switch command (#51)

* switch cmd to interactively switch to another branch in the stack

* add switch to docs

* addressing review comments

* bump skill file version

* default to current branch

Author: skarim

README.md

@@ -197,7 +197,7 @@ Pull from remote and do a cascading rebase across the stack.
 gh stack rebase [flags] [branch]
 ```
 
-Fetches the latest changes from `origin`, then ensures each branch in the stack has the tip of the previous layer in its commit history. Rebases branches in order from trunk upward. If a branch's PR has been squash-merged, the rebase automatically switches to `--onto` mode to correctly replay commits on top of the merge target.
+Fetches the latest changes from `origin`, then ensures each branch in the stack has the tip of the previous layer in its commit history. Rebases branches in order from trunk upward. If a branch's PR has been merged, the rebase automatically switches to `--onto` mode to correctly replay commits on top of the merge target.
 
 If a rebase conflict occurs, the operation pauses and prints the conflicted files with line numbers. Resolve the conflicts, stage with `git add`, and continue with `--continue`. To undo the entire rebase, use `--abort` to restore all branches to their pre-rebase state.
 

cmd/rebase.go

@@ -188,7 +188,7 @@ func runRebase(cfg *config.Config, opts *rebaseOptions) error {
 		return ErrSilent
 	}
 
-	// Track --onto rebase state for squash-merged branches.
+	// Track --onto rebase state for merged branches.
 	needsOnto := false
 	var ontoOldBase string
 
@@ -201,7 +201,7 @@ func runRebase(cfg *config.Config, opts *rebaseOptions) error {
 			base = s.Branches[absIdx-1].Branch
 		}
 
-		// Skip branches whose PRs have already been merged (e.g. via squash).
+		// Skip branches whose PRs have already been merged.
 		// Record state so subsequent branches can use --onto rebase.
 		if br.IsMerged() {
 			ontoOldBase = originalRefs[br.Branch]
@@ -252,7 +252,7 @@ func runRebase(cfg *config.Config, opts *rebaseOptions) error {
 				return ErrConflict
 			}
 
-			cfg.Successf("Rebased %s onto %s (squash-merge detected)", br.Branch, newBase)
+			cfg.Successf("Rebased %s onto %s (adjusted for merged PR)", br.Branch, newBase)
 			// Keep --onto mode; update old base for the next branch.
 			ontoOldBase = originalRefs[br.Branch]
 		} else {
@@ -450,7 +450,7 @@ func continueRebase(cfg *config.Config, gitDir string) error {
 				return ErrConflict
 			}
 
-			cfg.Successf("Rebased %s onto %s (squash-merge detected)", branchName, newBase)
+			cfg.Successf("Rebased %s onto %s (adjusted for merged PR)", branchName, newBase)
 			state.OntoOldBase = state.OriginalRefs[branchName]
 		} else {
 			var rebaseErr error

cmd/rebase_test.go

@@ -106,10 +106,10 @@ func TestRebase_CascadeRebase(t *testing.T) {
 	assert.Contains(t, output, "rebased locally")
 }
 
-// TestRebase_SquashMergedBranch_UsesOnto verifies that when b1 has a merged PR,
+// TestRebase_MergedBranch_UsesOnto verifies that when b1 has a merged PR,
 // it is skipped and b2 uses RebaseOnto with trunk as newBase and b1's original
 // SHA as oldBase. b3 also uses --onto (propagation).
-func TestRebase_SquashMergedBranch_UsesOnto(t *testing.T) {
+func TestRebase_MergedBranch_UsesOnto(t *testing.T) {
 	s := stack.Stack{
 		Trunk: stack.BranchRef{Branch: "main"},
 		Branches: []stack.BranchRef{
@@ -171,7 +171,7 @@ func TestRebase_SquashMergedBranch_UsesOnto(t *testing.T) {
 }
 
 // TestRebase_OntoPropagatesToSubsequentBranches verifies that when multiple
-// branches are squash-merged, --onto propagates correctly through the chain.
+// branches are merged, --onto propagates correctly through the chain.
 func TestRebase_OntoPropagatesToSubsequentBranches(t *testing.T) {
 	s := stack.Stack{
 		Trunk: stack.BranchRef{Branch: "main"},
@@ -651,7 +651,7 @@ func TestRebase_Continue_RebasesRemainingBranches(t *testing.T) {
 }
 
 // TestRebase_Continue_OntoMode verifies the --continue path when UseOnto is
-// set (squash-merged branches upstream). With no remaining branches, only
+// set (merged branches upstream). With no remaining branches, only
 // RebaseContinue runs and the state is cleaned up.
 func TestRebase_Continue_OntoMode(t *testing.T) {
 	s := stack.Stack{

cmd/sync.go

@@ -129,10 +129,16 @@ func runSync(cfg *config.Config, opts *syncOptions) error {
 		// Sync PR state to detect merged PRs before rebasing.
 		syncStackPRs(cfg, s)
 
-		// Save original refs so we can restore on conflict
-		branchNames := make([]string, len(s.Branches))
-		for i, b := range s.Branches {
-			branchNames[i] = b.Branch
+		// Save original refs so we can restore on conflict.
+		// Merged branches that no longer exist locally have no ref to
+		// resolve. They are always skipped during rebase but we must
+		// also exclude them here to avoid a rev-parse error.
+		branchNames := make([]string, 0, len(s.Branches))
+		for _, b := range s.Branches {
+			if b.IsMerged() && !git.BranchExists(b.Branch) {
+				continue
+			}
+			branchNames = append(branchNames, b.Branch)
 		}
 		originalRefs, _ := git.RevParseMap(branchNames)
 
@@ -191,7 +197,7 @@ func runSync(cfg *config.Config, opts *syncOptions) error {
 					break
 				}
 
-				cfg.Successf("Rebased %s onto %s (squash-merge detected)", br.Branch, newBase)
+				cfg.Successf("Rebased %s onto %s (adjusted for merged PR)", br.Branch, newBase)
 				ontoOldBase = originalRefs[br.Branch]
 			} else {
 				var rebaseErr error

cmd/sync_test.go

@@ -522,10 +522,10 @@ func TestSync_PushForceFlagDependsOnRebase(t *testing.T) {
 	}
 }
 
-// TestSync_SquashMergedBranch_UsesOnto verifies that when a squash-merged
+// TestSync_MergedBranch_UsesOnto verifies that when a merged
 // branch exists in the stack, sync's cascade rebase correctly uses --onto
 // to skip the merged branch and rebase subsequent branches onto the right base.
-func TestSync_SquashMergedBranch_UsesOnto(t *testing.T) {
+func TestSync_MergedBranch_UsesOnto(t *testing.T) {
 	s := stack.Stack{
 		Trunk: stack.BranchRef{Branch: "main"},
 		Branches: []stack.BranchRef{
@@ -549,6 +549,7 @@ func TestSync_SquashMergedBranch_UsesOnto(t *testing.T) {
 	}
 
 	mock := newSyncMock(tmpDir, "b2")
+	mock.BranchExistsFn = func(name string) bool { return true }
 	// Trunk behind remote to trigger rebase
 	mock.RevParseFn = func(ref string) (string, error) {
 		if ref == "main" {
@@ -765,8 +766,8 @@ func TestSync_BranchFastForward_WithTrunkUpdate(t *testing.T) {
 	writeStackFile(t, tmpDir, s)
 
 	var updateBranchRefCalls []struct{ branch, sha string }
-	var rebaseCalls []rebaseCall
-	var pushCalls []pushCall
+	var rebaseCalls2 []rebaseCall
+	var pushCalls2 []pushCall
 
 	mock := newSyncMock(tmpDir, "b1")
 	// Trunk and b2 both behind remote
