Fix bug caused by accidentally pinning a var in tuple_infer

Summary: Fixes #1535.

Reviewed By: stroxler

Differential Revision: D86928878

fbshipit-source-id: 51ce5a6c03ecc872333f0ecd0f4a557ca0cea363

Author: rchen152

pyrefly/lib/alt/expr.rs

@@ -657,7 +657,14 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             }
             if !nontuples.is_empty() {
                 // The non-tuple options may contain a type like Sequence[T] that provides an additional default hint.
-                let nontuple_hint = self.unions(nontuples.into_iter().cloned().collect());
+                // Filter out top-level Vars: they don't provide any hints, and we don't want to pin them.
+                let nontuple_hint = self.unions(
+                    nontuples
+                        .into_iter()
+                        .filter(|t| !matches!(t, Type::Var(_)))
+                        .cloned()
+                        .collect(),
+                );
                 let nontuple_element_hint =
                     self.decompose_tuple(HintRef::new(&nontuple_hint, hint.errors()));
                 if let Some(nontuple_element_hint) = nontuple_element_hint {

pyrefly/lib/alt/unwrap.rs

@@ -23,7 +23,7 @@ use crate::types::types::Var;
 // Soft type hints are used for `e1 or e1` expressions.
 pub struct Hint<'a>(Type, Option<&'a ErrorCollector>);
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, Debug)]
 pub struct HintRef<'a, 'b>(&'b Type, Option<&'a ErrorCollector>);
 
 impl<'a> Hint<'a> {

pyrefly/lib/test/generic_basic.rs

@@ -434,3 +434,13 @@ def f[T](x: T | None = None) -> tuple[T, T | int]: ...
 assert_type(f(), tuple[Any, int])
     "#,
 );
+
+testcase!(
+    test_pass_tuple_literal_through_identity_function,
+    r#"
+def f[T](x: T) -> T:
+    return x
+def g() -> int:
+    return f((1, "hello world"))[0]
+    "#,
+);

pyrefly/lib/test/natural.rs

@@ -124,13 +124,12 @@ assert_type(C(42), C[int])
 );
 
 testcase!(
-    bug = "Broken literal promotion for tuples, something happening during contextual typing against T",
     test_generic_tuple,
     r#"
 from typing import assert_type
 
 def f[T](x: T) -> list[T]: ...
 
-assert_type(f((1,)), list[tuple[int]]) # E: assert_type(list[tuple[Literal[1], ...]], list[tuple[int]]) failed
+assert_type(f((1,)), list[tuple[int]])
 "#,
 );