Fork resolution when Python requirement is narrowed

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

@@ -555,7 +555,7 @@ impl<InstalledPackages: InstalledPackagesProvider> ResolverState<InstalledPackag
                                     .map(ToString::to_string)
                                     .join(", ")
                             );
-                            assert!(forks.len() >= 2);
+
                             // This is a somewhat tortured technique to ensure
                             // that our resolver state is only cloned as much
                             // as it needs to be. We basically move the state
@@ -2422,7 +2422,6 @@ impl Dependencies {
                     continue;
                 }
             };
-            assert!(fork_groups.forks.len() >= 2, "expected definitive fork");
             let mut new_forks: Vec<Fork> = vec![];
             for group in fork_groups.forks {
                 let mut new_forks_for_group = forks.clone();
@@ -2600,7 +2599,7 @@ impl<'a> PossibleForks<'a> {
         let PossibleForks::PossiblyForking(ref fork_groups) = *self else {
             return false;
         };
-        fork_groups.forks.len() > 1
+        fork_groups.has_fork()
     }
 
     /// Consumes this possible set of forks and converts a "possibly forking"
@@ -2613,9 +2612,8 @@ impl<'a> PossibleForks<'a> {
         let PossibleForks::PossiblyForking(ref fork_groups) = self else {
             return self;
         };
-        if fork_groups.forks.len() == 1 {
+        if !fork_groups.has_fork() {
             self.make_no_forks_possible();
-            return self;
         }
         self
     }
@@ -2678,6 +2676,23 @@ impl<'a> PossibleForkGroups<'a> {
             .iter_mut()
             .find(|fork| fork.is_overlapping(marker))
     }
+
+    /// Returns `true` if the fork group has a fork.
+    fn has_fork(&self) -> bool {
+        if self.forks.len() > 1 {
+            return true;
+        }
+
+        if self.forks.iter().any(|fork| {
+            fork.packages
+                .iter()
+                .any(|(_, markers)| requires_python_marker(markers).is_some())
+        }) {
+            return true;
+        }
+
+        false
+    }
 }
 
 /// Intermediate state representing a single possible fork.

crates/uv/tests/branching_urls.rs

@@ -90,7 +90,7 @@ fn branching_urls_overlapping() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    error: Requirements contain conflicting URLs for package `iniconfig`:
+    error: Requirements contain conflicting URLs for package `iniconfig` in split `python_version < '3.12' and python_version >= '3.11'`:
     - https://files.pythonhosted.org/packages/9b/dd/b3c12c6d707058fa947864b67f0c4e0c39ef8610988d7baea9578f3c48f3/iniconfig-1.1.1-py2.py3-none-any.whl
     - https://files.pythonhosted.org/packages/ef/a6/62565a6e1cf69e10f5727360368e451d4b7f58beeac6173dc9db836a5b46/iniconfig-2.0.0-py3-none-any.whl
     "###
@@ -381,7 +381,7 @@ fn root_package_splits_other_dependencies_too() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    Resolved 9 packages in [TIME]
+    Resolved 7 packages in [TIME]
     "###
     );
 
@@ -397,7 +397,6 @@ fn root_package_splits_other_dependencies_too() -> Result<()> {
         { name = "anyio", version = "4.2.0", source = { registry = "https://pypi.org/simple" }, marker = "python_version < '3.12'" },
         { name = "anyio", version = "4.3.0", source = { registry = "https://pypi.org/simple" }, marker = "python_version >= '3.12'" },
         { name = "b1", marker = "python_version < '3.12'" },
-        { name = "b2", marker = "python_version >= '3.12'" },
     ]
 
     [[distribution]]
@@ -431,15 +430,7 @@ fn root_package_splits_other_dependencies_too() -> Result<()> {
     version = "0.1.0"
     source = { directory = "b1" }
     dependencies = [
-        { name = "iniconfig", version = "1.1.1", source = { registry = "https://pypi.org/simple" } },
-    ]
-
-    [[distribution]]
-    name = "b2"
-    version = "0.1.0"
-    source = { directory = "b2" }
-    dependencies = [
-        { name = "iniconfig", version = "2.0.0", source = { registry = "https://pypi.org/simple" } },
+        { name = "iniconfig" },
     ]
 