@@ -803,15 +804,15 @@ func TestSync_BranchFastForward_WithTrunkUpdate(t *testing.T) {
 	}
 	mock.CheckoutBranchFn = func(string) error { return nil }
 	mock.RebaseFn = func(base string) error {
-		rebaseCalls = append(rebaseCalls, rebaseCall{branch: "(rebase)" + base})
+		rebaseCalls2 = append(rebaseCalls2, rebaseCall{branch: "(rebase)" + base})
 		return nil
 	}
 	mock.RebaseOntoFn = func(newBase, oldBase, branch string) error {
-		rebaseCalls = append(rebaseCalls, rebaseCall{newBase, oldBase, branch})
+		rebaseCalls2 = append(rebaseCalls2, rebaseCall{newBase, oldBase, branch})
 		return nil
 	}
 	mock.PushFn = func(remote string, branches []string, force, atomic bool) error {
-		pushCalls = append(pushCalls, pushCall{remote, branches, force, atomic})
+		pushCalls2 = append(pushCalls2, pushCall{remote, branches, force, atomic})
 		return nil
 	}
 
@@ -829,7 +830,6 @@ func TestSync_BranchFastForward_WithTrunkUpdate(t *testing.T) {
 	output := string(errOut)
 
 	assert.NoError(t, err)
-
 	// Both trunk and b2 should be updated
 	branchUpdates := make(map[string]string)
 	for _, c := range updateBranchRefCalls {
@@ -839,7 +839,83 @@ func TestSync_BranchFastForward_WithTrunkUpdate(t *testing.T) {
 	assert.Equal(t, "b2-remote", branchUpdates["b2"], "b2 should be fast-forwarded")
 
 	assert.Contains(t, output, "fast-forwarded")
-	assert.NotEmpty(t, rebaseCalls, "rebase should occur")
-	require.Len(t, pushCalls, 1)
-	assert.True(t, pushCalls[0].force, "push should use force after rebase")
+	assert.NotEmpty(t, rebaseCalls2, "rebase should occur")
+	require.Len(t, pushCalls2, 1)
+	assert.True(t, pushCalls2[0].force, "push should use force after rebase")
+}
+
+func TestSync_MergedBranchDeletedFromRemote(t *testing.T) {
+	s := stack.Stack{
+		Trunk: stack.BranchRef{Branch: "main"},
+		Branches: []stack.BranchRef{
+			{Branch: "b1", PullRequest: &stack.PullRequestRef{Number: 1, Merged: true}},
+			{Branch: "b2"},
+		},
+	}
+
+	tmpDir := t.TempDir()
+	writeStackFile(t, tmpDir, s)
+
+	var rebaseOntoCalls []rebaseCall
+
+	mock := newSyncMock(tmpDir, "b2")
+	mock.BranchExistsFn = func(name string) bool {
+		// b1 does not exist locally (deleted from remote after merge)
+		return name != "b1"
+	}
+	mock.RevParseMultiFn = func(refs []string) ([]string, error) {
+		shas := make([]string, len(refs))
+		for i, r := range refs {
+			if r == "b1" {
+				t.Fatalf("RevParseMulti should not be called with non-existent branch b1")
+			}
+			if r == "main" {
+				shas[i] = "local-sha"
+			} else if r == "origin/main" {
+				shas[i] = "remote-sha"
+			} else {
+				shas[i] = "sha-" + r
+			}
+		}
+		return shas, nil
+	}
+	// Trunk behind remote to trigger rebase
+	mock.RevParseFn = func(ref string) (string, error) {
+		if ref == "main" {
+			return "local-sha", nil
+		}
+		if ref == "origin/main" {
+			return "remote-sha", nil
+		}
+		return "sha-" + ref, nil
+	}
+	mock.IsAncestorFn = func(a, d string) (bool, error) {
+		return a == "local-sha" && d == "remote-sha", nil
+	}
+	mock.UpdateBranchRefFn = func(string, string) error { return nil }
+	mock.CheckoutBranchFn = func(string) error { return nil }
+	mock.RebaseOntoFn = func(newBase, oldBase, branch string) error {
+		rebaseOntoCalls = append(rebaseOntoCalls, rebaseCall{newBase, oldBase, branch})
+		return nil
+	}
+
+	restore := git.SetOps(mock)
+	defer restore()
+
+	cfg, _, errR := config.NewTestConfig()
+	cmd := SyncCmd(cfg)
+	cmd.SetOut(io.Discard)
