test(scenarios): show that squash-merging a pull loses the sync point

README.md

@@ -123,6 +123,9 @@ More scenarios:
   subtree inside another is refused.
 - [`shared-remote-url`](test/scenarios/shared-remote-url/README.md): two
   subtrees with the same remote URL act like two clones of one repo.
+- [`squash-merged-pull`](test/scenarios/squash-merged-pull/README.md): a
+  known bug. After a pull on a branch that was squash-merged, `status`
+  reports `diverged` instead of `push`.
 
 [How the last sync point is found](docs/last-synced-commit/README.md)
 explains how the states are worked out and lists known limitations.

test/scenarios/squash-merged-pull/README.md

@@ -0,0 +1,89 @@
+# Scenario: squash-merged-pull
+
+A known bug: the remote's change was pulled on a feature branch, and the
+feature branch was squash-merged into `main`. Since then, the tool
+misreads `main`.
+
+- **Monorepo (`vendor/a`)**: added at `seed` on `main`. On `feature`,
+  `git subtree pull --squash` brought in `upstream change`. `feature` was
+  squash-merged into `main` and deleted. Then a local commit on `main`
+  added `local.txt`.
+- **Remote**: `seed`, then `upstream change`. It has never seen
+  `local.txt`.
+
+Only the local side changed since `upstream change`, so the right state is
+`push`. But the pull's squash commit, which records the sync point, was
+only reachable through the pull's merge commit on `feature`. A squash
+merge keeps the content and drops that history, so `main` still sees the
+sync point from `git subtree add`. Measured against it, both sides changed.
+
+## Output
+
+`scenario_squash_merged_pull` in [`setup.bash`](setup.bash)
+builds this state. [How scenarios work](../README.md).
+
+<!--
+```scrut {fail_fast: true, output_stream: combined}
+$ source "$TESTDIR/../readme-setup.sh" && build_scenario scenario_squash_merged_pull
+```
+-->
+
+`status` reports `diverged` instead of `push`:
+
+```scrut
+$ git subtrees status
+ok   vendor/a -> $UPSTREAM (diverged)
+ local.txt | 1 +
+ 1 file changed, 1 insertion(+)
+```
+
+`push` is rejected, since the history it rebuilds starts at the old sync
+point:
+
+```scrut
+$ git subtrees push
+git push using:  vendor/a main
+To $UPSTREAM
+ ! [rejected]        bd16e2c5cc3967a824b55916f9d66708a8fda514 -> main (non-fast-forward)
+error: failed to push some refs to '$UPSTREAM'
+hint: Updates were rejected because the tip of your current branch is behind
+hint: its remote counterpart. If you want to integrate the remote changes,
+hint: use 'git pull' before pushing again.
+hint: See the 'Note about fast-forwards' in 'git push --help' for details.
+!!   vendor/a: push failed
+!!   Failed: vendor/a
+[1]
+```
+
+`pull` recovers: it merges `upstream change` again, which changes no file,
+and records a new sync point:
+
+```scrut
+$ git subtrees pull
+ok   vendor/a fetched
+Merge made by the 'ort' strategy.
+ok   vendor/a: pulled
+```
+
+But the push after it rebuilds the squash-merged commit too, so the remote
+gets `feature (squash-merged)`, which repeats `upstream change`, next to
+the original:
+
+```scrut
+$ git subtrees push
+git push using:  vendor/a main
+To $UPSTREAM
+   6045a98..90babaf  90babaf10d3e73b0251a158cb615b5c563b07c80 -> main
+ok   vendor/a: pushed
+```
+
+```scrut
+$ git -C "$UPSTREAM" log --graph --format='%s' main
+*   Merge commit '00a22274fa7c6e2d9b734b9eefe569b3f8cdd6f8'
+|\  
+| * upstream change
+* | local change
+* | feature (squash-merged)
+|/  
+* seed
+```

test/scenarios/squash-merged-pull/setup.bash

@@ -0,0 +1,22 @@
+# See README.md in this directory.
+scenario_squash_merged_pull() {
+  local monorepo="$1" upstream="$2"
+  make_bare_repo "$upstream"
+  seed_bare_repo "$upstream" "seed"
+  init_monorepo "$monorepo"
+  add_subtree "$monorepo" "$upstream" "vendor/a"
+  seed_bare_repo "$upstream" "upstream change"
+  (
+    cd "$monorepo"
+    git switch -q -c feature
+    git fetch -q vendor/a
+    git subtree pull -q --prefix=vendor/a vendor/a main --squash -m "pull vendor/a" >/dev/null 2>&1
+    git switch -q main
+    git merge -q --squash feature >/dev/null
+    git commit -q -m "feature (squash-merged)"
+    git branch -q -D feature
+    echo "local change" >vendor/a/local.txt
+    git add vendor/a/local.txt
+    git commit -q -m "local change"
+  )
+}

test/status.bats

@@ -10,6 +10,7 @@ setup() {
   load 'scenarios/feature-branch-unchanged/setup'
   load 'scenarios/feature-branch-changed/setup'
   load 'scenarios/diverged-then-pulled/setup'
+  load 'scenarios/squash-merged-pull/setup'
   monorepo="$BATS_TEST_TMPDIR/monorepo"
   upstream="$BATS_TEST_TMPDIR/upstream.git"
 }
@@ -174,3 +175,10 @@ setup() {
   [ "$status" -eq 1 ]
   [[ "$output" == *"--base needs a branch name"* ]]
 }
+
+@test "status: reports push after a pull on a squash-merged branch" {
+  scenario_squash_merged_pull "$monorepo" "$upstream"
+  cd "$monorepo"
+  run cmd_status
+  [[ "$output" == *"(push)"* ]]
+}