Allow assigning sentinel to variable with different name

This seems to be what is being decided in the python typing spec, see
the PR description: python/typing#2277

Although some typecheckers, like pyright, still have the old behaviour
recommended in the PEP of giving an error when a different name is
assigned.

Author: hrolfurgylfa

pyrefly/lib/alt/expr.rs

@@ -1791,24 +1791,15 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
 
     pub fn sentinel_from_call(
         &self,
-        name: Identifier,
+        assignment_name: Identifier,
         x: &ExprCall,
         errors: &ErrorCollector,
     ) -> Sentinel {
+        let mut sentinel_name = assignment_name;
         let mut iargs = x.arguments.args.iter();
         if let Some(arg) = iargs.next() {
             if let Expr::StringLiteral(lit) = arg {
-                if lit.value.to_str() != name.id.as_str() {
-                    self.error(
-                        errors,
-                        x.range,
-                        ErrorKind::InvalidSentinel,
-                        format!(
-                            "Sentinel must be assigned to a variable named `{}`",
-                            lit.value.to_str()
-                        ),
-                    );
-                }
+                sentinel_name = Identifier::new(lit.value.to_str(), lit.range());
             } else {
                 self.error(
                     errors,
@@ -1877,7 +1868,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             }
         }
 
-        Sentinel::new(name, self.module().dupe())
+        Sentinel::new(sentinel_name, self.module().dupe())
     }
 
     pub fn typevar_from_call(

pyrefly/lib/test/sentinel.rs

@@ -67,11 +67,35 @@ A = Sentinel(name="A")  # E: Sentinel requires a name as the first argument # E:
 );
 
 testcase!(
-    test_sentinel_construction_different_names,
+    test_sentinel_construction_different_names_allowed,
     r#"
 from typing_extensions import Sentinel
 
-A = Sentinel("B")  # E: Sentinel must be assigned to a variable named `B`
+A = Sentinel("<A>")
+    "#,
+);
+
+testcase!(
+    test_sentinel_uses_sentinel_string_literal_name_in_error_messages,
+    r#"
+from typing_extensions import Sentinel
+
+A = Sentinel("<A>")
+
+def foo(a: A):
+    b: int = a  # E: `<A>` is not assignable to `int`
+    "#,
+);
+
+testcase!(
+    test_sentinel_defaults_to_assignment_name_if_not_constructed_with_name,
+    r#"
+from typing_extensions import Sentinel
+
+A = Sentinel()  # E: Sentinel requires a name as the first argument
+
+def foo(a: A):
+    b: int = a  # E: `A` is not assignable to `int`
     "#,
 );