Infer unions for ternary expressions (python#17427)

Ref python#12056

cc @JukkaL 

Again, let's check the primer...

mypy/checkexpr.py

@@ -5766,16 +5766,15 @@ def visit_conditional_expr(self, e: ConditionalExpr, allow_none_return: bool = F
                 context=if_type_fallback,
                 allow_none_return=allow_none_return,
             )
-
-        # Only create a union type if the type context is a union, to be mostly
-        # compatible with older mypy versions where we always did a join.
-        #
-        # TODO: Always create a union or at least in more cases?
-        if isinstance(get_proper_type(self.type_context[-1]), UnionType):
-            res: Type = make_simplified_union([if_type, full_context_else_type])
-        else:
-            res = join.join_types(if_type, else_type)
-
+        res: Type = make_simplified_union([if_type, else_type])
+        if has_uninhabited_component(res) and not isinstance(
+            get_proper_type(self.type_context[-1]), UnionType
+        ):
+            # In rare cases with empty collections join may give a better result.
+            alternative = join.join_types(if_type, else_type)
+            p_alt = get_proper_type(alternative)
+            if not isinstance(p_alt, Instance) or p_alt.type.fullname != "builtins.object":
+                res = alternative
         return res
 
     def analyze_cond_branch(

mypyc/test-data/irbuild-any.test

@@ -151,7 +151,9 @@ def f4(a, n, b):
     a :: object
     n :: int
     b :: bool
-    r0, r1, r2, r3 :: object
+    r0 :: union[object, int]
+    r1, r2 :: object
+    r3 :: union[int, object]
     r4 :: int
 L0:
     if b goto L1 else goto L2 :: bool

test-data/unit/check-errorcodes.test

@@ -460,7 +460,7 @@ a: D = {'x': ''}  # E: Incompatible types (expression has type "str", TypedDict
 b: D = {'y': ''}  # E: Missing key "x" for TypedDict "D"  [typeddict-item] \
                   # E: Extra key "y" for TypedDict "D"  [typeddict-unknown-key]
 c = D(x=0) if int() else E(x=0, y=0)
-c = {}  # E: Expected TypedDict key "x" but found no keys  [typeddict-item]
+c = {}  # E: Missing key "x" for TypedDict "D"  [typeddict-item]
 d: D = {'x': '', 'y': 1}  # E: Extra key "y" for TypedDict "D"  [typeddict-unknown-key] \
                           # E: Incompatible types (expression has type "str", TypedDict item "x" has type "int")  [typeddict-item]
 

test-data/unit/check-expressions.test

@@ -1470,10 +1470,9 @@ if int():
 
 [case testConditionalExpressionUnion]
 from typing import Union
-reveal_type(1 if bool() else 2) # N: Revealed type is "builtins.int"
-reveal_type(1 if bool() else '') # N: Revealed type is "builtins.object"
-x: Union[int, str] = reveal_type(1 if bool() else '') \
-    # N: Revealed type is "Union[Literal[1]?, Literal['']?]"
+reveal_type(1 if bool() else 2) # N: Revealed type is "Union[Literal[1]?, Literal[2]?]"
+reveal_type(1 if bool() else '') # N: Revealed type is "Union[Literal[1]?, Literal['']?]"
+x: Union[int, str] = reveal_type(1 if bool() else '') # N: Revealed type is "Union[Literal[1]?, Literal['']?]"
 class A:
     pass
 class B(A):
@@ -1487,17 +1486,17 @@ b = B()
 c = C()
 d = D()
 reveal_type(a if bool() else b) # N: Revealed type is "__main__.A"
-reveal_type(b if bool() else c) # N: Revealed type is "builtins.object"
-reveal_type(c if bool() else b) # N: Revealed type is "builtins.object"
-reveal_type(c if bool() else a) # N: Revealed type is "builtins.object"
-reveal_type(d if bool() else b) # N: Revealed type is "__main__.A"
+reveal_type(b if bool() else c) # N: Revealed type is "Union[__main__.B, __main__.C]"
+reveal_type(c if bool() else b) # N: Revealed type is "Union[__main__.C, __main__.B]"
+reveal_type(c if bool() else a) # N: Revealed type is "Union[__main__.C, __main__.A]"
+reveal_type(d if bool() else b) # N: Revealed type is "Union[__main__.D, __main__.B]"
 [builtins fixtures/bool.pyi]
 
 [case testConditionalExpressionUnionWithAny]
 from typing import Union, Any
 a: Any
 x: Union[int, str] = reveal_type(a if int() else 1)  # N: Revealed type is "Union[Any, Literal[1]?]"
-reveal_type(a if int() else 1)  # N: Revealed type is "Any"
+reveal_type(a if int() else 1)  # N: Revealed type is "Union[Any, Literal[1]?]"
 
