Refine parent type when narrowing "lookup" expressions

This diff adds support for the following pattern:

```python
from typing import Enum, List
from enum import Enum

class Key(Enum):
    A = 1
    B = 2

class Foo:
    key: Literal[Key.A]
    blah: List[int]

class Bar:
    key: Literal[Key.B]
    something: List[str]

x: Union[Foo, Bar]
if x.key is Key.A:
    reveal_type(x)  # Revealed type is 'Foo'
else:
    reveal_type(x)  # Revealed type is 'Bar'
```

In short, when we do `x.key is Key.A`, we "propagate" the information
we discovered about `x.key` up one level to refine the type of `x`.

We perform this propagation only when `x` is a Union and only when we
are doing member or index lookups into instances, typeddicts,
namedtuples, and tuples. For indexing operations, we have one additional
limitation: we *must* use a literal expression in order for narrowing
to work at all. Using Literal types or Final instances won't work;
See python#7905 for more details.

To put it another way, this adds support for tagged unions, I guess.

This more or less resolves python#7344.
We currently don't have support for narrowing based on string or int
literals, but that's a separate issue and should be resolved by
python#7169 (which I resumed work
on earlier this week).

mypy/checker.py

@@ -6,7 +6,7 @@
 
 from typing import (
     Dict, Set, List, cast, Tuple, TypeVar, Union, Optional, NamedTuple, Iterator, Iterable,
-    Sequence
+    Mapping, Sequence
 )
 from typing_extensions import Final
 
@@ -47,7 +47,9 @@
 )
 from mypy.typeops import (
     map_type_from_supertype, bind_self, erase_to_bound, make_simplified_union,
-    erase_def_to_union_or_bound, erase_to_union_or_bound,
+    erase_def_to_union_or_bound, erase_to_union_or_bound, coerce_to_literal,
+    try_getting_str_literals_from_type, try_getting_int_literals_from_type,
+    tuple_fallback, lookup_attribute_type, is_singleton_type, try_expanding_enum_to_union,
     true_only, false_only, function_type,
 )
 from mypy import message_registry
@@ -72,9 +74,6 @@
 from mypy.plugin import Plugin, CheckerPluginInterface
 from mypy.sharedparse import BINARY_MAGIC_METHODS
 from mypy.scope import Scope
-from mypy.typeops import (
-    tuple_fallback, coerce_to_literal, is_singleton_type, try_expanding_enum_to_union
-)
 from mypy import state, errorcodes as codes
 from mypy.traverser import has_return_statement, all_return_statements
 from mypy.errorcodes import ErrorCode
@@ -3709,6 +3708,12 @@ def find_isinstance_check(self, node: Expression
 
         Guaranteed to not return None, None. (But may return {}, {})
         """
+        if_map, else_map = self.find_isinstance_check_helper(node)
+        new_if_map = propagate_up_typemap_info(self.type_map, if_map)
+        new_else_map = propagate_up_typemap_info(self.type_map, else_map)
+        return new_if_map, new_else_map
+
+    def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeMap]:
         type_map = self.type_map
         if is_true_literal(node):
             return {}, None
@@ -3835,23 +3840,23 @@ def find_isinstance_check(self, node: Expression
                         else None)
             return if_map, else_map
         elif isinstance(node, OpExpr) and node.op == 'and':
-            left_if_vars, left_else_vars = self.find_isinstance_check(node.left)
-            right_if_vars, right_else_vars = self.find_isinstance_check(node.right)
+            left_if_vars, left_else_vars = self.find_isinstance_check_helper(node.left)
+            right_if_vars, right_else_vars = self.find_isinstance_check_helper(node.right)
 
             # (e1 and e2) is true if both e1 and e2 are true,
             # and false if at least one of e1 and e2 is false.
             return (and_conditional_maps(left_if_vars, right_if_vars),
                     or_conditional_maps(left_else_vars, right_else_vars))
         elif isinstance(node, OpExpr) and node.op == 'or':
-            left_if_vars, left_else_vars = self.find_isinstance_check(node.left)
-            right_if_vars, right_else_vars = self.find_isinstance_check(node.right)
+            left_if_vars, left_else_vars = self.find_isinstance_check_helper(node.left)
+            right_if_vars, right_else_vars = self.find_isinstance_check_helper(node.right)
 
             # (e1 or e2) is true if at least one of e1 or e2 is true,
             # and false if both e1 and e2 are false.
             return (or_conditional_maps(left_if_vars, right_if_vars),
                     and_conditional_maps(left_else_vars, right_else_vars))
         elif isinstance(node, UnaryExpr) and node.op == 'not':
-            left, right = self.find_isinstance_check(node.expr)
+            left, right = self.find_isinstance_check_helper(node.expr)
             return right, left
 
         # Not a supported isinstance check
@@ -4780,3 +4785,96 @@ def has_bool_item(typ: ProperType) -> bool:
         return any(is_named_instance(item, 'builtins.bool')
                    for item in typ.items)
     return False
+
+
+def propagate_up_typemap_info(existing_types: Mapping[Expression, Type],
+                              new_types: TypeMap) -> TypeMap:
+    """Attempts refining parent expressions of any MemberExpr or IndexExprs in new_types.
+
+    Specifically, this function accepts two mappings of expression to original types:
+    the original mapping (existing_types), and a new mapping (new_types) intended to
+    update the original.
+
+    This function iterates through new_types and attempts to use the information to try
+    refining the parent type if
+    """
+    if new_types is None:
+        return None
+    output_map = {}
+    for expr, typ in new_types.items():
+        # The original inferred type should always be present in the output map, of course
+        output_map[expr] = typ
+
+        # Next, check and see if this expression is one that's attempting to
+        # "index" into the parent type. If so, grab both the parent and the "key".
+        keys = []  # type: Sequence[Union[str, int]]
+        if isinstance(expr, MemberExpr):
+            parent_expr = expr.expr
+            parent_type = existing_types.get(parent_expr)
+            variant_name = expr.name
+            keys = [variant_name]
+        elif isinstance(expr, IndexExpr):
+            parent_expr = expr.base
+            parent_type = existing_types.get(parent_expr)
+
+            variant_type = existing_types.get(expr.index)
+            if variant_type is None:
+                continue
+
+            str_literals = try_getting_str_literals_from_type(variant_type)
+            if str_literals is not None:
+                keys = str_literals
+            else:
+                int_literals = try_getting_int_literals_from_type(variant_type)
+                if int_literals is not None:
+                    keys = int_literals
+                else:
+                    continue
+        else:
+            continue
+
+        # We don't try inferring anything if we've either already inferred something for
+        # the parent expression or if the parent somehow doesn't already have an existing type
+        if parent_expr in new_types or parent_type is None:
+            continue
+
+        # If the parent isn't a union, we won't be able to perform any useful refinements.
+        # So, give up and carry on.
+        #
+        # TODO: We currently refine just the immediate parent. Should we also try refining
+        # any parents of the parents?
+        #
+        # One quick-and-dirty way of doing this would be to have the caller repeatedly run
+        # this function until we seem fixpoint, but that seems expensive.
+        parent_type = get_proper_type(parent_type)
+        if not isinstance(parent_type, UnionType):
+            continue
+
+        # Take each potential parent type in the union and try "indexing" into it using.
+        # Does the resulting type overlap with the deduced type of the original expression?
+        # If so, keep the parent type in the union.
+        new_parent_types = []
+        for item in parent_type.items:
+            item = get_proper_type(item)
+            member_types = []
+            for key in keys:
+                t = lookup_attribute_type(item, key)
+                if t is not None:
+                    member_types.append(t)
+            member_type_for_item = make_simplified_union(member_types)
+            if member_type_for_item is None:
+                # We were unable to obtain the member type. So, we give up on refining this
+                # parent type entirely.
+                new_parent_types = []
+                break
+
+            if is_overlapping_types(member_type_for_item, typ):
+                new_parent_types.append(item)
+
+        # If none of the parent types overlap (if we derived an empty union), either
+        # we deliberately aborted or something went wrong. Deriving the uninhabited
+        # type seems unhelpful, so let's just skip refining the parent expression.
+        if new_parent_types:
+            output_map[parent_expr] = make_simplified_union(new_parent_types)
+
+    return output_map

mypy/checkexpr.py

@@ -2704,6 +2704,9 @@ def visit_index_with_type(self, left_type: Type, e: IndexExpr,
         index = e.index
         left_type = get_proper_type(left_type)
 
+        # Visit the index, just to make sure we have a type for it available
+        self.accept(index)
+
         if isinstance(left_type, UnionType):
             original_type = original_type or left_type
             return make_simplified_union([self.visit_index_with_type(typ, e,

mypy/test/testcheck.py

@@ -46,6 +46,7 @@
     'check-isinstance.test',
     'check-lists.test',
     'check-namedtuple.test',
+    'check-narrowing.test',
     'check-typeddict.test',
     'check-type-aliases.test',
     'check-ignore.test',

mypy/typeops.py

@@ -5,12 +5,12 @@
       since these may assume that MROs are ready.
 """
 
-from typing import cast, Optional, List, Sequence, Set
+from typing import cast, Optional, List, Sequence, Set, Union, TypeVar, Type as TypingType
 import sys
 
 from mypy.types import (
     TupleType, Instance, FunctionLike, Type, CallableType, TypeVarDef, Overloaded,
-    TypeVarType, UninhabitedType, FormalArgument, UnionType, NoneType,
+    TypeVarType, UninhabitedType, FormalArgument, UnionType, NoneType, TypedDictType,
     AnyType, TypeOfAny, TypeType, ProperType, LiteralType, get_proper_type, get_proper_types,
     copy_type, TypeAliasType
 )
@@ -43,6 +43,25 @@ def tuple_fallback(typ: TupleType) -> Instance:
     return Instance(info, [join_type_list(typ.items)])
 
