Add a SYMBOL_FUNCBASE_TYPES constant to avoid multiple-inheritance bug (#6481)

A mypy bug (#3603) causes mypy to think that SymbolNodes cannot be
FuncBases, which causes mypy-mypyc to crash in code that tests
that.
Currently we work around this in a couple gross ways (like turning the
SymbolNode into Any). Work around it instead by testing directly
against a tuple of the subtypes instead of FuncBase itself.

This is the better fix promised in #6480.

Author: msullivan

mypy/checkmember.py

@@ -1,6 +1,6 @@
 """Type checking of attribute access"""
 
-from typing import cast, Callable, List, Optional, TypeVar, Any
+from typing import cast, Callable, List, Optional, TypeVar
 
 from mypy.types import (
     Type, Instance, AnyType, TupleType, TypedDictType, CallableType, FunctionLike, TypeVarDef,
@@ -10,7 +10,7 @@
 from mypy.nodes import (
     TypeInfo, FuncBase, Var, FuncDef, SymbolNode, Context, MypyFile, TypeVarExpr,
     ARG_POS, ARG_STAR, ARG_STAR2, Decorator, OverloadedFuncDef, TypeAlias, TempNode,
-    is_final_node
+    is_final_node, SYMBOL_FUNCBASE_TYPES,
 )
 from mypy.messages import MessageBuilder
 from mypy.maptype import map_instance_to_supertype
@@ -624,12 +624,10 @@ def analyze_class_attribute_access(itype: Instance,
             return mx.chk.handle_partial_var_type(t, mx.is_lvalue, symnode, mx.context)
 
         # Find the class where method/variable was defined.
-        # mypyc hack to workaround mypy misunderstanding multiple inheritance (#3603)
-        node_node = node.node  # type: Any
-        if isinstance(node_node, Decorator):
-            super_info = node_node.var.info  # type: Optional[TypeInfo]
-        elif isinstance(node_node, (Var, FuncBase)):
-            super_info = node_node.info
+        if isinstance(node.node, Decorator):
+            super_info = node.node.var.info  # type: Optional[TypeInfo]
+        elif isinstance(node.node, (Var, SYMBOL_FUNCBASE_TYPES)):
+            super_info = node.node.info
         else:
             super_info = None
 

mypy/nodes.py

@@ -403,7 +403,18 @@ def __str__(self) -> str:
 
 
 class FuncBase(Node):
-    """Abstract base class for function-like nodes"""
+    """Abstract base class for function-like nodes.
+
+    N.B: Although this has SymbolNode subclasses (FuncDef,
+    OverloadedFuncDef), avoid calling isinstance(..., FuncBase) on
+    something that is typed as SymbolNode.  This is to work around
+    mypy bug #3603, in which mypy doesn't understand multiple
+    inheritance very well, and will assume that a SymbolNode
+    cannot be a FuncBase.
+
+    Instead, test against SYMBOL_FUNCBASE_TYPES, which enumerates
+    SymbolNode subclasses that are also FuncBase subclasses.
+    """
 
     __slots__ = ('type',
                  'unanalyzed_type',
@@ -668,6 +679,11 @@ def deserialize(cls, data: JsonDict) -> 'FuncDef':
         return ret
 
 
+# All types that are both SymbolNodes and FuncBases. See the FuncBase
+# docstring for the rationale.
+SYMBOL_FUNCBASE_TYPES = (OverloadedFuncDef, FuncDef)
+
+
 class Decorator(SymbolNode, Statement):
     """A decorated function.
 
@@ -2857,10 +2873,7 @@ def fullname(self) -> Optional[str]:
     @property
     def type(self) -> 'Optional[mypy.types.Type]':
         node = self.node
-        if (isinstance(node, Var) and node.type is not None):
-            return node.type
-        # mypy thinks this branch is unreachable but it is wrong (#3603)
-        elif (isinstance(node, FuncBase) and node.type is not None):
+        if isinstance(node, (Var, SYMBOL_FUNCBASE_TYPES)) and node.type is not None:
             return node.type
         elif isinstance(node, Decorator):
             return node.var.type

mypy/plugins/common.py

@@ -1,7 +1,7 @@
 from typing import List, Optional, Any
 
 from mypy.nodes import (
-    ARG_POS, MDEF, Argument, Block, CallExpr, Expression, FuncBase,
+    ARG_POS, MDEF, Argument, Block, CallExpr, Expression, FuncBase, SYMBOL_FUNCBASE_TYPES,
     FuncDef, PassStmt, RefExpr, SymbolTableNode, Var, StrExpr,
 )
 from mypy.plugin import ClassDefContext
@@ -51,9 +51,8 @@ def _get_argument(call: CallExpr, name: str) -> Optional[Expression]:
         return None
 
     callee_type = None
-    # mypyc hack to workaround mypy misunderstanding multiple inheritance (#3603)
-    callee_node = call.callee.node  # type: Any
-    if (isinstance(callee_node, (Var, FuncBase))
+    callee_node = call.callee.node
+    if (isinstance(callee_node, (Var, SYMBOL_FUNCBASE_TYPES))
             and callee_node.type):
         callee_node_type = callee_node.type
         if isinstance(callee_node_type, Overloaded):