Disallow yield [from] in async def.

Author: Guido van Rossum

mypy/semanal.py

@@ -1821,7 +1821,10 @@ def visit_yield_from_expr(self, e: YieldFromExpr) -> None:
         if not self.is_func_scope():  # not sure
             self.fail("'yield from' outside function", e, True, blocker=True)
         else:
-            self.function_stack[-1].is_generator = True
+            if self.function_stack[-1].is_coroutine:
+                self.fail("'yield from' in async function", e, True, blocker=True)
+            else:
+                self.function_stack[-1].is_generator = True
         if e.expr:
             e.expr.accept(self)
 
@@ -2074,7 +2077,10 @@ def visit_yield_expr(self, expr: YieldExpr) -> None:
         if not self.is_func_scope():
             self.fail("'yield' outside function", expr, True, blocker=True)
         else:
-            self.function_stack[-1].is_generator = True
+            if self.function_stack[-1].is_coroutine:
+                self.fail("'yield' in async function", expr, True, blocker=True)
+            else:
+                self.function_stack[-1].is_generator = True
         if expr.expr:
             expr.expr.accept(self)
 

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

@@ -79,3 +79,33 @@ async def f() -> None:
 [builtins fixtures/async_await.py]
 [out]
 main: note: In function "f":
+
+[case testNoYieldInAsyncDef]
+# options: fast_parser
+async def f():
+    yield None
+async def g():
+    yield
+async def h():
+    x = yield
+[builtins fixtures/async_await.py]
+[out]
+main: note: In function "f":
+main:3: error: 'yield' in async function
+main: note: In function "g":
+main:5: error: 'yield' in async function
+main: note: In function "h":
+main:7: error: 'yield' in async function
+
+[case testNoYieldFromInAsyncDef]
+# options: fast_parser
+async def f():
+    yield from []
+async def g():
+    x = yield from []
+[builtins fixtures/async_await.py]
+[out]
+main: note: In function "f":
+main:3: error: 'yield from' in async function
+main: note: In function "g":
+main:5: error: 'yield from' in async function

test-data/unit/fixtures/async_await.py

@@ -5,3 +5,4 @@ class type: pass
 class function: pass
 class int: pass
 class str: pass
+class list: pass