Checkout branch looks to remote tracking branches as fallback

LibGit2Sharp.Tests/CheckoutFixture.cs

@@ -1028,6 +1028,52 @@ public void CanCheckoutPathFromCurrentBranch(string fileName)
             }
         }
 
+        [Theory]
+        [InlineData("br2", "origin")]
+        [InlineData("unique/branch", "another/remote")]
+        public void CheckoutBranchTriesRemoteTrackingBranchAsFallbackAndSucceedsIfOnlyOne(string branchName, string expectedRemoteName)
+        {
+            string path = SandboxStandardTestRepo();
+            using (var repo = new Repository(path))
+            {
+                ResetAndCleanWorkingDirectory(repo);
+
+                // Define another remote
+                var otherRemote = "another/remote";
+                repo.Network.Remotes.Add(otherRemote, "https://github.com/libgit2/TestGitRepository");
+
+                // Define an extra remote tracking branch that does not conflict
+                repo.Refs.Add($"refs/remotes/{otherRemote}/unique/branch", repo.Head.Tip.Sha);
+
+                Branch branch = Commands.Checkout(repo, branchName);
+
+                Assert.NotNull(branch);
+                Assert.True(branch.IsTracking);
+                Assert.Equal($"refs/remotes/{expectedRemoteName}/{branchName}", branch.TrackedBranch.CanonicalName);
+            }
+        }
+
+        [Fact]
+        public void CheckoutBranchTriesRemoteTrackingBranchAsFallbackAndThrowsIfMoreThanOne()
+        {
+            string path = SandboxStandardTestRepo();
+            using (var repo = new Repository(path))
+            {
+                ResetAndCleanWorkingDirectory(repo);
+
+                // Define another remote
+                var otherRemote = "another/remote";
+                repo.Network.Remotes.Add(otherRemote, "https://github.com/libgit2/TestGitRepository");
+
+                // Define remote tracking branches that conflict
+                var branchName = "conflicting/branch";
+                repo.Refs.Add($"refs/remotes/origin/{branchName}", repo.Head.Tip.Sha);
+                repo.Refs.Add($"refs/remotes/{otherRemote}/{branchName}", repo.Head.Tip.Sha);
+
+                Assert.Throws<AmbiguousSpecificationException>(() => Commands.Checkout(repo, branchName));
+            }
+        }
+
         /// <summary>
         /// Helper method to populate a simple repository with
         /// a single file and two branches.

LibGit2Sharp/Commands/Checkout.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Linq;
 using LibGit2Sharp.Core;
 
 namespace LibGit2Sharp
@@ -37,18 +38,49 @@ public static Branch Checkout(IRepository repository, string committishOrBranchS
             Ensure.ArgumentNotNullOrEmptyString(committishOrBranchSpec, "committishOrBranchSpec");
             Ensure.ArgumentNotNull(options, "options");
 
-            Reference reference;
-            GitObject obj;
+            Reference reference = null;
+            GitObject obj = null;
+            Branch branch = null;
+
+            try
+            {
+                repository.RevParse(committishOrBranchSpec, out reference, out obj);
+            }
+            catch (NotFoundException)
+            {
+                // If committishOrBranchSpec is not a local branch but matches a tracking branch
+                // in exactly one remote, use it. This is the "git checkout" command's default behavior.
+                // https://git-scm.com/docs/git-checkout#Documentation/git-checkout.txt-emgitcheckoutemltbranchgt
+                var remoteBranches = repository.Network.Remotes
+                    .SelectMany(r => repository.Branches.Where(b =>
+                        b.IsRemote &&
+                        b.CanonicalName == $"refs/remotes/{r.Name}/{committishOrBranchSpec}"))
+                    .ToList();
+
+                if (remoteBranches.Count == 1)
+                {
+                    branch = repository.CreateBranch(committishOrBranchSpec, remoteBranches[0].Tip);
+                    repository.Branches.Update(branch, b => b.TrackedBranch = remoteBranches[0].CanonicalName);
+
+                    return Checkout(repository, branch, options);
+                }
+
+                if (remoteBranches.Count > 1)
+                {
+                    throw new AmbiguousSpecificationException($"'{committishOrBranchSpec}' matched multiple ({remoteBranches.Count}) remote tracking branches");
+                }
+
+                throw;
+            }
 
-            repository.RevParse(committishOrBranchSpec, out reference, out obj);
             if (reference != null && reference.IsLocalBranch)
             {
-                Branch branch = repository.Branches[reference.CanonicalName];
+                branch = repository.Branches[reference.CanonicalName];
                 return Checkout(repository, branch, options);
             }
 
             Commit commit = obj.Peel<Commit>(true);
-            Checkout(repository, commit.Tree,  options, committishOrBranchSpec);
+            Checkout(repository, commit.Tree, options, committishOrBranchSpec);
 
             return repository.Head;
         }

LibGit2Sharp/Commands/Remove.cs

@@ -1,7 +1,6 @@
-using System.Linq;
+using System.Collections.Generic;
 using System.IO;
-using System.Collections.Generic;
-using LibGit2Sharp;
+using System.Linq;
 using LibGit2Sharp.Core;
 
 namespace LibGit2Sharp