Improve async detection mechanism with blacklist

Because the async repl works by wrapping any code that raises
SyntaxError in an async function and trying to execute it again, cell
bodies that are invalid at the top level but valid in functions and
methods (e.g. return and yield statements) currently allow executing
invalid code.

This patch blacklists return and yield statements outside of a function
or method to restore the proper SyntaxError behavior.

IPython/core/async_helpers.py

@@ -1,107 +1,145 b''
 """
 Async helper function that are invalid syntax on Python 3.5 and below.
 
 Known limitation and possible improvement.
 
 Top level code that contain a return statement (instead of, or in addition to
 await) will be detected as requiring being wrapped in async calls. This should
 be prevented as early return will not work.
 """
 
 
 import ast
 import sys
 import inspect
 from textwrap import dedent, indent
 from types import CodeType
 
 
 def _asyncio_runner(coro):
     """
     Handler for asyncio autoawait
     """
     import asyncio
 
     return asyncio.get_event_loop().run_until_complete(coro)
 
 
 def _curio_runner(coroutine):
     """
     handler for curio autoawait
     """
     import curio
 
     return curio.run(coroutine)
 
 
 def _trio_runner(async_fn):
     import trio
     async def loc(coro):
         """
         We need the dummy no-op async def to protect from
         trio's internal. See https://github.com/python-trio/trio/issues/89
         """
         return await coro
     return trio.run(loc, async_fn)
 
 
 def _pseudo_sync_runner(coro):
     """
     A runner that does not really allow async execution, and just advance the coroutine.
 
     See discussion in https://github.com/python-trio/trio/issues/608,
 
     Credit to Nathaniel Smith
 
     """
     try:
         coro.send(None)
     except StopIteration as exc:
         return exc.value
     else:
         # TODO: do not raise but return an execution result with the right info.
         raise RuntimeError("{coro_name!r} needs a real async loop".format(coro_name=coro.__name__))
 
 
 def _asyncify(code: str) -> str:
     """wrap code in async def definition.
 
     And setup a bit of context to run it later.
     """
     res = dedent(
         """
         async def __wrapper__():
             try:
                 {usercode}
             finally:
                 locals()
     """
     ).format(usercode=indent(code, " " * 8)[8:])
     return res
 
 
+class _AsyncSyntaxErrorVisitor(ast.NodeVisitor):
+    """
+    Find syntax errors that would be an error in an async repl, but because
+    the implementation involves wrapping the repl in an async function, it
+    is erroneously allowed (e.g. yield or return at the top level)
+    """
+    def generic_visit(self, node):
+        func_types = (ast.FunctionDef, ast.AsyncFunctionDef)
+        invalid_types = (ast.Return, ast.Yield, ast.YieldFrom)
+
+        if isinstance(node, func_types):
+            return      # Don't recurse into functions
+        elif isinstance(node, invalid_types):
+            raise SyntaxError()
+        else:
+            super().generic_visit(node)
+
+
+def _async_parse_cell(cell: str) -> ast.AST:
+    """
+    This is a compatibility shim for pre-3.7 when async outside of a function
+    is a syntax error at the parse stage.
+
+    It will return an abstract syntax tree parsed as if async and await outside
+    of a function were not a syntax error.
+    """
+    if sys.version_info < (3, 7):
+        # Prior to 3.7 you need to asyncify before parse
+        wrapped_parse_tree = ast.parse(_asyncify(cell))
+        return wrapped_parse_tree.body[0].body[0]
+    else:
+        return ast.parse(cell)
+
+
 def _should_be_async(cell: str) -> bool:
     """Detect if a block of code need to be wrapped in an `async def`
 
     Attempt to parse the block of code, it it compile we're fine.
     Otherwise we  wrap if and try to compile.
 
     If it works, assume it should be async. Otherwise Return False.
 
     Not handled yet: If the block of code has a return statement as  the top
     level, it will be seen as async. This is a know limitation.
     """
 
     try:
         # we can't limit ourself to ast.parse, as it __accepts__ to parse on
         # 3.7+, but just does not _compile_
         compile(cell, "<>", "exec")
         return False
     except SyntaxError:
         try:
-            ast.parse(_asyncify(cell))
-            # TODO verify ast has not "top level" return or yield.
+            parse_tree = _async_parse_cell(cell)
+
+            # Raise a SyntaxError if there are top-level return or yields
+            v = _AsyncSyntaxErrorVisitor()
+            v.visit(parse_tree)
+
         except SyntaxError:
             return False
         return True
     return False