fix: coalesce split Myers hunks to prevent false merge conflicts

gix-merge/src/blob/builtin_driver/text/function.rs

@@ -3,9 +3,10 @@ use std::ops::Range;
 use crate::blob::{
     builtin_driver::text::{
         utils::{
-            assure_ends_with_nl, contains_lines, detect_line_ending, detect_line_ending_or_nl, fill_ancestor,
-            hunks_differ_in_diff3, take_intersecting, tokens, write_ancestor, write_conflict_marker, write_hunks,
-            zealously_contract_hunks, CollectHunks, Hunk, Side,
+            assure_ends_with_nl, coalesce_empty_insertions_with_nearest_same_side_hunk, contains_lines,
+            detect_line_ending, detect_line_ending_or_nl, fill_ancestor, hunks_differ_in_diff3, take_intersecting,
+            tokens, write_ancestor, write_conflict_marker, write_hunks, zealously_contract_hunks, CollectHunks, Hunk,
+            Side,
         },
         Conflict, ConflictStyle, Labels, Options,
     },
@@ -72,6 +73,7 @@ pub fn merge<'a>(
     }
 
     hunks.sort_by(|a, b| a.before.start.cmp(&b.before.start));
+    coalesce_empty_insertions_with_nearest_same_side_hunk(&mut hunks);
     let mut hunks = hunks.into_iter().peekable();
     let mut intersecting = Vec::new();
     let mut ancestor_integrated_until = 0;

gix-merge/src/blob/builtin_driver/text/utils.rs

@@ -457,6 +457,60 @@ pub fn take_intersecting(
     Some(())
 }
 
+/// Absorb empty-range insertion hunks into an adjacent same-side non-empty hunk.
+///
+/// Different diff algorithms (Myers vs Histogram) can produce different hunk boundaries
+/// for the same edit. Myers sometimes splits what is logically one change into a deletion
+/// plus a separate empty insertion, with one or more unchanged lines between them. When
+/// the empty insertion lands at a position that the other side also touches, the merge
+/// sees a false conflict.
+///
+/// This function detects the pattern: a non-empty same-side hunk `H` followed (after at
+/// most one unchanged base line) by an empty insertion `I`. It extends `H` to cover the
+/// gap and the insertion point, effectively re-joining the split hunk. This makes the
+/// merge insensitive to the diff algorithm's alignment choices for these cases.
+///
+/// Requires `hunks` to be sorted by `before.start`.
+pub fn coalesce_empty_insertions_with_nearest_same_side_hunk(hunks: &mut Vec<Hunk>) {
+    let mut i = 0;
+    while i < hunks.len() {
+        let hunk = &hunks[i];
+        // Only process empty insertions (before range is empty).
+        if !hunk.before.is_empty() {
+            i += 1;
+            continue;
+        }
+        let ins_pos = hunk.before.start;
+        let ins_side = hunk.side;
+        let ins_after_end = hunk.after.end;
+
+        // Look backwards for the nearest same-side non-empty hunk within a gap of ≤ 1 base line.
+        let mut found = false;
+        for j in (0..i).rev() {
+            let candidate = &hunks[j];
+            if candidate.side != ins_side {
+                continue;
+            }
+            if candidate.before.is_empty() {
+                // Skip other empty insertions from the same side.
+                continue;
+            }
+            let gap = ins_pos.saturating_sub(candidate.before.end);
+            if gap <= 1 {
+                // Extend the candidate to cover the gap and the insertion.
+                hunks[j].before.end = ins_pos;
+                hunks[j].after.end = ins_after_end;
+                hunks.remove(i);
+                found = true;
+            }
+            break;
+        }
+        if !found {
+            i += 1;
+        }
+    }
+}
+
 pub fn tokens(input: &[u8]) -> imara_diff::sources::ByteLines<'_, true> {
     imara_diff::sources::byte_lines_with_terminator(input)
 }

gix-merge/tests/merge/blob/false_conflict.rs

@@ -0,0 +1,64 @@
+use gix_merge::blob::{builtin_driver, builtin_driver::text::Conflict, Resolution};
+use imara_diff::intern::InternedInput;
+
+/// Minimal reproduction: Myers produces a false conflict where git merge-file resolves cleanly.
+///
+/// base:   alpha_x / (blank) / bravo_x / charlie_x / (blank)
+/// ours:   (blank) / (blank) / bravo_x / charlie_x
+/// theirs: alpha_x / (blank) / charlie_x / (blank)
+///
+/// base→ours:  alpha_x deleted (replaced by blank), trailing blank removed
+/// base→theirs: bravo_x deleted
+///
+/// These are non-overlapping changes that git merges cleanly.
+/// See https://github.com/GitoxideLabs/gitoxide/issues/2475
+#[test]
+fn myers_false_conflict_with_blank_line_ambiguity() {
+    let base = b"alpha_x\n\nbravo_x\ncharlie_x\n\n";
+    let ours = b"\n\nbravo_x\ncharlie_x\n";
+    let theirs = b"alpha_x\n\ncharlie_x\n\n";
+
+    let labels = builtin_driver::text::Labels {
+        ancestor: Some("base".into()),
+        current: Some("ours".into()),
+        other: Some("theirs".into()),
+    };
+
+    // Histogram resolves cleanly.
+    {
+        let options = builtin_driver::text::Options {
+            diff_algorithm: imara_diff::Algorithm::Histogram,
+            conflict: Conflict::Keep {
+                style: builtin_driver::text::ConflictStyle::Merge,
+                marker_size: 7.try_into().unwrap(),
+            },
+        };
+        let mut out = Vec::new();
+        let mut input = InternedInput::default();
+        let res = builtin_driver::text(&mut out, &mut input, labels, ours, base, theirs, options);
+        assert_eq!(res, Resolution::Complete, "Histogram should resolve cleanly");
+    }
+
+    // Myers should also resolve cleanly (it used to produce a false conflict because
+    // imara-diff's Myers splits the ours change into two hunks — a deletion at base[0]
+    // and an empty insertion at base[2] — and the insertion collided with theirs'
+    // deletion at base[2]).
+    {
+        let options = builtin_driver::text::Options {
+            diff_algorithm: imara_diff::Algorithm::Myers,
+            conflict: Conflict::Keep {
+                style: builtin_driver::text::ConflictStyle::Merge,
+                marker_size: 7.try_into().unwrap(),
+            },
+        };
+        let mut out = Vec::new();
+        let mut input = InternedInput::default();
+        let res = builtin_driver::text(&mut out, &mut input, labels, ours, base, theirs, options);
+        assert_eq!(
+            res,
+            Resolution::Complete,
+            "Myers should resolve cleanly (git merge-file does). Output:\n{}",
+            String::from_utf8_lossy(&out)
+        );
+    }
+}

gix-merge/tests/merge/blob/mod.rs

@@ -1,4 +1,5 @@
 mod builtin_driver;
+mod false_conflict;
 mod pipeline;
 mod platform;