Don't always infer unions for conditional expressions (#5095)

Use a join for type of a conditional expression unless the type context
is a union type.

A recent change (#5041) modified the type to always use a union, but this
caused many problems in internal Dropbox repos that we aren't ready to
fix yet.

mypy/checkexpr.py

@@ -2357,7 +2357,14 @@ 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 = UnionType.make_simplified_union([if_type, else_type])
+        # 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(self.type_context[-1], UnionType):
+            res = UnionType.make_simplified_union([if_type, else_type])
+        else:
+            res = join.join_types(if_type, else_type)
 
         return res
 

test-data/unit/check-expressions.test

@@ -1449,8 +1449,11 @@ x = ['x'] # E: List item 0 has incompatible type "str"; expected "int"
 [builtins fixtures/list.pyi]
 
 [case testConditionalExpressionUnion]
+from typing import Union
 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]'
+reveal_type(1 if bool() else '') # E: Revealed type is 'builtins.object'
+x: Union[int, str] = reveal_type(1 if bool() else '') \
+    # E: Revealed type is 'Union[builtins.int, builtins.str]'
 class A:
     pass
 class B(A):
@@ -1464,12 +1467,18 @@ 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]'
+reveal_type(b if bool() else c) # E: Revealed type is 'builtins.object'
+reveal_type(c if bool() else b) # E: Revealed type is 'builtins.object'
+reveal_type(c if bool() else a) # E: Revealed type is 'builtins.object'
+reveal_type(d if bool() else b) # E: Revealed type is '__main__.A'
 [builtins fixtures/bool.pyi]
 
+[case testConditionalExpressionUnionWithAny]
+from typing import Union, Any
+a: Any
+x: Union[int, str] = reveal_type(a if int() else 1)  # E: Revealed type is 'Union[Any, builtins.int]'
+reveal_type(a if int() else 1)  # E: Revealed type is 'Any'
+
 
 -- 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 'Union[def (), def (x: builtins.int)]'
-h(7) # E: Too many arguments for "f"
+reveal_type(h) # E: Revealed type is 'builtins.function'
+h(7) # E: Cannot call function of unknown type
 [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[None, builtins.int]'
+reveal_type(None if bool() else 0)  # E: Revealed type is 'Union[builtins.int, None]'
 [builtins fixtures/bool.pyi]
 
 [case testListWithNone]