Make TypeQuery more general, handling nonboolean queries.

Instead of TypeQuery always returning a boolean and having the strategy be an
enum, the strategy is now a Callable describing how to combine partial results,
and the two default strategies are plain old funcitons.

To preserve the short-circuiting behavior of the previous code, this PR uses an
exception.

This is a pure refactor that I am using in my experimentation regarding fixing
python#1551.  It should result in exactly no
change to current behavior. It's separable from the other things I'm
experimenting with, so I'm filing it as a separate pull request now. It enables
me to rewrite the code that pulls type variables out of types as a TypeQuery.

Consider waiting to merge this PR until I have some code that uses it ready for
review.  Or merge it now, if you think it's a pleasant cleanup instead of an
ugly complication.  I'm of two minds on that particular question.

mypy/checkexpr.py

@@ -2332,7 +2332,7 @@ def replace_callable_return_type(c: CallableType, new_ret_type: Type) -> Callabl
     return c.copy_modified(ret_type=new_ret_type)
 
 
-class ArgInferSecondPassQuery(types.TypeQuery):
+class ArgInferSecondPassQuery(types.TypeQuery[bool]):
     """Query whether an argument type should be inferred in the second pass.
 
     The result is True if the type has a type variable in a callable return
@@ -2346,7 +2346,7 @@ def visit_callable_type(self, t: CallableType) -> bool:
         return self.query_types(t.arg_types) or t.accept(HasTypeVarQuery())
 
 
-class HasTypeVarQuery(types.TypeQuery):
+class HasTypeVarQuery(types.TypeQuery[bool]):
     """Visitor for querying whether a type has a type variable component."""
     def __init__(self) -> None:
         super().__init__(False, types.ANY_TYPE_STRATEGY)
@@ -2359,7 +2359,7 @@ def has_erased_component(t: Type) -> bool:
     return t is not None and t.accept(HasErasedComponentsQuery())
 
 
-class HasErasedComponentsQuery(types.TypeQuery):
+class HasErasedComponentsQuery(types.TypeQuery[bool]):
     """Visitor for querying whether a type has an erased component."""
     def __init__(self) -> None:
         super().__init__(False, types.ANY_TYPE_STRATEGY)

mypy/constraints.py

@@ -250,7 +250,7 @@ def is_complete_type(typ: Type) -> bool:
     return typ.accept(CompleteTypeVisitor())
 
 
-class CompleteTypeVisitor(TypeQuery):
+class CompleteTypeVisitor(TypeQuery[bool]):
     def __init__(self) -> None:
         super().__init__(default=True, strategy=ALL_TYPES_STRATEGY)
 

mypy/stats.py

@@ -226,7 +226,7 @@ def is_imprecise(t: Type) -> bool:
     return t.accept(HasAnyQuery())
 
 
-class HasAnyQuery(TypeQuery):
+class HasAnyQuery(TypeQuery[bool]):
     def __init__(self) -> None:
         super().__init__(False, ANY_TYPE_STRATEGY)
 

mypy/types.py

@@ -5,7 +5,7 @@
 from collections import OrderedDict
 from typing import (
     Any, TypeVar, Dict, List, Tuple, cast, Generic, Set, Sequence, Optional, Union, Iterable,
-    NamedTuple,
+    NamedTuple, Callable,
 )
 
 import mypy.nodes
@@ -1486,112 +1486,112 @@ def keywords_str(self, a: Iterable[Tuple[str, Type]]) -> str:
         ])
 
 
-# These constants define the method used by TypeQuery to combine multiple
-# query results, e.g. for tuple types. The strategy is not used for empty
-# result lists; in that case the default value takes precedence.
-ANY_TYPE_STRATEGY = 0   # Return True if any of the results are True.
-ALL_TYPES_STRATEGY = 1  # Return True if all of the results are True.
+# Combination strategies for boolean type queries
+def ANY_TYPE_STRATEGY(current: bool, accumulated: bool) -> bool:
+    """True if any type's result is True"""
+    if accumulated:
+        raise ShortCircuitQuery()
+    return current
 
 
-class TypeQuery(TypeVisitor[bool]):
-    """Visitor for performing simple boolean queries of types.
+def ALL_TYPES_STRATEGY(current: bool, accumulated: bool) -> bool:
+    """True if all types' results are True"""
+    if not accumulated:
+        raise ShortCircuitQuery()
+    return current
 
-    This class allows defining the default value for leafs to simplify the
-    implementation of many queries.
-    """
 
-    default = False  # Default result
-    strategy = 0     # Strategy for combining multiple values (ANY_TYPE_STRATEGY or ALL_TYPES_...).
+class ShortCircuitQuery(Exception):
+    pass
 
-    def __init__(self, default: bool, strategy: int) -> None:
-        """Construct a query visitor.
 
