FastForward merge should block if there are checkout conflicts.

LibGit2Sharp.Tests/MergeFixture.cs

@@ -572,6 +572,32 @@ public void CanMergeCommittish(string committish, FastForwardStrategy strategy,
             }
         }
 
+        [Theory]
+        [InlineData(true, FastForwardStrategy.FastForwardOnly)]
+        [InlineData(false, FastForwardStrategy.FastForwardOnly)]
+        [InlineData(true, FastForwardStrategy.NoFastFoward)]
+        [InlineData(false, FastForwardStrategy.NoFastFoward)]
+        public void MergeWithWorkDirConflictsThrows(bool shouldStage, FastForwardStrategy strategy)
+        {
+            // Merging the fast_forward branch results in a change to file
+            // b.txt. In this test we modify the file in the working directory
+            // and then attempt to perform a merge. We expect the merge to fail
+            // due to merge conflicts.
+            string committishToMerge = "fast_forward";
+
+            using (var repo = new Repository(CloneMergeTestRepo()))
+            {
+                Touch(repo.Info.WorkingDirectory, "b.txt", "this is an alternate change");
+
+                if (shouldStage)
+                {
+                    repo.Index.Stage("b.txt");
+                }
+
+                Assert.Throws<MergeConflictException>(() => repo.Merge(committishToMerge, Constants.Signature, new MergeOptions() { FastForwardStrategy = strategy }));
+            }
+        }
+
         [Theory]
         [InlineData(CheckoutFileConflictStrategy.Ours)]
         [InlineData(CheckoutFileConflictStrategy.Theirs)]

LibGit2Sharp/CherryPickOptions.cs

@@ -104,7 +104,6 @@ CheckoutStrategy IConvertableToGitCheckoutOpts.CheckoutStrategy
             get
             {
                 return CheckoutStrategy.GIT_CHECKOUT_SAFE |
-                       CheckoutStrategy.GIT_CHECKOUT_ALLOW_CONFLICTS |
                        GitCheckoutOptsWrapper.CheckoutStrategyFromFileConflictStrategy(FileConflictStrategy);
             }
         }

LibGit2Sharp/Core/GitCheckoutOpts.cs

@@ -160,4 +160,50 @@ internal interface IConvertableToGitCheckoutOpts
 
         CheckoutNotifyFlags CheckoutNotifyFlags { get; }
     }
+
+    /// <summary>
+    /// This wraps an IConvertableToGitCheckoutOpts object and can tweak the
+    /// properties so that they are appropriate for a checkout performed as
+    /// part of a FastForward merge. Most properties are passthrough to the
+    /// wrapped object.
+    /// </summary>
+    internal class FastForwardCheckoutOptionsAdapter : IConvertableToGitCheckoutOpts
+    {
+        private IConvertableToGitCheckoutOpts internalOptions;
+
+        internal FastForwardCheckoutOptionsAdapter(IConvertableToGitCheckoutOpts internalOptions)
+        {
+            this.internalOptions = internalOptions;
+        }
+
+        /// <summary>
+        /// Passthrough to the wrapped object.
+        /// </summary>
+        /// <returns></returns>
+        public CheckoutCallbacks GenerateCallbacks()
+        {
+            return internalOptions.GenerateCallbacks();
+        }
+
+        /// <summary>
+        /// There should be no resolvable conflicts in a FastForward merge.
+        /// Just perform checkout with the safe checkout strategy.
+        /// </summary>
+        public CheckoutStrategy CheckoutStrategy
+        {
+            get
+            {
+                return CheckoutStrategy.GIT_CHECKOUT_SAFE;
+            }
+        }
+
+        /// <summary>
+        /// Passthrough to the wrapped object.
+        /// </summary>
+        /// <returns></returns>
+        public CheckoutNotifyFlags CheckoutNotifyFlags
+        {
+            get { return internalOptions.CheckoutNotifyFlags; }
+        }
+    }
 }

LibGit2Sharp/MergeOptions.cs

@@ -95,8 +95,7 @@ CheckoutStrategy IConvertableToGitCheckoutOpts.CheckoutStrategy
         {
             get
             {
-                return CheckoutStrategy.GIT_CHECKOUT_SAFE|
-                       CheckoutStrategy.GIT_CHECKOUT_ALLOW_CONFLICTS |
+                return CheckoutStrategy.GIT_CHECKOUT_SAFE |
                        GitCheckoutOptsWrapper.CheckoutStrategyFromFileConflictStrategy(FileConflictStrategy);
             }
         }

LibGit2Sharp/Repository.cs

@@ -1351,7 +1351,7 @@ private MergeResult FastForwardMerge(GitMergeHeadHandle mergeHead, Signature mer
             Commit fastForwardCommit = (Commit) Lookup(id, ObjectType.Commit);
             Ensure.GitObjectIsNotNull(fastForwardCommit, id.Sha);
 
-            CheckoutTree(fastForwardCommit.Tree, null, options);
+            CheckoutTree(fastForwardCommit.Tree, null, new FastForwardCheckoutOptionsAdapter(options));
 
             var reference = Refs.Head.ResolveToDirectReference();
 

LibGit2Sharp/RevertOptions.cs

@@ -104,7 +104,6 @@ CheckoutStrategy IConvertableToGitCheckoutOpts.CheckoutStrategy
             get
             {
                 return CheckoutStrategy.GIT_CHECKOUT_SAFE |
-                       CheckoutStrategy.GIT_CHECKOUT_ALLOW_CONFLICTS |
                        GitCheckoutOptsWrapper.CheckoutStrategyFromFileConflictStrategy(FileConflictStrategy);
             }
         }