Forward-referencing type application with type variable in a value expression in stubs emits confusing error

Bug Report, To Reproduce, & Actual Behaviour

In .pyi stub files, the spec mandates type-checkers to recognise forward references in annotation expressions without string-quoting. It is however silent about value expressions, but in mypy I've used symbols defined later in a stub file in a value context before without any issues, except recently when hitting the following example:

# stub.pyi
from typing_extensions import Generic, TypeVar

R = TypeVar("R")
T = TypeVar("T")

class A(Generic[R]):
    var = B[R]()  # E: "R" is a type variable and only valid in type context

class B(Generic[T]): ...

Expected Behavior

Either:

1. no errors, or
2. mypy doesn't support forward references in value contexts for stub files, and this emits a used-before-def error at B[R]().

Your Environment

• Mypy version used: 1.17
• Python version used: 3.11, 3.12

从 stub.pyi 复现用例开始，检查 mypy 如何处理 stub 文件值表达式中的前向引用。将当前诊断结果与 issue 中描述的两种预期结果进行比较；当该行为要么被接受，要么在 B[R]() 处报告为 used-before-def 时，即表示完成。