     [[distribution]]
@@ -460,15 +451,6 @@ fn root_package_splits_other_dependencies_too() -> Result<()> {
         { url = "https://files.pythonhosted.org/packages/9b/dd/b3c12c6d707058fa947864b67f0c4e0c39ef8610988d7baea9578f3c48f3/iniconfig-1.1.1-py2.py3-none-any.whl", hash = "sha256:011e24c64b7f47f6ebd835bb12a743f2fbe9a26d4cecaa7f53bc4f35ee9da8b3", size = 4990 },
     ]
 
-    [[distribution]]
-    name = "iniconfig"
-    version = "2.0.0"
-    source = { registry = "https://pypi.org/simple" }
-    sdist = { url = "https://files.pythonhosted.org/packages/d7/4b/cbd8e699e64a6f16ca3a8220661b5f83792b3017d0f79807cb8708d33913/iniconfig-2.0.0.tar.gz", hash = "sha256:2d91e135bf72d31a410b17c16da610a82cb55f6b0477d1a902134b24a455b8b3", size = 4646 }
-    wheels = [
-        { url = "https://files.pythonhosted.org/packages/ef/a6/62565a6e1cf69e10f5727360368e451d4b7f58beeac6173dc9db836a5b46/iniconfig-2.0.0-py3-none-any.whl", hash = "sha256:b6a85871a79d2e3b22d2d1b94ac2824226a63c6b741c88f7ae975f18b6778374", size = 5892 },
-    ]
-
     [[distribution]]
     name = "sniffio"
     version = "1.3.1"
@@ -637,7 +619,7 @@ fn branching_urls_of_different_sources_conflict() -> Result<()> {
     ----- stdout -----
 
     ----- stderr -----
-    error: Requirements contain conflicting URLs for package `iniconfig`:
+    error: Requirements contain conflicting URLs for package `iniconfig` in split `python_version < '3.12' and python_version >= '3.11'`:
     - git+https://github.com/pytest-dev/iniconfig@93f5930e668c0d1ddf4597e38dd0dea4e2665e7a
     - https://files.pythonhosted.org/packages/9b/dd/b3c12c6d707058fa947864b67f0c4e0c39ef8610988d7baea9578f3c48f3/iniconfig-1.1.1-py2.py3-none-any.whl
     "###

crates/uv/tests/pip_compile.rs

@@ -6567,7 +6567,7 @@ fn universal_multi_version() -> Result<()> {
 /// Perform a universal resolution that requires narrowing the supported Python range in one of the
 /// fork branches.
 #[test]
-fn universal_requires_python() -> Result<()> {
+fn universal_requires_python_fork() -> Result<()> {
     let context = TestContext::new("3.12");
     let requirements_in = context.temp_dir.child("requirements.in");
     requirements_in.write_str(indoc::indoc! {r"
@@ -6599,6 +6599,38 @@ fn universal_requires_python() -> Result<()> {
     Ok(())
 }
 
+/// Perform a universal resolution that requires narrowing the supported Python range without
+/// forking.
+#[test]
+fn universal_requires_python_non_fork() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let requirements_in = context.temp_dir.child("requirements.in");
+    requirements_in.write_str(indoc::indoc! {r"
+        numpy >=1.26 ; python_version >= '3.9'
+    "})?;
+
+    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
+    numpy==1.26.4 ; python_version >= '3.9'
+        # via -r requirements.in
+
+    ----- stderr -----
+    warning: The requested Python version 3.8 is not available; 3.12.[X] will be used to build dependencies instead.
+    Resolved 1 package in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// Resolve a package from a `requirements.in` file, with a `constraints.txt` file pinning one of
 /// its transitive dependencies to a specific version.
 #[test]