list.__add__ isn't permissive enough

[Akuli]

Works at runtime, doesn't type check:

class Foo:
 def asd(self) -> int:
 return 1

class Bar:
 def asd(self) -> int:
 return 2

combined = [Foo()] + [Bar()]
for item in combined:
 print(item.asd())

I would expect [Foo()] + [Bar()] to produce list[Foo | Bar].

Converting to tuples works as expected:

combined = tuple([Foo()]) + tuple([Bar()])

But invariance problems don't apply here, because we're constructing a new list that doesn't refer to the old lists in any way.

There has previously been discussion about how mypy doesn't understand [Foo(), Bar()], but this is slightly different: I'm adding lists, not mixing types within different elements of a single list literal.

[AlexWaygood]

It seems reasonable to change it to:

_T = TypeVar("_T")
_S = TypeVar("_S")

class list(MutableSequence[_T]):
 def __add__(self, __x: list[_S]) -> list[_T | _S]:...

We already do a similar thing with dict.__or__:

typeshed/stdlib/builtins.pyi

Line 1051 in 82745be

def __or__(self, __value: Mapping[_T1, _T2]) -> dict[_KT | _T1, _VT | _T2]: ...

[JelleZijlstra]

This is trickier than it seems, see #3183 for the last attempt.

[Akuli]

I'll try again. The last attempt was done mostly before mypy_primer was a thing, so this time we should at least know if something breaks.