-        Use the given default result and strategy for combining
-        multiple results. The strategy must be either
-        ANY_TYPE_STRATEGY or ALL_TYPES_STRATEGY.
-        """
+class TypeQuery(Generic[T], TypeVisitor[T]):
+    """Visitor for performing queries of types.
+
+    default is used as the query result unless a method for that type is
+    overridden.
+
+    strategy is used to combine a partial result with a result for a particular
+    type in a series of types.
+
+    Common use cases involve a boolean query using ANY_TYPE_STRATEGY and a
+    default of False or ALL_TYPES_STRATEGY and a default of True.
+    """
+
+    def __init__(self, default: T, strategy: Callable[[T, T], T]) -> None:
         self.default = default
         self.strategy = strategy
 
-    def visit_unbound_type(self, t: UnboundType) -> bool:
+    def visit_unbound_type(self, t: UnboundType) -> T:
         return self.default
 
-    def visit_type_list(self, t: TypeList) -> bool:
+    def visit_type_list(self, t: TypeList) -> T:
         return self.default
 
-    def visit_error_type(self, t: ErrorType) -> bool:
+    def visit_error_type(self, t: ErrorType) -> T:
         return self.default
 
-    def visit_any(self, t: AnyType) -> bool:
+    def visit_any(self, t: AnyType) -> T:
         return self.default
 
-    def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
+    def visit_uninhabited_type(self, t: UninhabitedType) -> T:
         return self.default
 
-    def visit_none_type(self, t: NoneTyp) -> bool:
+    def visit_none_type(self, t: NoneTyp) -> T:
         return self.default
 
-    def visit_erased_type(self, t: ErasedType) -> bool:
+    def visit_erased_type(self, t: ErasedType) -> T:
         return self.default
 
-    def visit_deleted_type(self, t: DeletedType) -> bool:
+    def visit_deleted_type(self, t: DeletedType) -> T:
         return self.default
 
-    def visit_type_var(self, t: TypeVarType) -> bool:
+    def visit_type_var(self, t: TypeVarType) -> T:
         return self.default
 
-    def visit_partial_type(self, t: PartialType) -> bool:
+    def visit_partial_type(self, t: PartialType) -> T:
         return self.default
 
-    def visit_instance(self, t: Instance) -> bool:
+    def visit_instance(self, t: Instance) -> T:
         return self.query_types(t.args)
 
-    def visit_callable_type(self, t: CallableType) -> bool:
+    def visit_callable_type(self, t: CallableType) -> T:
         # FIX generics
         return self.query_types(t.arg_types + [t.ret_type])
 
-    def visit_tuple_type(self, t: TupleType) -> bool:
+    def visit_tuple_type(self, t: TupleType) -> T:
         return self.query_types(t.items)
 
-    def visit_typeddict_type(self, t: TypedDictType) -> bool:
+    def visit_typeddict_type(self, t: TypedDictType) -> T:
         return self.query_types(t.items.values())
 
-    def visit_star_type(self, t: StarType) -> bool:
+    def visit_star_type(self, t: StarType) -> T:
         return t.type.accept(self)
 
-    def visit_union_type(self, t: UnionType) -> bool:
+    def visit_union_type(self, t: UnionType) -> T:
         return self.query_types(t.items)
 
-    def visit_overloaded(self, t: Overloaded) -> bool:
+    def visit_overloaded(self, t: Overloaded) -> T:
         return self.query_types(t.items())
 
-    def visit_type_type(self, t: TypeType) -> bool:
+    def visit_type_type(self, t: TypeType) -> T:
         return t.item.accept(self)
 
-    def query_types(self, types: Iterable[Type]) -> bool:
+    def query_types(self, types: Iterable[Type]) -> T:
         """Perform a query for a list of types.
 
-        Use the strategy constant to combine the results.
+        Use the strategy to combine the results.
         """
         if not types:
             # Use default result for empty list.
             return self.default
-        if self.strategy == ANY_TYPE_STRATEGY:
-            # Return True if at least one component is true.
-            res = False
+        res = self.default
+        try:
             for t in types:
-                res = res or t.accept(self)
-                if res:
-                    break
-            return res
-        else:
-            # Return True if all components are true.
-            res = True
-            for t in types:
-                res = res and t.accept(self)
-                if not res:
-                    break
-            return res
+                res = self.strategy(t.accept(self), res)
+        except ShortCircuitQuery:
+            pass
+        return res
 
 
 def strip_type(typ: Type) -> Type: