Make isinstance/issubclass generate ad-hoc intersections

This diff makes `isinstance(...)` and `issubclass(...)` try
generating ad-hoc intersections of Instances when possible.

For example, we previously concluded the if-branch is unreachable
in the following program. This PR makes mypy infer an ad-hoc
intersection instead.

    class A: pass
    class B: pass

    x: A
    if isinstance(x, B):
        reveal_type(x)  # N: Revealed type is 'test.<subclass of "A" and "B">'

If you try doing an `isinstance(...)` that legitimately is impossible
due to conflicting method signatures or MRO issues, we continue to
declare the branch unreachable. Passing in the `--warn-unreachable`
flag will now also report an error about this:

    # flags: --warn-unreachable
    x: str

    # E: Subclass of "str" and "bytes" cannot exist: would have
    #    incompatible method signatures
    if isinstance(x, bytes):
        reveal_type(x)  # E: Statement is unreachable

This error message has the same limitations as the other
`--warn-unreachable` ones: we suppress them if the isinstance check
is inside a function using TypeVars with multiple values.

However, we *do* end up always inferring an intersection type when
possible -- that logic is never suppressed.

I initially thought we might have to suppress the new logic as well
(see #3603 (comment)),
but it turns out this is a non-issue in practice once you add in
the check that disallows impossible intersections.

For example, when I tried running this PR on the larger of our two
internal codebases, I found about 25 distinct errors, all of which
were legitimate and unrelated to the problem discussed in the PR.

(And if we don't suppress the extra error message, we get about
100-120 errors, mostly due to tests repeatdly doing `result = blah()`
followed by `assert isinstance(result, X)` where X keeps changing.)

mypy/checker.py

@@ -38,7 +38,7 @@
     get_proper_types, is_literal_type, TypeAliasType)
 from mypy.sametypes import is_same_type
 from mypy.messages import (
-    MessageBuilder, make_inferred_type_note, append_invariance_notes,
+    MessageBuilder, make_inferred_type_note, append_invariance_notes, pretty_seq,
     format_type, format_type_bare, format_type_distinctly, SUGGESTED_TEST_FIXTURES
 )
 import mypy.checkexpr
@@ -63,7 +63,7 @@
 from mypy.maptype import map_instance_to_supertype
 from mypy.typevars import fill_typevars, has_no_typevars, fill_typevars_with_any
 from mypy.semanal import set_callable_name, refers_to_fullname
-from mypy.mro import calculate_mro
+from mypy.mro import calculate_mro, MroError
 from mypy.erasetype import erase_typevars, remove_instance_last_known_values, erase_type
 from mypy.expandtype import expand_type, expand_type_by_instance
 from mypy.visitor import NodeVisitor
@@ -1963,13 +1963,15 @@ def visit_block(self, b: Block) -> None:
             return
         for s in b.body:
             if self.binder.is_unreachable():
-                if (self.options.warn_unreachable
-                        and not self.binder.is_unreachable_warning_suppressed()
-                        and not self.is_raising_or_empty(s)):
+                if self.should_report_unreachable_issues() and not self.is_raising_or_empty(s):
                     self.msg.unreachable_statement(s)
                 break
             self.accept(s)
 
+    def should_report_unreachable_issues(self) -> bool:
+        return (self.options.warn_unreachable
+                and not self.binder.is_unreachable_warning_suppressed())
+
     def is_raising_or_empty(self, s: Statement) -> bool:
         """Returns 'true' if the given statement either throws an error of some kind
         or is a no-op.
@@ -3636,6 +3638,78 @@ def visit_continue_stmt(self, s: ContinueStmt) -> None:
         self.binder.handle_continue()
         return None
 
