Do not delete FuncItem.arguments on deserialize

When FuncDef is deserialized we don't reconstruct the arguments.
Previous code was deleting the attribute, leading to python#11899; instead,
always set the attribute, maybe to the empty list, and get argument
names from arg_names if arguments is empty.

mypy/messages.py

@@ -1975,10 +1975,12 @@ def [T <: int] f(self, x: int, y: T) -> None
             s += ' = ...'
 
     # If we got a "special arg" (i.e: self, cls, etc...), prepend it to the arg list
-    if isinstance(tp.definition, FuncDef) and tp.definition.name is not None:
+    if isinstance(tp.definition, FuncDef) and \
+       tp.definition.name is not None and \
+       tp.definition.arguments:
         definition_args = [arg.variable.name for arg in tp.definition.arguments]
         if definition_args and tp.arg_names != definition_args \
-                and len(definition_args) > 0 and definition_args[0]:
+           and len(definition_args) > 0 and definition_args[0]:
             if s:
                 s = ', ' + s
             s = definition_args[0] + s

mypy/nodes.py

@@ -649,7 +649,7 @@ def set_line(self,
 class FuncItem(FuncBase):
     """Base class for nodes usable as overloaded function items."""
 
-    __slots__ = ('arguments',  # Note that can be None if deserialized (type is a lie!)
+    __slots__ = ('arguments',  # Note that can be empty if deserialized
                  'arg_names',  # Names of arguments
                  'arg_kinds',  # Kinds of arguments
                  'min_args',  # Minimum number of arguments
@@ -665,7 +665,7 @@ class FuncItem(FuncBase):
                  'expanded',  # Variants of function with type variables with values expanded
                  )
 
-    __deletable__ = ('arguments', 'max_pos', 'min_args')
+    __deletable__ = ('max_pos', 'min_args')
 
     def __init__(self,
                  arguments: Optional[List[Argument]] = None,
@@ -780,7 +780,6 @@ def deserialize(cls, data: JsonDict) -> 'FuncDef':
         ret.arg_names = data['arg_names']
         ret.arg_kinds = [ArgKind(x) for x in data['arg_kinds']]
         # Leave these uninitialized so that future uses will trigger an error
-        del ret.arguments
         del ret.max_pos
         del ret.min_args
         return ret

mypy/suggestions.py

@@ -345,6 +345,7 @@ def get_args(self, is_method: bool,
         return types
 
     def get_default_arg_types(self, fdef: FuncDef) -> List[Optional[Type]]:
+        assert fdef.arguments is not None
         return [
             self.manager.all_types[arg.initializer] if arg.initializer else None
             for arg in fdef.arguments

mypy/types.py

@@ -1524,16 +1524,13 @@ def __init__(self,
             # after serialization, but it is useful in error messages.
             # TODO: decide how to add more info here (file, line, column)
             # without changing interface hash.
-            self.def_extras = {
-                'first_arg': (
-                    definition.arguments[0].variable.name
-                    if (getattr(definition, 'arguments', None)
-                        and definition.arg_names
-                        and definition.info
-                        and not definition.is_static)
-                    else None
-                ),
-            }
+            first_arg: Optional[str] = None
+            if definition.info and not definition.is_static:
+                if definition.arguments:
+                    first_arg = definition.arguments[0].variable.name
+                elif definition.arg_names:
+                    first_arg = definition.arg_names[0]
+            self.def_extras = {'first_arg': first_arg}
         else:
             self.def_extras = {}
         self.type_guard = type_guard

mypyc/irbuild/mapper.py

@@ -145,7 +145,7 @@ def fdef_to_sig(self, fdef: FuncDef) -> FuncSignature:
         # deserialized FuncDef that lacks arguments. We won't ever
         # need to use those inside of a FuncIR, so we just make up
         # some crap.
-        if hasattr(fdef, 'arguments'):
+        if fdef.arguments:
             arg_names = [arg.variable.name for arg in fdef.arguments]
         else:
             arg_names = [name or '' for name in fdef.arg_names]