pr merge: print instructions for merge conflicts (#5330)

Co-authored-by: Sam Coe <samcoe@users.noreply.github.com>
Co-authored-by: Mislav Marohnić <mislav@github.com>

pkg/cmd/pr/merge/merge.go

@@ -11,6 +11,7 @@ import (
 	"github.com/cli/cli/v2/context"
 	"github.com/cli/cli/v2/git"
 	"github.com/cli/cli/v2/internal/config"
+	"github.com/cli/cli/v2/internal/ghrepo"
 	"github.com/cli/cli/v2/pkg/cmd/pr/shared"
 	"github.com/cli/cli/v2/pkg/cmdutil"
 	"github.com/cli/cli/v2/pkg/iostreams"
@@ -72,7 +73,7 @@ func NewCmdMerge(f *cmdutil.Factory, runF func(*MergeOptions) error) *cobra.Comm
 			Merge a pull request on GitHub.
 
 			Without an argument, the pull request that belongs to the current branch
-			is selected.			
+			is selected.
     	`),
 		Args: cobra.MaximumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
@@ -130,6 +131,7 @@ func NewCmdMerge(f *cmdutil.Factory, runF func(*MergeOptions) error) *cobra.Comm
 			); err != nil {
 				return err
 			}
+
 			if bodyProvided || bodyFileProvided {
 				opts.BodySet = true
 				if bodyFileProvided {
@@ -139,7 +141,6 @@ func NewCmdMerge(f *cmdutil.Factory, runF func(*MergeOptions) error) *cobra.Comm
 					}
 					opts.Body = string(b)
 				}
-
 			}
 
 			opts.Editor = &userEditor{
@@ -211,6 +212,16 @@ func mergeRun(opts *MergeOptions) error {
 	if reason := blockedReason(pr.MergeStateStatus, opts.UseAdmin); !opts.AutoMergeEnable && !isPRAlreadyMerged && reason != "" {
 		fmt.Fprintf(opts.IO.ErrOut, "%s Pull request #%d is not mergeable: %s.\n", cs.FailureIcon(), pr.Number, reason)
 		fmt.Fprintf(opts.IO.ErrOut, "To have the pull request merged after all the requirements have been met, add the `--auto` flag.\n")
+		if remote := remoteForMergeConflictResolution(baseRepo, pr, opts); remote != nil {
+			mergeOrRebase := "merge"
+			if opts.MergeMethod == PullRequestMergeMethodRebase {
+				mergeOrRebase = "rebase"
+			}
+			fetchBranch := fmt.Sprintf("%s %s", remote.Name, pr.BaseRefName)
+			mergeBranch := fmt.Sprintf("%s %s/%s", mergeOrRebase, remote.Name, pr.BaseRefName)
+			cmd := fmt.Sprintf("gh pr checkout %d && git fetch %s && git %s", pr.Number, fetchBranch, mergeBranch)
+			fmt.Fprintf(opts.IO.ErrOut, "Run the following to resolve the merge conflicts locally:\n  %s\n", cs.Bold(cmd))
+		}
 		if !opts.UseAdmin && allowsAdminOverride(pr.MergeStateStatus) {
 			// TODO: show this flag only to repo admins
 			fmt.Fprintf(opts.IO.ErrOut, "To use administrator privileges to immediately merge the pull request, add the `--admin` flag.\n")
@@ -562,6 +573,25 @@ func allowsAdminOverride(status string) bool {
 	}
 }
 
+func remoteForMergeConflictResolution(baseRepo ghrepo.Interface, pr *api.PullRequest, opts *MergeOptions) *context.Remote {
+	if !mergeConflictStatus(pr.MergeStateStatus) || !opts.CanDeleteLocalBranch {
+		return nil
+	}
+	remotes, err := opts.Remotes()
+	if err != nil {
+		return nil
+	}
+	remote, err := remotes.FindByRepo(baseRepo.RepoOwner(), baseRepo.RepoName())
+	if err != nil {
+		return nil
+	}
+	return remote
+}
+
+func mergeConflictStatus(status string) bool {
+	return status == "DIRTY"
+}
+
 func isImmediatelyMergeable(status string) bool {
 	switch status {
 	case "CLEAN", "HAS_HOOKS", "UNSTABLE":

pkg/cmd/pr/merge/merge_test.go

@@ -333,6 +333,39 @@ func TestPrMerge_blocked(t *testing.T) {
 		`, "`"), output.Stderr())
 }
 
+func TestPrMerge_dirty(t *testing.T) {
+	http := initFakeHTTP()
+	defer http.Verify(t)
+
+	shared.RunCommandFinder(
+		"1",
+		&api.PullRequest{
+			ID:               "THE-ID",
+			Number:           123,
+			State:            "OPEN",
+			Title:            "The title of the PR",
+			MergeStateStatus: "DIRTY",
+			BaseRefName:      "trunk",
+			HeadRefName:      "feature",
+		},
+		baseRepo("OWNER", "REPO", "master"),
+	)
+
+	_, cmdTeardown := run.Stub()
+	defer cmdTeardown(t)
+
+	output, err := runCommand(http, "master", true, "pr merge 1 --merge")
+	assert.EqualError(t, err, "SilentError")
+
+	assert.Equal(t, "", output.String())
+	assert.Equal(t, heredoc.Docf(`
+		X Pull request #123 is not mergeable: the merge commit cannot be cleanly created.
+		To have the pull request merged after all the requirements have been met, add the %[1]s--auto%[1]s flag.
+		Run the following to resolve the merge conflicts locally:
+		  gh pr checkout 123 && git fetch origin trunk && git merge origin/trunk
+	`, "`"), output.Stderr())
+}
+
 func TestPrMerge_nontty(t *testing.T) {
 	http := initFakeHTTP()
 	defer http.Verify(t)