+    def make_fake_typeinfo(self,
+                           curr_module_fullname: str,
+                           class_gen_name: str,
+                           class_short_name: str,
+                           bases: List[Instance],
+                           ) -> Tuple[ClassDef, TypeInfo]:
+        # Build the fake ClassDef and TypeInfo together.
+        # The ClassDef is full of lies and doesn't actually contain a body.
+        # Use format_bare to generate a nice name for error messages.
+        # We skip fully filling out a handful of TypeInfo fields because they
+        # should be irrelevant for a generated type like this:
+        # is_protocol, protocol_members, is_abstract
+        cdef = ClassDef(class_short_name, Block([]))
+        cdef.fullname = curr_module_fullname + '.' + class_gen_name
+        info = TypeInfo(SymbolTable(), cdef, curr_module_fullname)
+        cdef.info = info
+        info.bases = bases
+        calculate_mro(info)
+        info.calculate_metaclass_type()
+        return cdef, info
+
+    def intersect_instances(self,
+                            instances: Sequence[Instance],
+                            ctx: Context,
+                            ) -> Optional[Instance]:
+        curr_module = self.scope.stack[0]
+        assert isinstance(curr_module, MypyFile)
+
+        base_classes = []
+        formatted_names = []
+        for inst in instances:
+            expanded = [inst]
+            if inst.type.is_intersection:
+                expanded = inst.type.bases
+
+            for expanded_inst in expanded:
+                base_classes.append(expanded_inst)
+                formatted_names.append(format_type_bare(expanded_inst))
+
+        pretty_names_list = pretty_seq(format_type_distinctly(*base_classes, bare=True), "and")
+        short_name = '<subclass of {}>'.format(pretty_names_list)
+        full_name = gen_unique_name(short_name, curr_module.names)
+
+        old_msg = self.msg
+        new_msg = self.msg.clean_copy()
+        self.msg = new_msg
+        try:
+            cdef, info = self.make_fake_typeinfo(
+                curr_module.fullname,
+                full_name,
+                short_name,
+                base_classes,
+            )
+            self.check_multiple_inheritance(info)
+            info.is_intersection = True
+        except MroError:
+            if self.should_report_unreachable_issues():
+                old_msg.impossible_intersection(
+                    pretty_names_list, "inconsistent method resolution order", ctx)
+            return None
+        finally:
+            self.msg = old_msg
+
+        if new_msg.is_errors():
+            if self.should_report_unreachable_issues():
+                self.msg.impossible_intersection(
+                    pretty_names_list, "incompatible method signatures", ctx)
+            return None
+
+        curr_module.names[full_name] = SymbolTableNode(GDEF, info)
+        return Instance(info, [])
+
     def intersect_instance_callable(self, typ: Instance, callable_type: CallableType) -> Instance:
         """Creates a fake type that represents the intersection of an Instance and a CallableType.
 
@@ -3650,20 +3724,9 @@ def intersect_instance_callable(self, typ: Instance, callable_type: CallableType
         gen_name = gen_unique_name("<callable subtype of {}>".format(typ.type.name),
                                    cur_module.names)
 
-        # Build the fake ClassDef and TypeInfo together.
-        # The ClassDef is full of lies and doesn't actually contain a body.
-        # Use format_bare to generate a nice name for error messages.
-        # We skip fully filling out a handful of TypeInfo fields because they
-        # should be irrelevant for a generated type like this:
-        # is_protocol, protocol_members, is_abstract
+        # Synthesize a fake TypeInfo
         short_name = format_type_bare(typ)
-        cdef = ClassDef(short_name, Block([]))
-        cdef.fullname = cur_module.fullname + '.' + gen_name
-        info = TypeInfo(SymbolTable(), cdef, cur_module.fullname)
-        cdef.info = info
-        info.bases = [typ]
-        calculate_mro(info)
-        info.calculate_metaclass_type()
+        cdef, info = self.make_fake_typeinfo(cur_module.fullname, gen_name, short_name, [typ])
 
