Comprehensions trigger is_func_scope() (#20540)

Fixes #20530

Author: A5rocks

mypy/semanal.py

@@ -6297,21 +6297,33 @@ def visit_type_application(self, expr: TypeApplication) -> None:
 
     def visit_list_comprehension(self, expr: ListComprehension) -> None:
         if any(expr.generator.is_async):
-            if not self.is_func_scope() or not self.function_stack[-1].is_coroutine:
+            if (
+                not self.is_func_scope()
+                or not self.function_stack
+                or not self.function_stack[-1].is_coroutine
+            ):
                 self.fail(message_registry.ASYNC_FOR_OUTSIDE_COROUTINE, expr, code=codes.SYNTAX)
 
         expr.generator.accept(self)
 
     def visit_set_comprehension(self, expr: SetComprehension) -> None:
         if any(expr.generator.is_async):
-            if not self.is_func_scope() or not self.function_stack[-1].is_coroutine:
+            if (
+                not self.is_func_scope()
+                or not self.function_stack
+                or not self.function_stack[-1].is_coroutine
+            ):
                 self.fail(message_registry.ASYNC_FOR_OUTSIDE_COROUTINE, expr, code=codes.SYNTAX)
 
         expr.generator.accept(self)
 
     def visit_dictionary_comprehension(self, expr: DictionaryComprehension) -> None:
         if any(expr.is_async):
-            if not self.is_func_scope() or not self.function_stack[-1].is_coroutine:
+            if (
+                not self.is_func_scope()
+                or not self.function_stack
+                or not self.function_stack[-1].is_coroutine
+            ):
                 self.fail(message_registry.ASYNC_FOR_OUTSIDE_COROUTINE, expr, code=codes.SYNTAX)
 
         with self.enter(expr):

test-data/unit/check-async-await.test

@@ -1080,3 +1080,14 @@ class Launcher(P):
 
 [builtins fixtures/async_await.pyi]
 [typing fixtures/typing-async.pyi]
+
+[case testInvalidAsyncForInComprehensionNoCrash]
+# a # is at the end to mark these not as section headers
+[[5 async for _ in [5]] for _ in []]#  # E: "async for" outside async function \
+                                       # E: "list[int]" has no attribute "__aiter__" (not async iterable)
+[{5 async for _ in [5]} for _ in []]#  # E: "async for" outside async function \
+                                       # E: "list[int]" has no attribute "__aiter__" (not async iterable)
+[{5: 5 async for _ in [5]} for _ in []]#  # E: "async for" outside async function \
+                                          # E: "list[int]" has no attribute "__aiter__" (not async iterable)
+[builtins fixtures/async_await.pyi]
+[typing fixtures/typing-async.pyi]