Support typing.NoReturn and typing.Never (#14559)

Fix #14558 
## Summary

- Add `typing.NoReturn` and `typing.Never` to known instances and infer
them as `Type::Never`
- Add `is_assignable_to` cases for `Type::Never`

I skipped emitting diagnostic for when a function is annotated as
`NoReturn` but it actually returns.

## Test Plan

Added tests from

https://github.com/python/typing/blob/main/conformance/tests/specialtypes_never.py
except from generics and checking if the return value of the function
and the annotations match.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>
This commit is contained in:
Shaygan Hooshyari
2024-11-25 22:37:55 +01:00
committed by GitHub
parent f98eebdbab
commit 557d583e32
5 changed files with 101 additions and 8 deletions

View File

@@ -0,0 +1,62 @@
# 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
```py
from typing import NoReturn
def stop() -> NoReturn:
raise RuntimeError("no way")
# revealed: Never
reveal_type(stop())
```
## Assignment
```py
from typing import NoReturn, Never, Any
# error: [invalid-type-parameter] "Type `typing.Never` expected no type parameter"
x: Never[int]
a1: NoReturn
# TODO: Test `Never` is only available in python >= 3.11
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 Extensions
```py
from typing_extensions import NoReturn, Never
x: NoReturn
y: Never
def f():
# revealed: Never
reveal_type(x)
# revealed: Never
reveal_type(y)
```