Prioritize forks based on upper bounds (#5643)

## Summary

Given a fork like:

```
pylint < 3 ; sys_platform == 'darwin'
pylint > 2 ; sys_platform != 'darwin'
```

Solving the top branch will typically yield a solution that also
satisfies the bottom branch, due to maximum version selection (while the
inverse isn't true).

To quote an example from the docs:

```rust
// If there's no difference, prioritize forks with upper bounds. We'd prefer to solve
// `numpy <= 2` before solving `numpy >= 1`, since the resolution produced by the former
// might work for the latter, but the inverse is unlikely to be true due to maximum
// version selection. (Selecting `numpy==2.0.0` would satisfy both forks, but selecting
// the latest `numpy` would not.)
```

Closes https://github.com/astral-sh/uv/issues/4926 for now.

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

@@ -2844,7 +2844,34 @@ impl Ord for Fork {
         // work for the latter, but the inverse is unlikely to be true.
         let self_bound = requires_python_marker(&self.markers).unwrap_or_default();
         let other_bound = requires_python_marker(&other.markers).unwrap_or_default();
-        other_bound.cmp(&self_bound)
+
+        other_bound.cmp(&self_bound).then_with(|| {
+            // If there's no difference, prioritize forks with upper bounds. We'd prefer to solve
+            // `numpy <= 2` before solving `numpy >= 1`, since the resolution produced by the former
+            // might work for the latter, but the inverse is unlikely to be true due to maximum
+            // version selection. (Selecting `numpy==2.0.0` would satisfy both forks, but selecting
+            // the latest `numpy` would not.)
+            let self_upper_bounds = self
+                .dependencies
+                .iter()
+                .filter(|dep| {
+                    dep.version
+                        .bounding_range()
+                        .is_some_and(|(_, upper)| !matches!(upper, Bound::Unbounded))
+                })
+                .count();
+            let other_upper_bounds = other
+                .dependencies
+                .iter()
+                .filter(|dep| {
+                    dep.version
+                        .bounding_range()
+                        .is_some_and(|(_, upper)| !matches!(upper, Bound::Unbounded))
+                })
+                .count();
+
+            self_upper_bounds.cmp(&other_upper_bounds)
+        })
     }
 }
 

crates/uv/tests/pip_compile.rs

@@ -7949,6 +7949,63 @@ fn universal_no_repeated_unconditional_distributions() -> Result<()> {
     Ok(())
 }
 
+/// Solve for upper bounds before solving for lower bounds. A solution that satisfies `pylint < 3`
+/// can also work for `pylint > 2`, but the inverse isn't true (due to maximum version selection).
+#[test]
+fn universal_prefer_upper_bounds() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        pylint < 3 ; sys_platform == 'darwin'
+        pylint > 2 ; sys_platform != 'darwin'
+    "})?;
+
+    uv_snapshot!(context.filters(), windows_filters=false, context.pip_compile()
+            .arg("requirements.in")
+            .arg("-p")
+            .arg("3.8")
+            .arg("--universal"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] requirements.in -p 3.8 --universal
+    astroid==2.15.8 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via pylint
+    colorama==0.4.6 ; sys_platform == 'win32'
+        # via pylint
+    dill==0.3.8
+        # via pylint
+    isort==5.13.2 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via pylint
+    lazy-object-proxy==1.10.0 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via astroid
+    mccabe==0.7.0 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via pylint
+    platformdirs==4.2.0 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via pylint
+    pylint==2.17.7
+        # via -r requirements.in
+    tomli==2.0.1 ; python_version < '3.11'
+        # via pylint
+    tomlkit==0.12.4 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via pylint
+    typing-extensions==4.10.0 ; python_version < '3.11'
+        # via
+        #   astroid
+        #   pylint
+    wrapt==1.16.0 ; (python_version < '3.11' and sys_platform == 'darwin') or (python_version < '3.11' and sys_platform != 'darwin') or (python_version >= '3.11' and sys_platform == 'darwin') or (python_version >= '3.11' and sys_platform != 'darwin')
+        # via astroid
+
+    ----- stderr -----
+    warning: The requested Python version 3.8 is not available; 3.12.[X] will be used to build dependencies instead.
+    Resolved 12 packages in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Remove `python_version` markers that are always true.
 #[test]
 fn universal_unnecessary_python() -> Result<()> {