 [case testConditionalExpressionStatementNoReturn]
 from typing import List, Union

test-data/unit/check-functions.test

@@ -2250,13 +2250,26 @@ def dec(f: Callable[[A, str], None]) -> Callable[[A, int], None]: pass
 [out]
 
 [case testUnknownFunctionNotCallable]
+from typing import TypeVar
+
 def f() -> None:
     pass
 def g(x: int) -> None:
     pass
 h = f if bool() else g
-reveal_type(h) # N: Revealed type is "builtins.function"
-h(7) # E: Cannot call function of unknown type
+reveal_type(h) # N: Revealed type is "Union[def (), def (x: builtins.int)]"
+h(7) # E: Too many arguments for "f"
+
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
+h2 = join(f, g)
+reveal_type(h2)  # N: Revealed type is "builtins.function"
+h2(7)  # E: Cannot call function of unknown type
+
+h3 = join(g, f)
+reveal_type(h3)  # N: Revealed type is "builtins.function"
+h3(7)  # E: Cannot call function of unknown type
 [builtins fixtures/bool.pyi]
 
 [case testFunctionWithNameUnderscore]

test-data/unit/check-inference-context.test

@@ -701,7 +701,7 @@ class A: pass
 class B(A): pass
 class C(A): pass
 def f(func: Callable[[T], S], *z: T, r: Optional[S] = None) -> S: pass
-reveal_type(f(lambda x: 0 if isinstance(x, B) else 1))  # N: Revealed type is "builtins.int"
+reveal_type(f(lambda x: 0 if isinstance(x, B) else 1))  # N: Revealed type is "Union[Literal[0]?, Literal[1]?]"
 f(lambda x: 0 if isinstance(x, B) else 1, A())() # E: "int" not callable
 f(lambda x: x if isinstance(x, B) else B(), A(), r=B())() # E: "B" not callable
 f(
@@ -1391,15 +1391,15 @@ from typing import Union, List, Any
 
 def f(x: Union[List[str], Any]) -> None:
     a = x if x else []
-    reveal_type(a)  # N: Revealed type is "Union[builtins.list[Union[builtins.str, Any]], builtins.list[builtins.str], Any]"
+    reveal_type(a)  # N: Revealed type is "Union[builtins.list[builtins.str], Any, builtins.list[Union[builtins.str, Any]]]"
 [builtins fixtures/list.pyi]
 
 [case testConditionalExpressionWithEmptyIteableAndUnionWithAny]
 from typing import Union, Iterable, Any
 
 def f(x: Union[Iterable[str], Any]) -> None:
     a = x if x else []
-    reveal_type(a)  # N: Revealed type is "Union[builtins.list[Union[builtins.str, Any]], typing.Iterable[builtins.str], Any]"
+    reveal_type(a)  # N: Revealed type is "Union[typing.Iterable[builtins.str], Any, builtins.list[Union[builtins.str, Any]]]"
 [builtins fixtures/list.pyi]
 
 [case testInferMultipleAnyUnionCovariant]

test-data/unit/check-inference.test

@@ -1438,18 +1438,22 @@ class Wrapper:
 
 def f(cond: bool) -> Any:
     f = Wrapper if cond else lambda x: x
-    reveal_type(f)  # N: Revealed type is "def (x: Any) -> Any"
+    reveal_type(f)  # N: Revealed type is "Union[def (x: Any) -> __main__.Wrapper, def (x: Any) -> Any]"
     return f(3)
 
 def g(cond: bool) -> Any:
     f = lambda x: x if cond else Wrapper
-    reveal_type(f)  # N: Revealed type is "def (x: Any) -> Any"
+    reveal_type(f)  # N: Revealed type is "def (x: Any) -> Union[Any, def (x: Any) -> __main__.Wrapper]"
+    return f(3)
+
+def h(cond: bool) -> Any:
+    f = (lambda x: x) if cond else Wrapper
+    reveal_type(f)  # N: Revealed type is "Union[def (x: Any) -> Any, def (x: Any) -> __main__.Wrapper]"
     return f(3)
 
 -- Boolean operators
 -- -----------------
 
-
 [case testOrOperationWithGenericOperands]
 from typing import List
 a: List[A]

test-data/unit/check-optional.test

@@ -395,7 +395,7 @@ def lookup_field(name, obj):
         attr = None
 
 [case testTernaryWithNone]
-reveal_type(None if bool() else 0)  # N: Revealed type is "Union[Literal[0]?, None]"
+reveal_type(None if bool() else 0)  # N: Revealed type is "Union[None, Literal[0]?]"
 [builtins fixtures/bool.pyi]
 
 [case testListWithNone]

test-data/unit/check-tuples.test

@@ -1228,68 +1228,76 @@ x, y = g(z) # E: Argument 1 to "g" has incompatible type "int"; expected "Tuple[
 [out]
 
 [case testFixedTupleJoinVarTuple]
-from typing import Tuple
+from typing import Tuple, TypeVar
 
 class A: pass
 class B(A): pass
 
 fixtup: Tuple[B, B]
 
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
 vartup_b: 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, ...]"
+reveal_type(join(fixtup, vartup_b))  # N: Revealed type is "builtins.tuple[__main__.B, ...]"
+reveal_type(join(vartup_b, fixtup))  # N: Revealed type is "builtins.tuple[__main__.B, ...]"
 
 vartup_a: 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, ...]"
-
+reveal_type(join(fixtup, vartup_a))  # N: Revealed type is "builtins.tuple[__main__.A, ...]"
+reveal_type(join(vartup_a, fixtup))  # N: Revealed type is "builtins.tuple[__main__.A, ...]"
 
