update merge.md with new test cases

Author: mitchellwrosen

unison-cli/src/Unison/Cli/UniqueTypeGuidLookup.hs

@@ -51,15 +51,15 @@ loadUniqueTypeGuid pp name0 = do
             -- a new GUID because it's not clear whether alice's or bob's should be preferred.
             (Just aliceGuid, Just bobGuid) -> do
               case (aliceMaybeBranchId, bobMaybeBranchId) of
-                (Just aliceBranchId, Just bobBranchId) ->
-                  Queries.loadProjectBranchParent pp.project.projectId aliceBranchId >>= \case
-                    Just aliceParentBranchId ->
-                      if aliceParentBranchId == bobBranchId
-                        then pure (Just bobGuid)
-                        else
-                          Queries.loadProjectBranchParent pp.project.projectId bobBranchId <&> \case
-                            Just bobParentBranchId | bobParentBranchId == aliceBranchId -> Just aliceGuid
-                            _ -> Nothing
-                    Nothing -> pure Nothing
+                (Just aliceBranchId, Just bobBranchId) -> do
+                  aliceParentBranchId <- Queries.loadProjectBranchParent pp.project.projectId aliceBranchId
+                  if aliceParentBranchId == Just bobBranchId
+                    then pure (Just bobGuid)
+                    else do
+                      bobParentBranchId <- Queries.loadProjectBranchParent pp.project.projectId bobBranchId
+                      pure
+                        if bobParentBranchId == Just aliceBranchId
+                          then Just aliceGuid
+                          else Nothing
                 _ -> pure Nothing
     Just guid -> pure (Just guid)

unison-src/transcripts/merge.md

@@ -1748,6 +1748,105 @@ scratch/alice> names Bar
 scratch/main> project.delete scratch
 ```
 
+### Unique type GUID reuse – parent branch
+
+When on a merge branch, if the parser has to pick between two different GUIDs, and the two merge parents themselves have
+a branch parent-child relationship, it will prefer the parent's GUID. If they don't the parser will make a new GUID.
+
+``` ucm :hide
+scratch/main> builtins.mergeio lib.builtins
+```
+
+``` unison
+type Foo = Bar
+```
+
+``` ucm
+scratch/main> branch topic
+scratch/main> branch topic2
+scratch/main> add
+scratch/main> switch /topic
+```
+
+``` unison
+type Foo = Bar
+```
+
+``` ucm
+scratch/topic> add
+scratch/topic> switch /topic2
+```
+
+``` unison
+type Foo = Bar
+```
+
+``` ucm
+scratch/topic2> add
+scratch/main> switch /main
+```
+
+``` ucm :error
+scratch/main> merge /topic
+```
+
+``` ucm
+scratch/main> switch /merge-topic-into-main
+```
+
+``` unison
+type Foo = Bar
+```
+
+``` ucm
+scratch/merge-topic-into-main> update
+scratch/main> names Foo
+scratch/topic> names Foo
+scratch/merge-topic-into-main> names Foo
+```
+
+``` ucm :error
+scratch/topic> merge /main
+```
+
+``` ucm
+scratch/merge-main-into-topic>
+```
+
+``` unison
+type Foo = Bar
+```
+
+``` ucm
+scratch/merge-main-into-topic> update
+scratch/main> names Foo
+scratch/topic> names Foo
+scratch/merge-main-into-topic> names Foo
+```
+
+``` ucm :error
+scratch/topic> merge /topic2
+```
+
+``` ucm
+scratch/merge-topic2-into-topic>
+```
+
+``` unison
+type Foo = Bar
+```
+
+``` ucm
+scratch/merge-topic2-into-topic> update
+scratch/topic> names Foo
+scratch/topic2> names Foo
+scratch/merge-topic2-into-topic> names Foo
+```
+
+``` ucm :hide
+scratch/main> project.delete scratch
+```
+
 ### Using Alice's names for Bob's things
 
 Previously, we'd render Alice's stuff with her names and Bob's stuff with his. But because Alice is doing the merge,