improve NamedTuple field checks

Summary:
Based on some testing in the CLI
- methods are never counted as fields (but lambdas work, unlike enums)
- it's a runtime error if named tuple fields start with an underscore, with some caveats
  - if it's a method, then it's not a field so there's no runtime error if it starts with underscore.
  - if a field is renamed to _0, _1 using the functional syntax rename=True param, then there's no error if it starts with underscore
  - if a user-written field starts with an underscore and rename=True is set, it will be renamed to _0, _1, etc.

This diff implements all of the above functionality, EXCEPT disallowing fields starting with underscore for the class syntax. This is because we don't have a way to differentiate a renamed field from a field originally written by person.

I think we could probably do something about this by moving every field on a synthesized class into the same structure that we're using for synthesized methods, but that's a bigger change for a separate diff. For now, I left a testcase with some comments.

fixes #101

Reviewed By: stroxler

Differential Revision: D73113784

fbshipit-source-id: 2f20e6b949e2e17dbfedf36f153d333cae98a786

Author: yangdanny97

Diff for: pyrefly/lib/alt/class/named_tuple.rs

@@ -30,6 +30,9 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
     pub fn get_named_tuple_elements(&self, cls: &Class) -> Vec<Name> {
         let mut elements = Vec::new();
         for name in cls.fields() {
+            if !cls.is_field_annotated(name) {
+                continue;
+            }
             if let Some(range) = cls.field_decl_range(name) {
                 elements.push((name.clone(), range));
             }

Diff for: pyrefly/lib/binding/class.rs

@@ -388,9 +388,13 @@ impl<'a> BindingsBuilder<'a> {
                 );
                 continue;
             }
+            // Synthesized fields for named tuples are always considered annotated
             fields.insert(
                 member_name.clone(),
-                ClassFieldProperties::new(member_annotation.is_some(), range),
+                ClassFieldProperties::new(
+                    member_annotation.is_some() || class_kind == SynthesizedClassKind::NamedTuple,
+                    range,
+                ),
             );
             let initial_value = if force_class_initialization || member_value.is_some() {
                 ClassFieldInitialValue::Class(member_value.clone())

Diff for: pyrefly/lib/test/named_tuple.rs

@@ -197,3 +197,34 @@ x = 2
 Tup = namedtuple("Tup", ["a", "b"], defaults=(None, x))
 "#,
 );
+
+testcase!(
+    test_named_tuple_dunder_unpack,
+    r#"
+from typing import NamedTuple
+class A(NamedTuple):
+    a: int
+    b: str
+    def __repr__(self) -> str:
+        return "A"
+
+def test(x: A) -> None:
+    a, b = x
+"#,
+);
+
+testcase!(
+    bug =
+        "Field names cannot start with an underscore, unless they were generated with rename=True",
+    test_named_tuple_underscore_field_name,
+    r#"
+from typing import NamedTuple
+from collections import namedtuple
+class A(NamedTuple):
+    a: int
+    b: str
+    _c: str  # Not OK
+B = namedtuple("B", ["a", "b", "_c"])  # E: NamedTuple field name may not start with an underscore
+C = namedtuple("C", ["a", "b", "_c"], rename=True)  # OK
+"#,
+);