 [builtins fixtures/tuple.pyi]
 [out]
 
 [case testFixedTupleJoinList]
-from typing import Tuple, List
+from typing import Tuple, List, TypeVar
 
 class A: pass
 class B(A): pass
 
 fixtup: Tuple[B, B]
 
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
 lst_b: 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]"
+reveal_type(join(fixtup, lst_b))  # N: Revealed type is "typing.Sequence[__main__.B]"
+reveal_type(join(lst_b, fixtup))  # N: Revealed type is "typing.Sequence[__main__.B]"
 
 lst_a: 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]"
+reveal_type(join(fixtup, lst_a))  # N: Revealed type is "typing.Sequence[__main__.A]"
+reveal_type(join(lst_a, fixtup))  # N: Revealed type is "typing.Sequence[__main__.A]"
 
 [builtins fixtures/tuple.pyi]
 [out]
 
 [case testEmptyTupleJoin]
-from typing import Tuple, List
+from typing import Tuple, List, TypeVar
 
 class A: pass
 
 empty = ()
 
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
 fixtup: 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, ...]"
+reveal_type(join(fixtup, empty))  # N: Revealed type is "builtins.tuple[__main__.A, ...]"
+reveal_type(join(empty, fixtup))  # N: Revealed type is "builtins.tuple[__main__.A, ...]"
 
 vartup: 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, ...]"
+reveal_type(join(vartup, empty))  # N: Revealed type is "builtins.tuple[__main__.A, ...]"
+reveal_type(join(empty, vartup))  # N: Revealed type is "builtins.tuple[__main__.A, ...]"
 
 lst: 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]"
+reveal_type(join(empty, lst))  # N: Revealed type is "typing.Sequence[__main__.A]"
+reveal_type(join(lst, empty))  # N: Revealed type is "typing.Sequence[__main__.A]"
 
 [builtins fixtures/tuple.pyi]
 [out]
 
 [case testTupleSubclassJoin]
-from typing import Tuple, NamedTuple
+from typing import Tuple, NamedTuple, TypeVar
 
 class NTup(NamedTuple):
     a: bool
@@ -1302,32 +1310,38 @@ ntup: NTup
 subtup: SubTuple
 vartup: SubVarTuple
 
-reveal_type(ntup if int() else vartup)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
-reveal_type(subtup if int() else vartup)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
+reveal_type(join(ntup, vartup))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(subtup, vartup))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
 
 [builtins fixtures/tuple.pyi]
 [out]
 
 [case testTupleJoinIrregular]
-from typing import Tuple
+from typing import Tuple, TypeVar
 
 tup1: Tuple[bool, int]
 tup2: Tuple[bool]
 
-reveal_type(tup1 if int() else tup2)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
-reveal_type(tup2 if int() else tup1)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
+reveal_type(join(tup1, tup2))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(tup2, tup1))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
 
-reveal_type(tup1 if int() else ())  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
-reveal_type(() if int() else tup1)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(tup1, ()))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join((), tup1))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
 
-reveal_type(tup2 if int() else ())  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
-reveal_type(() if int() else tup2)  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
+reveal_type(join(tup2, ()))  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
+reveal_type(join((), tup2))  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
 
 [builtins fixtures/tuple.pyi]
 [out]
 
 [case testTupleSubclassJoinIrregular]
-from typing import Tuple, NamedTuple
+from typing import Tuple, NamedTuple, TypeVar
 
 class NTup1(NamedTuple):
     a: bool
@@ -1342,14 +1356,17 @@ tup1: NTup1
 tup2: NTup2
 subtup: SubTuple
 
-reveal_type(tup1 if int() else tup2)  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
-reveal_type(tup2 if int() else tup1)  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
+T = TypeVar("T")
+def join(x: T, y: T) -> T: ...
+
+reveal_type(join(tup1, tup2))  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
+reveal_type(join(tup2, tup1))  # N: Revealed type is "builtins.tuple[builtins.bool, ...]"
 
-reveal_type(tup1 if int() else subtup)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
-reveal_type(subtup if int() else tup1)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(tup1, subtup))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(subtup, tup1))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
 
-reveal_type(tup2 if int() else subtup)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
-reveal_type(subtup if int() else tup2)  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(tup2, subtup))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
+reveal_type(join(subtup, tup2))  # N: Revealed type is "builtins.tuple[builtins.int, ...]"
 
 [builtins fixtures/tuple.pyi]
 [out]