constrained TypeVar does not allow default of one of the constraint choices

[asottile-sentry]

I suspect this is related to #3737 but seems to be a specific sub-case that should be unambigious (and pyright accepts the code unchanged)

Bug Report

a constrained TypeVar T: (U, V) "T must be either U or V" does not accept either U or V as a default value

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&gist=b7cc5486d300e5869277d59fa33a2b03

class U: pass
class V: pass

class C[T: (U, V)]:
    def __init__(self, tp: type[T] = U) -> None:
        self.x: dict[int, T] = {1: tp()}

(I kinda understand that mypy wouldn't know well enough that C() would actually be C[U] -- I've omitted overloads that make that work with mypy -- but the error on the default parameter still remains)

Expected Behavior

(no errors)

Actual Behavior

main.py:5: error: Incompatible default for argument "tp" (default has type "type[U]", argument has type "type[V]")  [assignment]
Found 1 error in 1 file (checked 1 source file)

I'm guessing that for a constrained typevar mypy is just checking each possible constraint against the default value?

Your Environment

• Mypy version used: 1.12.1
• Mypy command-line flags: n/a
• Mypy configuration options from mypy.ini (and other config files): n/a
• Python version used: 3.12.x

[asottile-sentry]

(fwiw I don't think this is PEP 696 related)

[apowers313]

I think I have another flavor of this same bug:

class Thing:
    pass

ThingCls = TypeVar("ThingCls", bound=type[Thing])

def thing_class_modifier(base: ThingCls = Thing) -> ThingCls:
    return base

Yields the error:

Incompatible default for argument "base" (default has type "type[Thing]", argument has type "ThingCls")

I've been told it works fine on pylance.

Mypy version: 1.11.1
Python version: 3.11.10
(can reproduce on mypy-play.net with mypy 1.13.0 and Python 3.12)