[ty] fix a bug with global symbol lookup from eager scopes

Author: mtshiba

crates/ty_python_semantic/resources/mdtest/regression/pr_20962_comprehension_panics.md renamed to crates/ty_python_semantic/resources/corpus/cyclic_symbol_in_comprehension.py

@@ -1,13 +1,7 @@
-# Documentation of two fuzzer panics involving comprehensions
+# Regression test for https://github.com/astral-sh/ruff/pull/20962
+# error message:
+# `place_by_id: execute: too many cycle iterations`
 
-Type inference for comprehensions was added in <https://github.com/astral-sh/ruff/pull/20962>. It
-added two new fuzzer panics that are documented here for regression testing.
-
-## Too many cycle iterations in `place_by_id`
-
-<!-- expect-panic: too many cycle iterations -->
-
-```py
 name_5(name_3)
 [0 for unique_name_0 in unique_name_1 for unique_name_2 in name_3]
 
@@ -34,4 +28,3 @@ def name_3():
 @name_3
 async def name_5():
     pass
-```

crates/ty_python_semantic/resources/mdtest/comprehensions/basic.md

@@ -58,6 +58,24 @@ Iterating over an unbound iterable yields `Unknown`:
 # error: [not-iterable] "Object of type `int` is not iterable"
 # revealed: tuple[int, Unknown]
 [reveal_type((x, z)) for x in range(3) for z in x]
+
+# error: [unresolved-reference] "Name `foo` used when not defined"
+foo
+foo = [
+    # revealed: tuple[int, Unknown]
+    reveal_type((x, z))
+    for x in range(3)
+    # error: [unresolved-reference] "Name `foo` used when not defined"
+    for z in [foo]
+]
+
+baz = [
+    # revealed: tuple[int, Unknown]
+    reveal_type((x, z))
+    for x in range(3)
+    # error: [unresolved-reference] "Name `baz` used when not defined"
+    for z in [baz]
+]
 ```
 
 ## Starred expressions

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

@@ -8022,6 +8022,9 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                                     FileScopeId::global(),
                                     ConstraintKey::NarrowingConstraint(constraint),
                                 ));
+                                // Reaching here means that no bindings are found in any scope.
+                                // Since `explicit_global_symbol` may return a cycle initial value, we return `Place::Undefined` here.
+                                return Place::Undefined.into();
                             }
                             EnclosingSnapshotResult::FoundBindings(bindings) => {
                                 let place = place_from_bindings(db, bindings).map_type(|ty| {