An empty generic class is covariant even though the doc says "all user-defined generic classes invariant by default"

[hyperkai]

*Memo:

• mypy --strict test.py
• mypy 1.19.0
• Python 3.14.0
• Windows 11

Invariance vs covariance says mypy considers all user-defined generic classes invariant by default as shown below:

Most mutable generic collections are invariant, and mypy considers all user-defined generic classes invariant by default (see Variance of generic types for motivation). This could lead to some unexpected errors when combined with type inference. For example:

But an empty generic class is covariant against what the doc says as shown below:

class A: ...
class B(A): ...
class C(B): ...

class MyCls[T]: ...

mycls1: MyCls[A] = MyCls[B]() # No error
mycls2: MyCls[B] = MyCls[B]() # No error
mycls3: MyCls[C] = MyCls[B]() # Error

[JelleZijlstra]

Yes the docs are wrong here. What's true is that under the old syntax T = TypeVar("T"), a TypeVar is invariant by default. But under the new PEP 695 syntax, classes are covariant by default if they don't use the TypeVar.