+	cmd.SetErr(io.Discard)
+	err := cmd.Execute()
+
+	cfg.Err.Close()
+	errOut, _ := io.ReadAll(errR)
+	output := string(errOut)
+
+	assert.NoError(t, err)
+	assert.Contains(t, output, "Skipping b1")
+
+	// Only b2 should be rebased
+	require.Len(t, rebaseOntoCalls, 1)
+	assert.Equal(t, "b2", rebaseOntoCalls[0].branch)
 }

docs/src/content/docs/reference/cli.md

@@ -223,7 +223,7 @@ gh stack rebase [flags] [branch]
 
 Fetches the latest changes from `origin`, then ensures each branch in the stack has the tip of the previous layer in its commit history. Rebases branches in order from trunk upward.
 
-If a branch's PR has been squash-merged, the rebase automatically switches to `--onto` mode to correctly replay commits on top of the merge target.
+If a branch's PR has been merged, the rebase automatically switches to `--onto` mode to correctly replay commits on top of the merge target.
 
 If a rebase conflict occurs, the operation pauses and prints the conflicted files with line numbers. Resolve the conflicts, stage with `git add`, and continue with `--continue`. To undo the entire rebase, use `--abort` to restore all branches to their pre-rebase state.
 

internal/git/git.go

@@ -179,7 +179,7 @@ func SaveRerereDeclined() error {
 //	git rebase --onto <newBase> <oldBase> <branch>
 //
 // This replays commits after oldBase from branch onto newBase. It is used
-// when a prior branch was squash-merged and the normal rebase cannot detect
+// when a prior branch was merged and the normal rebase cannot detect
 // which commits have already been applied.
 // If rerere resolves all conflicts automatically, the rebase continues
 // without user intervention.

skills/gh-stack/SKILL.md

@@ -570,7 +570,7 @@ gh stack sync [flags]
 
 1. **Fetch** latest changes from the remote
 2. **Fast-forward trunk** to match remote (skips if already up to date, warns if diverged)
-3. **Cascade rebase** all stack branches onto their updated parents (only if trunk moved). Handles squash-merged PRs automatically. If a conflict is detected, **all branches are restored** to their pre-rebase state and the command exits with code 3 — see [Handle rebase conflicts](#handle-rebase-conflicts-agent-workflow) for the resolution workflow
+3. **Cascade rebase** all stack branches onto their updated parents (only if trunk moved). Handles merged PRs automatically. If a conflict is detected, **all branches are restored** to their pre-rebase state and the command exits with code 3 — see [Handle rebase conflicts](#handle-rebase-conflicts-agent-workflow) for the resolution workflow
 4. **Push** all active branches atomically
 5. **Sync PR state** from GitHub and report the status of each PR
 
@@ -625,7 +625,7 @@ gh stack rebase --abort
 
 **Conflict handling:** See [Handle rebase conflicts](#handle-rebase-conflicts-agent-workflow) in the Workflows section for the full resolution workflow.
 
-**Squash-merge detection:** If a branch's PR was squash-merged on GitHub, the rebase automatically handles this and correctly replays commits on top of the merge target.
+**Merged PR detection:** If a branch's PR was merged on GitHub, the rebase automatically handles this using `--onto` mode and correctly replays commits on top of the merge target.
 
 **Rerere (conflict memory):** `git rerere` is enabled by `init` so previously resolved conflicts are auto-resolved in future rebases.