Always accept already-installed pre-releases (#5419)

## Summary

If a pre-release is already installed, we should allow it to be used
even if it doesn't match the strategy.

Closes https://github.com/astral-sh/uv/issues/5418.

crates/uv-resolver/src/candidate_selector.rs

@@ -120,14 +120,6 @@ impl CandidateSelector {
                     break 'preference;
                 }
 
-                // Respect the pre-release strategy for this fork.
-                if version.any_prerelease()
-                    && self.prerelease_strategy.allows(package_name, markers)
-                        != AllowPreRelease::Yes
-                {
-                    break 'preference;
-                }
-
                 // Check for a locally installed distribution that matches the preferred version
                 if !exclusions.contains(package_name) {
                     let installed_dists = installed_packages.get_packages(package_name);
@@ -155,6 +147,14 @@ impl CandidateSelector {
                     }
                 }
 
+                // Respect the pre-release strategy for this fork.
+                if version.any_prerelease()
+                    && self.prerelease_strategy.allows(package_name, markers)
+                        != AllowPreRelease::Yes
+                {
+                    break 'preference;
+                }
+
                 // Check for a remote distribution that matches the preferred version
                 if let Some(file) = version_maps
                     .iter()
@@ -183,14 +183,6 @@ impl CandidateSelector {
                         return None;
                     }
 
-                    // Respect the pre-release strategy for this fork.
-                    if version.any_prerelease()
-                        && self.prerelease_strategy.allows(package_name, markers)
-                            != AllowPreRelease::Yes
-                    {
-                        return None;
-                    }
-
                     debug!("Found installed version of {dist} that satisfies {range}");
                     return Some(Candidate {
                         name: package_name,

crates/uv/tests/pip_install.rs

@@ -6105,3 +6105,46 @@ fn local_index_fallback() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn accept_existing_prerelease() -> Result<()> {
+    let context = TestContext::new("3.12").with_filtered_counts();
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("Flask==2.0.0rc1")?;
+
+    // Install a pre-release version of `flask`.
+    uv_snapshot!(context.filters(), context.pip_install().arg("Flask==2.0.0rc1"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved [N] packages in [TIME]
+    Prepared [N] packages in [TIME]
+    Installed [N] packages in [TIME]
+     + click==8.1.7
+     + flask==2.0.0rc1
+     + itsdangerous==2.1.2
+     + jinja2==3.1.3
+     + markupsafe==2.1.5
+     + werkzeug==3.0.1
+    "###
+    );
+
+    // Install `flask-login`, without enabling pre-releases. The existing version of `flask` should
+    // still be accepted.
+    uv_snapshot!(context.filters(), context.pip_install().arg("flask-login==0.6.0"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved [N] packages in [TIME]
+    Prepared [N] packages in [TIME]
+    Installed [N] packages in [TIME]
+     + flask-login==0.6.0
+    "###
+    );
+
+    Ok(())
+}