         # Build up a fake FuncDef so we can populate the symbol table.
         func_def = FuncDef('__call__', [], Block([]), callable_type)
@@ -3828,9 +3891,11 @@ def find_isinstance_check_helper(self, node: Expression) -> Tuple[TypeMap, TypeM
                     return {}, {}
                 expr = node.args[0]
                 if literal(expr) == LITERAL_TYPE:
-                    vartype = type_map[expr]
-                    type = get_isinstance_type(node.args[1], type_map)
-                    return conditional_type_map(expr, vartype, type)
+                    return self.conditional_type_map_with_intersection(
+                        expr,
+                        type_map[expr],
+                        get_isinstance_type(node.args[1], type_map),
+                    )
             elif refers_to_fullname(node.callee, 'builtins.issubclass'):
                 if len(node.args) != 2:  # the error will be reported elsewhere
                     return {}, {}
@@ -4309,6 +4374,10 @@ def refine_identity_comparison_expression(self,
 
             if enum_name is not None:
                 expr_type = try_expanding_enum_to_union(expr_type, enum_name)
+
+            # We intentionally use 'conditional_type_map' directly here instead of
+            # 'self.conditional_type_map_with_intersection': we only compute ad-hoc
+            # intersections when working with pure instances.
             partial_type_maps.append(conditional_type_map(expr, expr_type, target_type))
 
         return reduce_conditional_maps(partial_type_maps)
@@ -4726,10 +4795,48 @@ def infer_issubclass_maps(self, node: CallExpr,
             # Any other object whose type we don't know precisely
             # for example, Any or a custom metaclass.
             return {}, {}  # unknown type
-        yes_map, no_map = conditional_type_map(expr, vartype, type)
+        yes_map, no_map = self.conditional_type_map_with_intersection(expr, vartype, type)
         yes_map, no_map = map(convert_to_typetype, (yes_map, no_map))
         return yes_map, no_map
 
+    def conditional_type_map_with_intersection(self,
+                                               expr: Expression,
+                                               expr_type: Type,
+                                               type_ranges: Optional[List[TypeRange]],
+                                               ) -> Tuple[TypeMap, TypeMap]:
+        yes_map, no_map = conditional_type_map(expr, expr_type, type_ranges)
+
+        if yes_map is not None or type_ranges is None:
+            return yes_map, no_map
+
+        # If we couldn't infer anything useful, try again by trying to compute an intersection
+        expr_type = get_proper_type(expr_type)
+        if isinstance(expr_type, UnionType):
+            possible_expr_types = get_proper_types(expr_type.relevant_items())
+        else:
+            possible_expr_types = [expr_type]
+
+        possible_target_types = []
+        for tr in type_ranges:
+            item = get_proper_type(tr.item)
+            if not isinstance(item, Instance) or tr.is_upper_bound:
+                return yes_map, no_map
+            possible_target_types.append(item)
+
+        out = []
+        for v in possible_expr_types:
+            if not isinstance(v, Instance):
+                return yes_map, no_map
+            for t in possible_target_types:
+                intersection = self.intersect_instances([v, t], expr)
+                if intersection is None:
+                    continue
+                out.append(intersection)
+        if len(out) == 0:
+            return None, {}
+        new_yes_type = make_simplified_union(out)
+        return {expr: new_yes_type}, {}
+
 
 def conditional_type_map(expr: Expression,
                          current_type: Optional[Type],

