Problem observed using current HEAD (0.730+dev.d2a7f05d27d9ac5c096c61ecf0455edda91f5d5a) but can confirmed on 0.660 as well, so it is nothing new.

Flags used:

• --python-version 3.7 --strict-optional --no-site-packages --show-traceback --no-implicit-optional
• Also confirmed with no flags at all (Python 3.7)

Problem occurs:

• With__mod__ /__rmod__ pair.
• When other in__mod__ isUnion[T, str], whereT can be self type, or some other type (let's sayint).
• When other in__rmod__ isstr.
• When return type annotation for__rmod__ is provided,

Reproducible example:

fromtypingimportUnionclassFoo:def__mod__(self,other:Union[Foo,str])->Foo: ...def__rmod__(self,other:str)->Foo: ...

yields

foo.py:5: error: Signatures of "__rmod__" of "Foo" and "__mod__" of "str" are unsafely overlapping

Issue is specific to__mod__ /__rmod__ pair, i.e. this works just fine:

fromtypingimportUnionclassFoo:def__mul__(self,other:Union[Foo,str])->Foo: ...def__rmul__(self,other:str)->Foo: ...

and I haven't found any other problematic pair.

As far as I can tell it strictly depends onother beingstr. For example this works just fine

fromtypingimportUnionclassFoo:def__mod__(self,other:Union[Foo,int])->Foo: ...def__rmod__(self,other:int)->Foo: ...

Additionally, to reproduce the problem, we need a return type annotation on__rmod__, and removing it is sufficient to pass type check (removing return type annotation from__mod__ has no impact at all):

fromtypingimportUnionclassFoo:def__mod__(self,other:Union[Foo,str])->Foo: ...def__rmod__(self,other:str): ...

though failure is not specifically caused by referencing self type, as for example this, will fail as well

fromtypingimportUnionclassFoo:def__mod__(self,other:Union[Foo,str])->int: ...def__rmod__(self,other:str)->int: ...

foo.py:5: error: Signatures of "__rmod__" of "Foo" and "__mod__" of "str" are unsafely overlapping