Support docstrings in class based TypedDict (#3749)

Author: miedzinski

mypy/semanal.py

@@ -1280,7 +1280,7 @@ def check_typeddict_classdef(self, defn: ClassDef,
                 # Still allow pass or ... (for empty TypedDict's).
                 if (not isinstance(stmt, PassStmt) and
                     not (isinstance(stmt, ExpressionStmt) and
-                         isinstance(stmt.expr, EllipsisExpr))):
+                         isinstance(stmt.expr, (EllipsisExpr, StrExpr)))):
                     self.fail(TPDICT_CLASS_ERROR, stmt)
             elif len(stmt.lvalues) > 1 or not isinstance(stmt.lvalues[0], NameExpr):
                 # An assignment, but an invalid one.

test-data/unit/semanal-typeddict.test

@@ -34,3 +34,24 @@ MypyFile:1(
   AssignmentStmt:2(
     NameExpr(Point* [__main__.Point])
     TypedDictExpr:2(Point)))
+
+[case testTypedDictWithDocString]
+from mypy_extensions import TypedDict
+class A(TypedDict):
+    """foo"""
+    x: str
+[builtins fixtures/dict.pyi]
+[out]
+MypyFile:1(
+  ImportFrom:1(mypy_extensions, [TypedDict])
+  ClassDef:2(
+    A
+    BaseTypeExpr(
+      NameExpr(TypedDict [mypy_extensions.TypedDict]))
+    ExpressionStmt:3(
+      StrExpr(foo))
+    AssignmentStmt:4(
+      NameExpr(x)
+      TempNode:-1(
+        Any)
+      str?)))