 
+def try_getting_instance_fallback(typ: ProperType) -> Optional[Instance]:
+    """Returns the Instance fallback for this type if one exists.
+
+    Otherwise, returns None.
+    """
+    if isinstance(typ, Instance):
+        return typ
+    elif isinstance(typ, TupleType):
+        return tuple_fallback(typ)
+    elif isinstance(typ, TypedDictType):
+        return typ.fallback
+    elif isinstance(typ, FunctionLike):
+        return typ.fallback
+    elif isinstance(typ, LiteralType):
+        return typ.fallback
+    else:
+        return None
+
+
 def type_object_type_from_function(signature: FunctionLike,
                                    info: TypeInfo,
                                    def_info: TypeInfo,
@@ -475,27 +494,66 @@ def try_getting_str_literals(expr: Expression, typ: Type) -> Optional[List[str]]
     2. 'typ' is a LiteralType containing a string
     3. 'typ' is a UnionType containing only LiteralType of strings
     """
-    typ = get_proper_type(typ)
-
     if isinstance(expr, StrExpr):
         return [expr.value]
 
+    # TODO: See if we can eliminate this function and call the below one directly
+    return try_getting_str_literals_from_type(typ)
+
+
+def try_getting_str_literals_from_type(typ: Type) -> Optional[List[str]]:
+    """If the given expression or type corresponds to a string Literal
+    or a union of string Literals, returns a list of the underlying strings.
+    Otherwise, returns None.
+
+    For example, if we had the type 'Literal["foo", "bar"]' as input, this function
+    would return a list of strings ["foo", "bar"].
+    """
+    return try_getting_literals_from_type(typ, str, "builtins.str")
+
+
+def try_getting_int_literals_from_type(typ: Type) -> Optional[List[int]]:
+    """If the given expression or type corresponds to an int Literal
+    or a union of int Literals, returns a list of the underlying ints.
+    Otherwise, returns None.
+
+    For example, if we had the type 'Literal[1, 2, 3]' as input, this function
+    would return a list of ints [1, 2, 3].
+    """
+    return try_getting_literals_from_type(typ, int, "builtins.int")
+
+
+T = TypeVar('T')
+
+
+def try_getting_literals_from_type(typ: Type,
+                                   target_literal_type: TypingType[T],
+                                   target_fullname: str) -> Optional[List[T]]:
+    """If the given expression or type corresponds to a Literal or
+    union of Literals where the underlying values corresponds to the given
+    target type, returns a list of those underlying values. Otherwise,
+    returns None.
+    """
+    typ = get_proper_type(typ)
+
     if isinstance(typ, Instance) and typ.last_known_value is not None:
         possible_literals = [typ.last_known_value]  # type: List[Type]
     elif isinstance(typ, UnionType):
         possible_literals = list(typ.items)
     else:
         possible_literals = [typ]
 
-    strings = []
+    literals = []  # type: List[T]
     for lit in get_proper_types(possible_literals):
-        if isinstance(lit, LiteralType) and lit.fallback.type.fullname() == 'builtins.str':
+        if isinstance(lit, LiteralType) and lit.fallback.type.fullname() == target_fullname:
             val = lit.value
-            assert isinstance(val, str)
-            strings.append(val)
+            if isinstance(val, target_literal_type):
+                literals.append(val)
+            else:
+                return None
         else:
             return None
-    return strings
+    return literals
 
 
 def get_enum_values(typ: Instance) -> List[str]:
@@ -587,3 +645,34 @@ def coerce_to_literal(typ: Type) -> ProperType:
             if len(enum_values) == 1:
                 return LiteralType(value=enum_values[0], fallback=typ)
     return typ
+
+
+def lookup_attribute_type(typ: Type, key: Union[str, int]) -> Optional[Type]:
+    typ = get_proper_type(typ)
+    if isinstance(key, int):
+        # Int keys apply to tuples and namedtuples
+        if isinstance(typ, TupleType):
+            try:
+                return typ.items[key]
+            except IndexError:
+                return None
+    else:
+        # Str keys apply to typed dicts, named tuples, instances, and anything that has
+        # an instance fallback
+        if isinstance(typ, TypedDictType):
+            return typ.items.get(key)
+
+        instance = try_getting_instance_fallback(typ)
+        if instance is None:
+            return None
+
+        symbol = instance.type.get(key)
+        if symbol is None:
+            return None
+
+        if symbol.type is None:
+            return None
+
+        return expand_type_by_instance(symbol.type, instance)
+
+    return None