For extending the type alias created with `TypeAlias` or an assignment statement, pyrefly should also give error for consistency

[hyperkai]

Describe the Bug

Memo:

• pyrefly check
• pyrefly 0.46.1
• Python 3.14.0

Extending the type alias created with a type statement or TypeAliasType, pyrefly gives the error as shown below:

''' Type statement '''
type TA = int
''' Type statement '''

''' TypeAliasType '''
# from typing import TypeAliasType

# TA = TypeAliasType('TA', value=int)
''' TypeAliasType '''

class Cls(TA): pass # Error

ERROR Cannot use scoped type alias TA as a base class. Use a legacy type alias instead: TA: TypeAlias = type[int]

But extending the type alias created with TypeAlias or an assignment statement, pyrefly doesn't give error as shown below so error should also occur for consistency:

''' TypeAlias '''
from typing import TypeAlias

TA: TypeAlias = int
''' TypeAlias '''

''' Assignment statement '''
# TA = int
''' Assignment statement '''

class Cls(TA): pass # No error

Sandbox Link

No response

(Only applicable for extension issues) IDE Information

No response

[javabster]

Thanks for reporting, I'm not sure what the expected behaviour should actually be here, perhaps @yangdanny97 can weigh in?

sandbox link here.

[grievejia]

@hyperkai Could you clarify why you think Pyrefly should treat legacy-style type alias and PEP695-style type alias consistently? They are treated differently now because their runtime behaviors are different:

type TA = int
class Foo(TA): pass  # Error at runtime!

from typing import TypeAlias
TB: TypeAlias = int
class Bar(TB): pass  # No error at runtime.