Apply lockfile marker normalization for fork markers (#18116)

While dependency markers get a roundtrip through simplify/complexify
([crates/uv-resolver/src/lock/mod.rs#L4846-L4848](https://github.com/astral-sh/uv/blob/3223b1c39f8011a4460f2b5d56ace19e5d26e16d/crates/uv-resolver/src/lock/mod.rs#L4846-L4848),
#15994), this treatment was missing
for fork markers, causing errors with `--locked --refresh` on a fresh
lockfile.

Fixes #16839
Closes #18024

Author: konstin

crates/uv-resolver/src/lock/mod.rs

@@ -326,12 +326,19 @@ impl Lock {
 
             // If there are multiple distributions for the same package, include the markers of all
             // forks that included the current distribution.
+            //
+            // Normalize with a simplify/complexify round-trip through `requires-python` to
+            // match markers deserialized from the lockfile (which get complexified on read).
             let fork_markers = if duplicates.contains(dist.name()) {
                 resolution
                     .fork_markers
                     .iter()
                     .filter(|fork_markers| !fork_markers.is_disjoint(dist.marker))
-                    .copied()
+                    .map(|marker| {
+                        let simplified =
+                            SimplifiedMarkerTree::new(&requires_python, marker.combined());
+                        UniversalMarker::from_combined(simplified.into_marker(&requires_python))
+                    })
                     .collect()
             } else {
                 vec![]
@@ -433,6 +440,18 @@ impl Lock {
             fork_strategy: resolution.options.fork_strategy,
             exclude_newer: resolution.options.exclude_newer.clone().into(),
         };
+        // Normalize fork markers with a simplify/complexify round-trip through
+        // `requires-python`. This ensures markers from the resolver (which don't include
+        // `requires-python` bounds) match markers deserialized from the lockfile (which get
+        // complexified on read).
+        let fork_markers = resolution
+            .fork_markers
+            .iter()
+            .map(|marker| {
+                let simplified = SimplifiedMarkerTree::new(&requires_python, marker.combined());
+                UniversalMarker::from_combined(simplified.into_marker(&requires_python))
+            })
+            .collect();
         let lock = Self::new(
             VERSION,
             REVISION,
@@ -443,7 +462,7 @@ impl Lock {
             Conflicts::empty(),
             vec![],
             vec![],
-            resolution.fork_markers.clone(),
+            fork_markers,
         )?;
         Ok(lock)
     }

crates/uv/tests/it/lock.rs

@@ -4597,6 +4597,50 @@ fn lock_upgrade_log_multi_version() -> Result<()> {
     Ok(())
 }
 
+/// `--upgrade --dry-run` should not report changes when the lockfile is up-to-date with
+/// fork markers.
+///
+/// Regression test for: <https://github.com/astral-sh/uv/issues/16839>
+#[test]
+fn lock_upgrade_dry_run_multi_version() -> Result<()> {
+    let context = uv_test::test_context!("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = [
+            "markupsafe<2 ; sys_platform != 'win32'",
+            "markupsafe==2.0.0 ; sys_platform == 'win32'"
+        ]
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    ");
+
+    uv_snapshot!(context.filters(), context.lock().arg("--upgrade").arg("--dry-run"), @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    No lockfile changes detected
+    ");
+
+    Ok(())
+}
+
 /// Respect the locked version in an existing lockfile.
 #[test]
 fn lock_preference() -> Result<()> {