fix: don't merge conflict nodes with distinct requirements

src/conflict.rs

@@ -440,14 +440,16 @@ impl<S: SolverId> ConflictGraph<S> {
                 | ConflictNode::Excluded(_) => continue,
             };
 
+            // Candidates that require different version sets stay separate so each
+            // distinct requirement is reported to the user (conda/rattler#2476).
             let predecessors: Vec<_> = graph
                 .edges_directed(node_id, Direction::Incoming)
-                .map(|e| e.source())
+                .map(|e| (e.weight().try_requires(), e.source()))
                 .sorted_unstable()
                 .collect();
             let successors: Vec<_> = graph
                 .edges(node_id)
-                .map(|e| e.target())
+                .map(|e| (e.weight().try_requires(), e.target()))
                 .sorted_unstable()
                 .collect();
 

tests/solver/main.rs

@@ -450,6 +450,20 @@ fn test_unsat_no_candidates_for_child_2() {
     insta::assert_snapshot!(error);
 }
 
+// Versions requiring different versions of a missing dependency are each
+// reported (conda/rattler#2476).
+#[test]
+fn test_unsat_no_candidates_distinct_requirements() {
+    let provider = BundleBoxProvider::from_packages(&[
+        ("a", 1, vec!["b 41..42"]),
+        ("a", 2, vec!["b 41..42"]),
+        ("a", 3, vec!["b 42..43"]),
+        ("a", 4, vec!["b 43..44"]),
+    ]);
+    let error = solve_unsat(provider, &["a"]);
+    insta::assert_snapshot!(error);
+}
+
 #[test]
 fn test_unsat_missing_top_level_dep_1() {
     let provider = BundleBoxProvider::from_packages(&[("asdf", 1, vec![])]);

tests/solver/snapshots/solver__unsat_no_candidates_distinct_requirements.snap

@@ -0,0 +1,11 @@
+---
+source: tests/solver/main.rs
+expression: error
+---
+a * cannot be installed because there are no viable options:
+├─ a 4 would require
+│  └─ b >=43, <44, for which no candidates were found.
+├─ a 3 would require
+│  └─ b >=42, <43, for which no candidates were found.
+└─ a 1 | 2 would require
+   └─ b >=41, <42, for which no candidates were found.