Handle adjacent decorated underscore functions in semanal

Instead of turning adjacent decorated underscore functions into a series
of FuncDefs instead of one OverloadedFuncDef in the parser, handle
OverloadedFuncDefs named '_' in semanal. That prevents us from needing
to make that change in fastparse2, and move the handling of the
underscore function case closer to where the error would be emitted.

Author: pranavrajpal

mypy/fastparse.py

@@ -455,7 +455,7 @@ def fix_function_overloads(self, stmts: List[Statement]) -> List[Statement]:
                 elif len(current_overload) > 1:
                     ret.append(OverloadedFuncDef(current_overload))
 
-                if isinstance(stmt, Decorator) and stmt.name != "_":
+                if isinstance(stmt, Decorator):
                     current_overload = [stmt]
                     current_overload_name = stmt.name
                 else:

mypy/semanal.py

@@ -812,7 +812,7 @@ def handle_missing_overload_decorators(self,
                 else:
                     self.fail("The implementation for an overloaded function "
                               "must come last", defn.items[idx])
-        else:
+        elif defn.name != "_":
             for idx in non_overload_indexes[1:]:
                 self.name_already_defined(defn.name, defn.items[idx], defn.items[0])
             if defn.impl: