Make join(None, T) produce Optional[T] in strict-optional (#1999)

Fixes #1820.

Author: ddfisher

mypy/join.py

@@ -66,6 +66,9 @@ def join_types(s: Type, t: Type) -> Type:
     if isinstance(s, ErasedType):
         return t
 
+    if isinstance(s, UnionType) and not isinstance(t, UnionType):
+        s, t = t, s
+
     if isinstance(s, NoneTyp) and not isinstance(t, NoneTyp):
         s, t = t, s
 
@@ -114,8 +117,12 @@ def visit_none_type(self, t: NoneTyp) -> Type:
         if experiments.STRICT_OPTIONAL:
             if isinstance(self.s, (NoneTyp, UninhabitedType)):
                 return t
+            elif isinstance(self.s, UnboundType):
+                return AnyType()
+            elif isinstance(self.s, Void) or isinstance(self.s, ErrorType):
+                return ErrorType()
             else:
-                return self.default(self.s)
+                return UnionType.make_simplified_union([self.s, t])
         else:
             if not isinstance(self.s, Void):
                 return self.s

test-data/unit/check-optional.test

@@ -330,3 +330,11 @@ def lookup_field(name, obj):
 [out]
 main: note: In function "lookup_field":
 main:10: error: Need type annotation for variable
+
+[case testTernaryWithNone]
+reveal_type(None if bool() else 0)  # E: Revealed type is 'Union[builtins.int, builtins.None]'
+[builtins fixtures/bool.py]
+
+[case testListWithNone]
+reveal_type([0, None, 0])    # E: Revealed type is 'builtins.list[Union[builtins.int, builtins.None]]'
+[builtins fixtures/list.py]