Incorrect default for constrained type variables

[decorator-factory]

From the docs:

https://adaptix.readthedocs.io/en/latest/loading-and-dumping/extended-usage.html

TypeVar	Derived implicit parameter
T = TypeVar('T')	Any
B = TypeVar('B', bound=Book)	Book
C = TypeVar('C', str, bytes)	Union[str, bytes]

The behaviour for the third case is wrong: Union[str, bytes] is not a valid value for C.

A constrained type variable can only be resolved to one of the options, so Thing[int] is valid, Thing[str] is valid, but Thing[str | int], Thing[Literal[0]], Thing[str | Literal[0]], and Thing[bool] are all invalid. (see the specification on TypeVars)

In other words, TypeVar("S", str, int) is not the same as TypeVar("S", bound=str | int).

---

Example code:

from dataclasses import dataclass
from typing import Generic, TypeVar

import adaptix

S = TypeVar("S", str, int)

@dataclass
class Thing(Generic[S]):
    first: S
    second: S

    def should_always_be_true(self) -> bool:
        return (
            isinstance(self.first, str) and isinstance(self.second, str)
            or
            isinstance(self.first, int) and isinstance(self.second, int)
        )


thing = adaptix.load({"first": "hmm", "second": 42}, Thing)
print(thing)  # Thing(first='hmm', second=42)
print(thing.should_always_be_true())  # False

Possible solutions:

• ban unqualified constrained typevars (i.e. Thing[str] is legal as the second argument to load, Thing is not)
• change Thing with no parameter to mean Thing[str] | Thing[int]