[red-knot] fix lookups of possibly-shadowed builtins (#12898)

If a builtin is conditionally shadowed by a global, we didn't correctly
fall back to builtins for the not-defined-in-globals path (see added
test for an example.)

crates/red_knot_python_semantic/src/types.rs

@@ -145,8 +145,33 @@ impl<'db> Type<'db> {
         matches!(self, Type::Unbound)
     }
 
-    pub const fn is_unknown(&self) -> bool {
-        matches!(self, Type::Unknown)
+    pub const fn is_never(&self) -> bool {
+        matches!(self, Type::Never)
+    }
+
+    pub fn may_be_unbound(&self, db: &'db dyn Db) -> bool {
+        match self {
+            Type::Unbound => true,
+            Type::Union(union) => union.contains(db, Type::Unbound),
+            // Unbound can't appear in an intersection, because an intersection with Unbound
+            // simplifies to just Unbound.
+            _ => false,
+        }
+    }
+
+    #[must_use]
+    pub fn replace_unbound_with(&self, db: &'db dyn Db, replacement: Type<'db>) -> Type<'db> {
+        match self {
+            Type::Unbound => replacement,
+            Type::Union(union) => union
+                .elements(db)
+                .into_iter()
+                .fold(UnionBuilder::new(db), |builder, ty| {
+                    builder.add(ty.replace_unbound_with(db, replacement))
+                })
+                .build(),
+            ty => *ty,
+        }
     }
 
     #[must_use]

crates/red_knot_python_semantic/src/types/builder.rs

@@ -201,6 +201,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 self.negative.retain(|elem| !pos.contains(elem));
             }
             Type::Never => {}
+            Type::Unbound => {}
             _ => {
                 if !self.positive.remove(&ty) {
                     self.negative.insert(ty);
@@ -214,9 +215,13 @@ impl<'db> InnerIntersectionBuilder<'db> {
 
         // Never is a subtype of all types
         if self.positive.contains(&Type::Never) {
-            self.positive.clear();
+            self.positive.retain(Type::is_never);
+            self.negative.clear();
+        }
+
+        if self.positive.contains(&Type::Unbound) {
+            self.positive.retain(Type::is_unbound);
             self.negative.clear();
-            self.positive.insert(Type::Never);
         }
     }
 
@@ -426,4 +431,26 @@ mod tests {
 
         assert_eq!(ty, Type::Never);
     }
+
+    #[test]
+    fn build_intersection_simplify_positive_unbound() {
+        let db = setup_db();
+        let ty = IntersectionBuilder::new(&db)
+            .add_positive(Type::Unbound)
+            .add_positive(Type::IntLiteral(1))
+            .build();
+
+        assert_eq!(ty, Type::Unbound);
+    }
+
+    #[test]
+    fn build_intersection_simplify_negative_unbound() {
+        let db = setup_db();
+        let ty = IntersectionBuilder::new(&db)
+            .add_negative(Type::Unbound)
+            .add_positive(Type::IntLiteral(1))
+            .build();
+
+        assert_eq!(ty, Type::IntLiteral(1));
+    }
 }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1354,18 +1354,22 @@ impl<'db> TypeInferenceBuilder<'db> {
                     let symbol = symbols.symbol_by_name(id).unwrap();
                     if !symbol.is_defined() || !self.scope.is_function_like(self.db) {
                         // implicit global
-                        let mut unbound_ty = if file_scope_id == FileScopeId::global() {
+                        let unbound_ty = if file_scope_id == FileScopeId::global() {
                             Type::Unbound
                         } else {
                             global_symbol_ty_by_name(self.db, self.file, id)
                         };
                         // fallback to builtins
-                        if matches!(unbound_ty, Type::Unbound)
+                        if unbound_ty.may_be_unbound(self.db)
                             && Some(self.scope) != builtins_scope(self.db)
                         {
-                            unbound_ty = builtins_symbol_ty_by_name(self.db, id);
+                            Some(unbound_ty.replace_unbound_with(
+                                self.db,
+                                builtins_symbol_ty_by_name(self.db, id),
+                            ))
+                        } else {
+                            Some(unbound_ty)
                         }
-                        Some(unbound_ty)
                     } else {
                         Some(Type::Unbound)
                     }
@@ -2163,6 +2167,38 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn conditionally_global_or_builtin() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "/src/a.py",
+            "
+            if flag:
+                copyright = 1
+            def f():
+                y = copyright
+            ",
+        )?;
+
+        let file = system_path_to_file(&db, "src/a.py").expect("Expected file to exist.");
+        let index = semantic_index(&db, file);
+        let function_scope = index
+            .child_scopes(FileScopeId::global())
+            .next()
+            .unwrap()
+            .0
+            .to_scope_id(&db, file);
+        let y_ty = symbol_ty_by_name(&db, function_scope, "y");
+
+        assert_eq!(
+            y_ty.display(&db).to_string(),
+            "Literal[1] | Literal[copyright]"
+        );
+
+        Ok(())
+    }
+
     /// Class name lookups do fall back to globals, but the public type never does.
     #[test]
     fn unbound_class_local() -> anyhow::Result<()> {