[ty] Use Top materializations for TypeIs special form (#20591)

crates/ty_python_semantic/src/types/infer/builder/type_expression.rs

@@ -1286,7 +1286,17 @@ impl<'db> TypeInferenceBuilder<'db, '_> {
 
                     Type::unknown()
                 }
-                _ => TypeIsType::unbound(self.db(), self.infer_type_expression(arguments_slice)),
+                _ => TypeIsType::unbound(
+                    self.db(),
+                    // N.B. Using the top materialization here is a pragmatic decision
+                    // that makes us produce more intuitive results given how
+                    // `TypeIs` is used in the real world (in particular, in typeshed).
+                    // However, there's some debate about whether this is really
+                    // fully correct. See <https://github.com/astral-sh/ruff/pull/20591>
+                    // for more discussion.
+                    self.infer_type_expression(arguments_slice)
+                        .top_materialization(self.db()),
+                ),
             },
             SpecialFormType::TypeGuard => {
                 self.infer_type_expression(arguments_slice);