fix: error when head and base refs are identical in pr create

When a user runs `gh pr create` with head and base refs pointing to
the same ref in the same repository, the command now exits with an
error instead of proceeding with an invalid PR creation.

This prevents accidental pushes to the main branch and provides clear
feedback when users attempt to create a PR from the base branch.

Cross-repository PRs (e.g., from forks) with the same branch name
continue to work as expected.

Fixes #11903

Signed-off-by: majiayu000 <1835304752@qq.com>

pkg/cmd/pr/create/create.go

@@ -166,6 +166,19 @@ func (r forkableRefs) UnqualifiedHeadRef() string {
 	return r.qualifiedHeadRef.BranchName()
 }
 
+// isSameRef checks if the head and base refs point to the same ref in the same repository.
+// For cross-repository PRs (e.g., from a fork), the qualified head ref will contain
+// an owner prefix (owner:branch), so even if branch names match, they refer to different repos.
+func isSameRef(refs creationRefs) bool {
+	// If the qualified head ref contains ":", it's a cross-repo PR (e.g., from a fork)
+	// and we should allow it even if branch names are the same
+	if strings.Contains(refs.QualifiedHeadRef(), ":") {
+		return false
+	}
+	// Same repository: check if branch names are identical
+	return refs.UnqualifiedHeadRef() == refs.BaseRef()
+}
+
 // CreateContext stores contextual data about the creation process and is for building up enough
 // data to create a pull request.
 type CreateContext struct {
@@ -367,6 +380,11 @@ func createRun(opts *CreateOptions) error {
 		return err
 	}
 
+	// Check if head and base refs point to the same ref in the same repository
+	if isSameRef(ctx.PRRefs) {
+		return fmt.Errorf("head branch %q is the same as base branch %q, cannot create a pull request", ctx.PRRefs.UnqualifiedHeadRef(), ctx.PRRefs.BaseRef())
+	}
+
 	httpClient, err := opts.HttpClient()
 	if err != nil {
 		return err

pkg/cmd/pr/create/create_test.go

@@ -384,6 +384,31 @@ func Test_createRun(t *testing.T) {
 			},
 			expectedOut: "https://github.com/OWNER/REPO/pull/12\n",
 		},
+		{
+			name: "same head and base branch should error",
+			setup: func(opts *CreateOptions, t *testing.T) func() {
+				opts.TitleProvided = true
+				opts.BodyProvided = true
+				opts.Title = "my title"
+				opts.Body = "my body"
+				opts.HeadBranch = "master"
+				return func() {}
+			},
+			wantErr: `head branch "master" is the same as base branch "master", cannot create a pull request`,
+		},
+		{
+			name: "same head and base branch with explicit base should error",
+			setup: func(opts *CreateOptions, t *testing.T) func() {
+				opts.TitleProvided = true
+				opts.BodyProvided = true
+				opts.Title = "my title"
+				opts.Body = "my body"
+				opts.HeadBranch = "feature"
+				opts.BaseBranch = "feature"
+				return func() {}
+			},
+			wantErr: `head branch "feature" is the same as base branch "feature", cannot create a pull request`,
+		},
 		{
 			name: "dry-run-nontty-with-default-base",
 			tty:  false,
@@ -2879,3 +2904,73 @@ func TestProjectsV1Deprecation(t *testing.T) {
 		})
 	})
 }
+
+func Test_isSameRef(t *testing.T) {
+	tests := []struct {
+		name     string
+		refs     creationRefs
+		expected bool
+	}{
+		{
+			name: "same branch in same repo",
+			refs: skipPushRefs{
+				qualifiedHeadRef: shared.NewQualifiedHeadRefWithoutOwner("main"),
+				baseRefs: baseRefs{
+					baseBranchName: "main",
+				},
+			},
+			expected: true,
+		},
+		{
+			name: "different branches in same repo",
+			refs: skipPushRefs{
+				qualifiedHeadRef: shared.NewQualifiedHeadRefWithoutOwner("feature"),
+				baseRefs: baseRefs{
+					baseBranchName: "main",
+				},
+			},
+			expected: false,
+		},
+		{
+			name: "same branch name in different repos (cross-repo PR)",
+			refs: skipPushRefs{
+				qualifiedHeadRef: shared.NewQualifiedHeadRef("other-owner", "main"),
+				baseRefs: baseRefs{
+					baseBranchName: "main",
+				},
+			},
+			expected: false,
+		},
+		{
+			name: "pushableRefs same branch same repo",
+			refs: pushableRefs{
+				headRepo:       ghrepo.New("OWNER", "REPO"),
+				headBranchName: "main",
+				baseRefs: baseRefs{
+					baseRepo:       api.InitRepoHostname(&api.Repository{Name: "REPO", Owner: api.RepositoryOwner{Login: "OWNER"}}, "github.com"),
+					baseBranchName: "main",
+				},
+			},
+			expected: true,
+		},
+		{
+			name: "pushableRefs same branch different repos (fork)",
+			refs: pushableRefs{
+				headRepo:       ghrepo.New("FORK-OWNER", "REPO"),
+				headBranchName: "main",
+				baseRefs: baseRefs{
+					baseRepo:       api.InitRepoHostname(&api.Repository{Name: "REPO", Owner: api.RepositoryOwner{Login: "OWNER"}}, "github.com"),
+					baseBranchName: "main",
+				},
+			},
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isSameRef(tt.refs)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}