From adefaea0cdc36de91d02c797818ed014f370db0c Mon Sep 17 00:00:00 2001
From: Jukka Lehtosalo <jukka.lehtosalo@iki.fi>
Date: Tue, 22 May 2018 12:23:26 -0700
Subject: [PATCH 1/2] Don't always infer unions for conditional expressions

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                     |  9 ++++++++-
 test-data/unit/check-expressions.test | 19 ++++++++++++++-----
 test-data/unit/check-functions.test   |  4 ++--
 test-data/unit/check-optional.test    |  2 +-
 4 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index 0e349a436dab..d7905ddb348b 100644
--- a/mypy/checkexpr.py
+++ b/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
+        # compatibile with older mypy versions where 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
 
diff --git a/test-data/unit/check-expressions.test b/test-data/unit/check-expressions.test
index 193b6cd5233f..5772238e3fa3 100644
--- a/test-data/unit/check-expressions.test
+++ b/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
 -- -------------
diff --git a/test-data/unit/check-functions.test b/test-data/unit/check-functions.test
index cbad4baa95a2..6817b110b3e0 100644
--- a/test-data/unit/check-functions.test
+++ b/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
diff --git a/test-data/unit/check-optional.test b/test-data/unit/check-optional.test
index 44c83f973a5e..55345d11b996 100644
--- a/test-data/unit/check-optional.test
+++ b/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]

From 0280a6494b8c5831ae4131074284f62a1970fda8 Mon Sep 17 00:00:00 2001
From: Jukka Lehtosalo <jukka.lehtosalo@iki.fi>
Date: Tue, 22 May 2018 13:41:49 -0700
Subject: [PATCH 2/2] Update comment

---
 mypy/checkexpr.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mypy/checkexpr.py b/mypy/checkexpr.py
index d7905ddb348b..463d274b02df 100644
--- a/mypy/checkexpr.py
+++ b/mypy/checkexpr.py
@@ -2357,8 +2357,8 @@ 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)
 
-        # Only create a union type if the type context is a union to be mostly
-        # compatibile with older mypy versions where always did a join.
+        # 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):