+		// the PR from the merge queue.
+		if m.opts.DeleteBranch {
+			fmt.Fprintf(m.opts.IO.ErrOut, "%s Cannot use `-d` or `--delete-branch` when merge queue enabled\n", m.cs.FailureIcon())
+			return cmdutil.SilentError
+		}
+		// Otherwise, a pull request can always be added to the merge queue
 		return nil
 	}
 

pkg/cmd/pr/merge/merge_test.go

@@ -659,6 +659,34 @@ func TestPrMerge_deleteBranch(t *testing.T) {
 	`), output.Stderr())
 }
 
+func TestPrMerge_deleteBranch_mergeQueue(t *testing.T) {
+	http := initFakeHTTP()
+	defer http.Verify(t)
+
+	shared.RunCommandFinder(
+		"",
+		&api.PullRequest{
+			ID:                  "PR_10",
+			Number:              10,
+			State:               "OPEN",
+			Title:               "Blueberries are a good fruit",
+			HeadRefName:         "blueberries",
+			BaseRefName:         "main",
+			MergeStateStatus:    "CLEAN",
+			IsMergeQueueEnabled: true,
+		},
+		baseRepo("OWNER", "REPO", "main"),
+	)
+
+	output, err := runCommand(http, nil, "blueberries", true, `pr merge --merge --delete-branch`)
+	assert.ErrorIs(t, err, cmdutil.SilentError)
+
+	assert.Equal(t, "", output.String())
+	assert.Equal(t, heredoc.Docf(`
+		X Cannot use %[1]s-d%[1]s or %[1]s--delete-branch%[1]s when merge queue enabled
+	`, "`"), output.Stderr())
+}
+
 func TestPrMerge_deleteBranch_nonDefault(t *testing.T) {
 	http := initFakeHTTP()
 	defer http.Verify(t)