Add support for operators with union operands

mypy/checkexpr.py

@@ -1188,14 +1188,14 @@ def check_overload_call(self,
                 # gives a narrower type.
                 if unioned_return:
                     returns, inferred_types = zip(*unioned_return)
-                    # Note that we use `union_overload_matches` instead of just returning
+                    # Note that we use `combine_function_signatures` instead of just returning
                     # a union of inferred callables because for example a call
                     # Union[int -> int, str -> str](Union[int, str]) is invalid and
                     # we don't want to introduce internal inconsistencies.
                     unioned_result = (UnionType.make_simplified_union(list(returns),
                                                                       context.line,
                                                                       context.column),
-                                      self.union_overload_matches(inferred_types))
+                                      self.combine_function_signatures(inferred_types))
 
         # Step 3: We try checking each branch one-by-one.
         inferred_result = self.infer_overload_return_type(plausible_targets, args, arg_types,
@@ -1492,8 +1492,8 @@ def type_overrides_set(self, exprs: Sequence[Expression],
             for expr in exprs:
                 del self.type_overrides[expr]
 
-    def union_overload_matches(self, types: Sequence[Type]) -> Union[AnyType, CallableType]:
-        """Accepts a list of overload signatures and attempts to combine them together into a
+    def combine_function_signatures(self, types: Sequence[Type]) -> Union[AnyType, CallableType]:
+        """Accepts a list of function signatures and attempts to combine them together into a
         new CallableType consisting of the union of all of the given arguments and return types.
 
         If there is at least one non-callable type, return Any (this can happen if there is
@@ -1507,7 +1507,7 @@ def union_overload_matches(self, types: Sequence[Type]) -> Union[AnyType, Callab
             return callables[0]
 
         # Note: we are assuming here that if a user uses some TypeVar 'T' in
-        # two different overloads, they meant for that TypeVar to mean the
+        # two different functions, they meant for that TypeVar to mean the
         # same thing.
         #
         # This function will make sure that all instances of that TypeVar 'T'
@@ -1525,7 +1525,7 @@ def union_overload_matches(self, types: Sequence[Type]) -> Union[AnyType, Callab
 
         too_complex = False
         for target in callables:
-            # We fall back to Callable[..., Union[<returns>]] if the overloads do not have
+            # We fall back to Callable[..., Union[<returns>]] if the functions do not have
             # the exact same signature. The only exception is if one arg is optional and
             # the other is positional: in that case, we continue unioning (and expect a
             # positional arg).
@@ -1820,19 +1820,12 @@ def check_op_reversible(self,
                             left_expr: Expression,
                             right_type: Type,
                             right_expr: Expression,
-                            context: Context) -> Tuple[Type, Type]:
-        # Note: this kludge exists mostly to maintain compatibility with
-        # existing error messages. Apparently, if the left-hand-side is a
-        # union and we have a type mismatch, we print out a special,
-        # abbreviated error message. (See messages.unsupported_operand_types).
-        unions_present = isinstance(left_type, UnionType)
-
+                            context: Context,
+                            msg: MessageBuilder) -> Tuple[Type, Type]:
         def make_local_errors() -> MessageBuilder:
             """Creates a new MessageBuilder object."""
-            local_errors = self.msg.clean_copy()
+            local_errors = msg.clean_copy()
             local_errors.disable_count = 0
-            if unions_present:
-                local_errors.disable_type_names += 1
             return local_errors
 
         def lookup_operator(op_name: str, base_type: Type) -> Optional[Type]:
@@ -2006,30 +1999,101 @@ def lookup_definer(typ: Instance, attr_name: str) -> Optional[str]:
                 # TODO: Remove this extra case
                 return result
 
-        self.msg.add_errors(errors[0])
+        msg.add_errors(errors[0])
         if len(results) == 1:
             return results[0]
         else:
             error_any = AnyType(TypeOfAny.from_error)
             result = error_any, error_any
             return result
 
-    def check_op(self, method: str, base_type: Type, arg: Expression,
-                 context: Context,
+    def check_op(self, method: str, base_type: Type,
+                 arg: Expression, context: Context,
                  allow_reverse: bool = False) -> Tuple[Type, Type]:
         """Type check a binary operation which maps to a method call.
 
         Return tuple (result type, inferred operator method type).
         """
 
         if allow_reverse:
-            return self.check_op_reversible(
-                op_name=method,
-                left_type=base_type,
-                left_expr=TempNode(base_type),
-                right_type=self.accept(arg),
-                right_expr=arg,
-                context=context)
+            left_variants = [base_type]
+            if isinstance(base_type, UnionType):
+                left_variants = [item for item in base_type.relevant_items()]
+            right_type = self.accept(arg)
+
+            # Step 1: We first try leaving the right arguments alone and destructure
+            # just the left ones. (Mypy can sometimes perform some more precise inference
+            # if we leave the right operands a union -- see testOperatorWithEmptyListAndSum.
+            msg = self.msg.clean_copy()
+            msg.disable_count = 0
+            all_results = []
+            all_inferred = []
+
+            for left_possible_type in left_variants:
+                result, inferred = self.check_op_reversible(
+                    op_name=method,
+                    left_type=left_possible_type,
+                    left_expr=TempNode(left_possible_type),
+                    right_type=right_type,
+                    right_expr=arg,
+                    context=context,
+                    msg=msg)
+                all_results.append(result)
+                all_inferred.append(inferred)
+
+            if not msg.is_errors():
+                results_final = UnionType.make_simplified_union(all_results)
+                inferred_final = UnionType.make_simplified_union(all_inferred)
+                return results_final, inferred_final
+
+            # Step 2: If that fails, we try again but also destructure the right argument.
+            # This is also necessary to make certain edge cases work -- see
+            # testOperatorDoubleUnionInterwovenUnionAdd, for example.
+
+            # Note: We want to pass in the original 'arg' for 'left_expr' and 'right_expr'
+            # whenever possible so that plugins and similar things can introspect on the original
+            # node if possible.
+            #
+            # We don't do the same for the base expression because it could lead to weird
+            # type inference errors -- e.g. see 'testOperatorDoubleUnionSum'.
+            # TODO: Can we use `type_overrides_set()` here?
+            right_variants = [(right_type, arg)]
+            if isinstance(right_type, UnionType):
+                right_variants = [(item, TempNode(item)) for item in right_type.relevant_items()]
+
+            msg = self.msg.clean_copy()
+            msg.disable_count = 0
+            all_results = []
+            all_inferred = []
+
+            for left_possible_type in left_variants:
+                for right_possible_type, right_expr in right_variants:
+                    result, inferred = self.check_op_reversible(
+                        op_name=method,
+                        left_type=left_possible_type,
+                        left_expr=TempNode(left_possible_type),
+                        right_type=right_possible_type,
+                        right_expr=right_expr,
+                        context=context,
+                        msg=msg)
+                    all_results.append(result)
+                    all_inferred.append(inferred)
+
+            if msg.is_errors():
+                self.msg.add_errors(msg)
+                if len(left_variants) >= 2 and len(right_variants) >= 2:
+                    self.msg.warn_both_operands_are_from_unions(context)
+                elif len(left_variants) >= 2:
+                    self.msg.warn_operand_was_from_union("Left", base_type, context)
+                elif len(right_variants) >= 2:
+                    self.msg.warn_operand_was_from_union("Right", right_type, context)
+
+            # See the comment in 'check_overload_call' for more details on why
+            # we call 'combine_function_signature' instead of just unioning the inferred
+            # callable types.
+            results_final = UnionType.make_simplified_union(all_results)
+            inferred_final = self.combine_function_signatures(all_inferred)
+            return results_final, inferred_final
         else:
             return self.check_op_local_by_name(
                 method=method,

mypy/messages.py

@@ -1003,6 +1003,12 @@ def overloaded_signatures_ret_specific(self, index: int, context: Context) -> No
         self.fail('Overloaded function implementation cannot produce return type '
                   'of signature {}'.format(index), context)
 
+    def warn_both_operands_are_from_unions(self, context: Context) -> None:
+        self.note('Both left and right operands are unions', context)
+
+    def warn_operand_was_from_union(self, side: str, original: Type, context: Context) -> None:
+        self.note('{} operand is of type {}'.format(side, self.format(original)), context)
+
     def operator_method_signatures_overlap(
             self, reverse_class: TypeInfo, reverse_method: str, forward_class: Type,
             forward_method: str, context: Context) -> None:

test-data/unit/check-callable.test

@@ -46,7 +46,8 @@ from typing import Callable, Union
 x = 5  # type: Union[int, Callable[[], str], Callable[[], int]]
 
 if callable(x):
-    y = x() + 2 # E: Unsupported operand types for + (likely involving Union)
+    y = x() + 2 # E: Unsupported operand types for + ("str" and "int") \
+                # N: Left operand is of type "Union[str, int]"
 else:
     z = x + 6
 
@@ -60,7 +61,8 @@ x = 5  # type: Union[int, str, Callable[[], str]]
 if callable(x):
     y = x() + 'test'
 else:
-    z = x + 6  # E: Unsupported operand types for + (likely involving Union)
+    z = x + 6  # E: Unsupported operand types for + ("str" and "int") \
+               # N: Left operand is of type "Union[int, str]"
 
 [builtins fixtures/callable.pyi]
 
@@ -153,7 +155,8 @@ x = 5  # type: Union[int, Callable[[], str]]
 if callable(x) and x() == 'test':
     x()
 else:
-    x + 5  # E: Unsupported left operand type for + (some union)
+    x + 5  # E: Unsupported left operand type for + ("Callable[[], str]") \
+           # N: Left operand is of type "Union[int, Callable[[], str]]"
 
 [builtins fixtures/callable.pyi]