mypy/messages.py

@@ -290,7 +290,11 @@ def has_no_attr(self,
                     if matches:
                         self.fail(
                             '{} has no attribute "{}"; maybe {}?{}'.format(
-                                format_type(original_type), member, pretty_or(matches), extra),
+                                format_type(original_type),
+                                member,
+                                pretty_seq(matches, "or"),
+                                extra,
+                            ),
                             context,
                             code=codes.ATTR_DEFINED)
                         failed = True
@@ -623,7 +627,7 @@ def unexpected_keyword_argument(self, callee: CallableType, name: str, arg_type:
         if not matches:
             matches = best_matches(name, not_matching_type_args)
         if matches:
-            msg += "; did you mean {}?".format(pretty_or(matches[:3]))
+            msg += "; did you mean {}?".format(pretty_seq(matches[:3], "or"))
         self.fail(msg, context, code=codes.CALL_ARG)
         module = find_defining_module(self.modules, callee)
         if module:
@@ -1263,6 +1267,14 @@ def redundant_condition_in_assert(self, truthiness: bool, context: Context) -> N
     def redundant_expr(self, description: str, truthiness: bool, context: Context) -> None:
         self.fail("{} is always {}".format(description, str(truthiness).lower()), context)
 
+    def impossible_intersection(self,
+                                formatted_base_class_list: str,
+                                reason: str,
+                                context: Context,
+                                ) -> None:
+        template = "Subclass of {} cannot exist: would have {}"
+        self.fail(template.format(formatted_base_class_list, reason), context)
+
     def report_protocol_problems(self,
                                  subtype: Union[Instance, TupleType, TypedDictType],
                                  supertype: Instance,
@@ -1995,13 +2007,14 @@ def best_matches(current: str, options: Iterable[str]) -> List[str]:
                   reverse=True, key=lambda v: (ratios[v], v))
 
 
-def pretty_or(args: List[str]) -> str:
+def pretty_seq(args: Sequence[str], conjunction: str) -> str:
     quoted = ['"' + a + '"' for a in args]
     if len(quoted) == 1:
         return quoted[0]
     if len(quoted) == 2:
-        return "{} or {}".format(quoted[0], quoted[1])
-    return ", ".join(quoted[:-1]) + ", or " + quoted[-1]
+        return "{} {} {}".format(quoted[0], conjunction, quoted[1])
+    last_sep = ", " + conjunction + " "
+    return ", ".join(quoted[:-1]) + last_sep + quoted[-1]
 
 
 def append_invariance_notes(notes: List[str], arg_type: Instance,

mypy/nodes.py

@@ -2379,13 +2379,17 @@ class is generic then it will be a type constructor of higher kind.
     # Is this a newtype type?
     is_newtype = False
 
+    # Is this a synthesized intersection type?
+    is_intersection = False
+
     # This is a dictionary that will be serialized and un-serialized as is.
     # It is useful for plugins to add their data to save in the cache.
     metadata = None  # type: Dict[str, JsonDict]
 
     FLAGS = [
         'is_abstract', 'is_enum', 'fallback_to_any', 'is_named_tuple',
         'is_newtype', 'is_protocol', 'runtime_protocol', 'is_final',
+        'is_intersection',
     ]  # type: Final[List[str]]
 
     def __init__(self, names: 'SymbolTable', defn: ClassDef, module_name: str) -> None:

mypy/semanal.py

@@ -81,7 +81,7 @@
 from mypy.typevars import fill_typevars
 from mypy.visitor import NodeVisitor
 from mypy.errors import Errors, report_internal_error
-from mypy.messages import best_matches, MessageBuilder, pretty_or, SUGGESTED_TEST_FIXTURES
+from mypy.messages import best_matches, MessageBuilder, pretty_seq, SUGGESTED_TEST_FIXTURES
 from mypy.errorcodes import ErrorCode
 from mypy import message_registry, errorcodes as codes
 from mypy.types import (
@@ -1802,7 +1802,7 @@ def report_missing_module_attribute(self, import_id: str, source_id: str, import
             alternatives = set(module.names.keys()).difference({source_id})
             matches = best_matches(source_id, alternatives)[:3]
             if matches:
-                suggestion = "; maybe {}?".format(pretty_or(matches))
+                suggestion = "; maybe {}?".format(pretty_seq(matches, "or"))
                 message += "{}".format(suggestion)
         self.fail(message, context, code=codes.ATTR_DEFINED)
         self.add_unknown_imported_symbol(imported_id, context)