Fix joining of Sequence (e.g. variadic tuple) and fixed-length tuple

For example:
* Tuple[int] + Tuple[bool, ...] becomes Tuple[int, ...]
* List[int] + Tuple[bool, ...] becomes Sequence[int]

Previously Mypy simply punted and returned `object`.

Author: intgr

mypy/join.py

@@ -177,6 +177,8 @@ def visit_instance(self, t: Instance) -> ProperType:
             return join_types(t, self.s)
         elif isinstance(self.s, TypedDictType):
             return join_types(t, self.s)
+        elif isinstance(self.s, TupleType):
+            return join_types(t, self.s)
         elif isinstance(self.s, LiteralType):
             return join_types(t, self.s)
         else:
@@ -260,6 +262,15 @@ def visit_overloaded(self, t: Overloaded) -> ProperType:
         return join_types(t.fallback, s)
 
     def visit_tuple_type(self, t: TupleType) -> ProperType:
+        # When given two fixed-length tuples:
+        # * If they have the same length, join their subtypes item-wise:
+        #   Tuple[int, bool] + Tuple[bool, bool] becomes Tuple[int, bool]
+        #
+        # Otherwise, `t` is a fixed-length tuple but `self.s` is NOT:
+        # * Joining with a variadic tuple returns variadic tuple:
+        #   Tuple[int, bool] + Tuple[bool, ...] becomes Tuple[int, ...]
+        # * Joining with any Sequence also returns a Sequence:
+        #   Tuple[int, bool] + List[bool] becomes Sequence[int]
         if isinstance(self.s, TupleType) and self.s.length() == t.length():
             items = []  # type: List[Type]
             for i in range(t.length()):
@@ -269,7 +280,7 @@ def visit_tuple_type(self, t: TupleType) -> ProperType:
             assert isinstance(fallback, Instance)
             return TupleType(items, fallback)
         else:
-            return self.default(self.s)
+            return join_types(self.s, mypy.typeops.tuple_fallback(t))
 
     def visit_typeddict_type(self, t: TypedDictType) -> ProperType:
         if isinstance(self.s, TypedDictType):

mypy/test/testtypes.py

@@ -501,10 +501,10 @@ def test_tuples(self) -> None:
 
         self.assert_join(self.tuple(self.fx.a, self.fx.a),
                          self.fx.std_tuple,
-                         self.fx.o)
+                         self.var_tuple(self.fx.anyt))
         self.assert_join(self.tuple(self.fx.a),
                          self.tuple(self.fx.a, self.fx.a),
-                         self.fx.o)
+                         self.var_tuple(self.fx.a))
 
     def test_function_types(self) -> None:
         self.assert_join(self.callable(self.fx.a, self.fx.b),
@@ -760,6 +760,10 @@ def assert_simple_join(self, s: Type, t: Type, join: Type) -> None:
     def tuple(self, *a: Type) -> TupleType:
         return TupleType(list(a), self.fx.std_tuple)
 
+    def var_tuple(self, t: Type) -> Instance:
+        """Construct a variable-length tuple type"""
+        return Instance(self.fx.std_tuplei, [t])
+
     def callable(self, *a: Type) -> CallableType:
         """callable(a1, ..., an, r) constructs a callable with argument types
         a1, ... an and return type r.

test-data/unit/check-tuples.test

@@ -1077,6 +1077,67 @@ x, y = g(z) # E: Argument 1 to "g" has incompatible type "int"; expected "Tuple[
 [builtins fixtures/tuple.pyi]
 [out]
 
+[case testFixedTupleJoinVarTuple]
+from typing import Tuple
+
+class A: pass
+class B(A): pass
+
+fixtup = None  # type: Tuple[B, B]
+
+vartup_b = None  # type: Tuple[B, ...]
+reveal_type(fixtup if int() else vartup_b)  # N: Revealed type is 'builtins.tuple[__main__.B]'
+reveal_type(vartup_b if int() else fixtup)  # N: Revealed type is 'builtins.tuple[__main__.B]'
+
+vartup_a = None  # type: Tuple[A, ...]
+reveal_type(fixtup if int() else vartup_a)  # N: Revealed type is 'builtins.tuple[__main__.A]'
+reveal_type(vartup_a if int() else fixtup)  # N: Revealed type is 'builtins.tuple[__main__.A]'
+
+
+[builtins fixtures/tuple.pyi]
+[out]
+
+[case testFixedTupleJoinList]
+from typing import Tuple, List
+
+class A: pass
+class B(A): pass
+
+fixtup = None  # type: Tuple[B, B]
+
+lst_b = None  # type: List[B]
+reveal_type(fixtup if int() else lst_b)  # N: Revealed type is 'typing.Sequence[__main__.B]'
+reveal_type(lst_b if int() else fixtup)  # N: Revealed type is 'typing.Sequence[__main__.B]'
+
+lst_a = None  # type: List[A]
+reveal_type(fixtup if int() else lst_a)  # N: Revealed type is 'typing.Sequence[__main__.A]'
+reveal_type(lst_a if int() else fixtup)  # N: Revealed type is 'typing.Sequence[__main__.A]'
+
+[builtins fixtures/tuple.pyi]
+[out]
+
+[case testEmptyTupleJoin]
+from typing import Tuple, List
+
+class A: pass
+
+empty = ()
+
+fixtup = None  # type: Tuple[A]
+reveal_type(fixtup if int() else empty)  # N: Revealed type is 'builtins.tuple[__main__.A]'
+reveal_type(empty if int() else fixtup)  # N: Revealed type is 'builtins.tuple[__main__.A]'
+
+vartup = None  # type: Tuple[A, ...]
+reveal_type(empty if int() else vartup)  # N: Revealed type is 'builtins.tuple[__main__.A]'
+reveal_type(vartup if int() else empty)  # N: Revealed type is 'builtins.tuple[__main__.A]'
+
+lst = None  # type: List[A]
+reveal_type(empty if int() else lst)  # N: Revealed type is 'typing.Sequence[__main__.A*]'
+reveal_type(lst if int() else empty)  # N: Revealed type is 'typing.Sequence[__main__.A*]'
+
+[builtins fixtures/tuple.pyi]
+[out]
+
 [case testTupleWithUndersizedContext]
 a = ([1], 'x')
 if int():