6b9f3d7d7c
## Summary `typing.Never` and `typing.LiteralString` are only conditionally exported from `typing` for Python versions 3.11 and later. We run the Markdown tests with the default Python version of 3.9, so here we change the import to `typing_extensions` instead, and add a new test to make sure we'll continue to understand the `typing`-version of these symbols for newer versions. This didn't cause problems so far, as we don't understand `sys.version_info` branches yet. ## Test Plan New Markdown tests to make sure this will continue to work in the future.
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NoReturn & Never
NoReturn is used to annotate the return type for functions that never return. Never is the
bottom type, representing the empty set of Python objects. These two annotations can be used
interchangeably.
Function Return Type Annotation
from typing import NoReturn
def stop() -> NoReturn:
raise RuntimeError("no way")
# revealed: Never
reveal_type(stop())
Assignment
from typing_extensions import NoReturn, Never, Any
# error: [invalid-type-parameter] "Type `typing.Never` expected no type parameter"
x: Never[int]
a1: NoReturn
a2: Never
b1: Any
b2: int
def f():
# revealed: Never
reveal_type(a1)
# revealed: Never
reveal_type(a2)
# Never is assignable to all types.
v1: int = a1
v2: str = a1
# Other types are not assignable to Never except for Never (and Any).
v3: Never = b1
v4: Never = a2
v5: Any = b2
# error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to `Never`"
v6: Never = 1
typing.Never
typing.Never is only available in Python 3.11 and later:
[environment]
target-version = "3.11"
from typing import Never
x: Never
def f():
reveal_type(x) # revealed: Never