Star operator gets confused when the type has multiple generics

[KotlinIsland]

Bug Report
When a type has multiple generics and is an iterable, the star operator results in the wrong type

from typing import Iterator, Generic, TypeVar
T = TypeVar("T")
T2 = TypeVar("T2")

class A(Iterator[T2], Generic[T, T2]):
    ...
a: A[int, Iterator[int]]
reveal_type((*a,))
b: list[list[int]] = [*a]

To Reproduce

(Write your steps here:)

1. Step 1...
2. Step 2...
3. Step 3...

Expected Behavior

(Write what you thought would happen.)

Actual Behavior

(Write what happened.)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: na
• Mypy configuration options from mypy.ini (and other config files): na
• Python version used: 3.10
• Operating system and version: mypy playground

[sobolevn]

@KotlinIsland this line does not seem to be correct: b: list[list[int]] = [*a]. That's exactly what error: List item 0 has incompatible type "A[int, Iterator[int]]"; expected "List[int]" tells you about.
Here b should be list[int]:

from typing import Iterator, Generic, TypeVar
T = TypeVar("T")
T2 = TypeVar("T2")

class A(Iterator[T2], Generic[T, T2]):
    ...
a: A[int, Iterator[int]] = A()
reveal_type((*a,))
reveal_type([*a])
b: list[int] = [*a]

It works: https://mypy-play.net/?mypy=latest&python=3.10&gist=e9ba2aada4295a07d24c5508eadfb5f0
Similar to reveal_type((*a,)) which reveals Revealed type is "builtins.tuple[builtins.int*]"

[KotlinIsland]

@sobolevn actually it should be a list[Iterator[int]]

[KotlinIsland]

I think this is clearer: https://mypy-play.net/?mypy=latest&python=3.10&gist=e7a3eb908190fd06c04032459aae0d70

[KotlinIsland]

from typing import Generic, TypeVar
T = TypeVar("T")
T2 = TypeVar("T2")

class A(Iterator[T2], Generic[T, T2]):
    ...
    
a: A[str, int]
reveal_type((*a,))  # note: Revealed type is "builtins.tuple[builtins.str*]"
b: list[int] = [*a]  # error: List item 0 has incompatible type "A[str, int]"; expected "int"

[JelleZijlstra]

@KotlinIsland's latest gist does look pretty buggy. For (*a,) it infers tuple[str, ...], which is wrong because a is an Iterator[Iterator[int]] instead.

[JelleZijlstra]

I like this example of the bug:

from typing import Generic, Iterator, TypeVar
T = TypeVar("T")
U = TypeVar("U")

class X(Iterator[U], Generic[T, U]):
    pass

x: X[str, int]
reveal_type(list(x))  # N: Revealed type is "builtins.list[builtins.int*]"
reveal_type([*x])  # N: Revealed type is "builtins.list[builtins.str*]"

[sobolevn]

Oh, I see. Will try to fix it tomorrow 👍

[sobolevn]

Yeap, solved using map_instance_to_supertype(actual_type, Iterable) here:

mypy/mypy/argmap.py

Lines 168 to 173 in b3ff2a6

	if actual_type.type.fullname == 'builtins.list':
	# List *arg.
	return actual_type.args[0]
	elif actual_type.args:
	# TODO: Try to map type arguments to Iterable
	return actual_type.args[0]

But, I need changes from #11151 to make it work correctly. Right now it is blocked.
@JelleZijlstra I would appreciate your review to get it merged! 🙏

[JelleZijlstra]

Nice catch! I figured we were assuming somewhere that the first generic arg was for Iterable, but couldn't find the right place in the code.