Fix type of conditional statement to Union and not Join (#5041)

Fixes #3487

mypy/checker.py

@@ -1328,9 +1328,9 @@ def check_protocol_variance(self, defn: ClassDef) -> None:
         tvars = info.defn.type_vars
         for i, tvar in enumerate(tvars):
             up_args = [object_type if i == j else AnyType(TypeOfAny.special_form)
-                       for j, _ in enumerate(tvars)]
+                       for j, _ in enumerate(tvars)]  # type: List[Type]
             down_args = [UninhabitedType() if i == j else AnyType(TypeOfAny.special_form)
-                         for j, _ in enumerate(tvars)]
+                         for j, _ in enumerate(tvars)]  # type: List[Type]
             up, down = Instance(info, up_args), Instance(info, down_args)
             # TODO: add advanced variance checks for recursive protocols
             if is_subtype(down, up, ignore_declared_variance=True):

mypy/checkexpr.py

@@ -2342,7 +2342,7 @@ def visit_conditional_expr(self, e: ConditionalExpr) -> Type:
             # branch's type.
             else_type = self.analyze_cond_branch(else_map, e.else_expr, context=if_type)
 
-        res = join.join_types(if_type, else_type)
+        res = UnionType.make_simplified_union([if_type, else_type])
 
         return res
 

test-data/unit/check-expressions.test

@@ -1448,6 +1448,28 @@ x = [1]
 x = ['x'] # E: List item 0 has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
 
+[case testConditionalExpressionUnion]
+reveal_type(1 if bool() else 2) # E: Revealed type is 'builtins.int'
+reveal_type(1 if bool() else '') # E: Revealed type is 'Union[builtins.int, builtins.str]'
+class A:
+    pass
+class B(A):
+    pass
+class C:
+    pass
+class D(A):
+    pass
+a = A()
+b = B()
+c = C()
+d = D()
+reveal_type(a if bool() else b) # E: Revealed type is '__main__.A'
+reveal_type(b if bool() else c) # E: Revealed type is 'Union[__main__.B, __main__.C]'
+reveal_type(c if bool() else b) # E: Revealed type is 'Union[__main__.C, __main__.B]'
+reveal_type(c if bool() else a) # E: Revealed type is 'Union[__main__.C, __main__.A]'
+reveal_type(d if bool() else b) # E: Revealed type is 'Union[__main__.D, __main__.B]'
+[builtins fixtures/bool.pyi]
+
 
 -- Special cases
 -- -------------

test-data/unit/check-functions.test

@@ -2064,8 +2064,8 @@ def f() -> None:
 def g(x: int) -> None:
     pass
 h = f if bool() else g
-reveal_type(h) # E: Revealed type is 'builtins.function'
-h(7) # E: Cannot call function of unknown type
+reveal_type(h) # E: Revealed type is 'Union[def (), def (x: builtins.int)]'
+h(7) # E: Too many arguments for "f"
 [builtins fixtures/bool.pyi]
 
 -- Positional-only arguments

test-data/unit/check-optional.test

@@ -385,7 +385,7 @@ def lookup_field(name, obj):
         attr = None
 
 [case testTernaryWithNone]
-reveal_type(None if bool() else 0)  # E: Revealed type is 'Union[builtins.int, builtins.None]'
+reveal_type(None if bool() else 0)  # E: Revealed type is 'Union[builtins.None, builtins.int]'
 [builtins fixtures/bool.pyi]
 
 [case testListWithNone]