fix(resolve_base): a main push whose HEAD is behind the tip is a push shape too — main red on 6157d1924

scripts/check_roadmap_diff_additive.sh

@@ -382,6 +382,16 @@ EOF
         *"never the tree against itself"*) printf 'ok    row %-2s push shape, parent not fetched (depth-1): refused by name, never HEAD itself\n' "$row" ;;
         *) printf 'FAIL  row %-2s push shape, parent not fetched: refused for the wrong reason (rc=%s): %s\n' "$row" "$rc5" "$err5"; fails=1 ;;
     esac
+    # Push shape, HEAD BEHIND the tip: a later merge landed before this run checked out, so
+    # merge-base(origin/main, HEAD) is HEAD itself (run 34747052599, main red 2026-09-13). The base
+    # is still HEAD's first parent; without this row the resolver named HEAD and the run was refused.
+    row=$((row + 1))
+    rm -rf "${R:?}.behind"; git clone -q "file://$R" "$R.behind" 2>/dev/null
+    ( cd "$R.behind" && git checkout -q main && prev=$(git rev-parse HEAD) && cp "$TD/base_dup.yaml" later.yaml && git add later.yaml && git -c user.name=t -c user.email=t@t commit -qm later-merge && git update-ref refs/remotes/origin/main HEAD && git checkout -q --detach "$prev" ) 2>/dev/null
+    got6=$( cd "$R.behind" && bash -c '. "$0" --lib-only; REPO_ROOT="$1"; resolve_base HEAD && printf "%s|%s" "$BASE_REF" "$BASE_HOW"' "$SELF" "$R.behind" 2>/dev/null ) || true
+    want6=$( cd "$R.behind" && git rev-parse 'HEAD^1' )
+    case "$got6" in "$want6|first parent of HEAD (HEAD is on origin/main, behind the tip"*) printf 'ok    row %-2s push shape, HEAD behind the tip (a later merge landed first) -> its first parent, never HEAD itself\n' "$row" ;;
+        *) printf 'FAIL  row %-2s push shape, HEAD behind the tip: wanted %s|first parent of HEAD (HEAD is on origin/main, behind the tip..., got %s\n' "$row" "$want6" "$got6"; fails=1 ;; esac
     # Rows 19-20: a STACKED merge-group entry at depth 1 — the head's single parent
     # is the previous entry's squash, not the origin/main tip. Under merge_group it
     # deepens and names that parent; with deepening disabled (the mutation) it is

scripts/lib/resolve_base.sh

@@ -23,12 +23,24 @@ resolve_base() {
     local PROG="${PROG:-resolve_base}"
     headid=$(git -C "$REPO_ROOT" rev-parse "$head^{commit}" 2>/dev/null || true)
     main_tip=$(git -C "$REPO_ROOT" rev-parse origin/main 2>/dev/null || true)
-    if [ -n "$headid" ] && [ "$headid" = "$main_tip" ]; then
-        # push shape: HEAD IS the origin/main tip. merge-base(origin/main, HEAD) is HEAD, and a
-        # differential of HEAD against itself passes vacuously. The change under judgment is
-        # what the tip's first parent lacks; a depth-1 checkout must deepen by one to hold it.
+    # push shape: HEAD is ON origin/main -- the tip, or a commit BEHIND the tip when a later merge
+    # landed before this run checked out (run 34747052599: origin/main had moved one squash past
+    # HEAD; merge-base(origin/main, HEAD) was HEAD itself and the run was refused "never the tree
+    # against itself"). Either way merge-base is HEAD and a differential of HEAD against itself
+    # passes vacuously. The change under judgment is what the first parent lacks; a depth-1
+    # checkout must deepen by one to hold it.
+    local on_main=0
+    if [ -n "$headid" ] && [ -n "$main_tip" ]; then
+        # On main's FIRST-PARENT line, not merely an ancestor: a feature commit merged into main is an
+        # ancestor too, and its base is the merge-base, not its own parent (case-table row 15).
+        if [ "$headid" = "$main_tip" ] || git -C "$REPO_ROOT" rev-list --first-parent "$main_tip" 2>/dev/null | grep -qx "$headid"; then on_main=1; fi
+    fi
+    if [ "$on_main" = 1 ]; then
         local p; p=$(git -C "$REPO_ROOT" rev-parse -q --verify "$head^1^{commit}" 2>/dev/null || true)
-        if [ -n "$p" ]; then BASE_REF="$p"; BASE_HOW="first parent of $head (HEAD is the origin/main tip: push shape)"; return 0; fi
+        if [ -n "$p" ]; then
+            if [ "$headid" = "$main_tip" ]; then BASE_HOW="first parent of $head (HEAD is the origin/main tip: push shape)"; else BASE_HOW="first parent of $head (HEAD is on origin/main, behind the tip: push shape, a later merge landed first)"; fi
+            BASE_REF="$p"; return 0
+        fi
         printf '%s: %s is the origin/main tip and its first parent is not fetched, so the only base would be %s itself; refused (never the tree against itself). Deepen the checkout: git fetch --deepen=1 origin +refs/heads/main:refs/remotes/origin/main\n' "$PROG" "$head" "$head" >&2
         return 1
     fi