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[ty] [experiment] Add ReturnsNever constraints only in function scopes. #19114

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[ty] [experiment] Add ReturnsNever constraints only in function scopes. #19114

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253e269
handle NoReturn
abhijeetbodas2001 May 27, 2025
7e45c45
only add ReturnsNever constraints in function scopes?
abhijeetbodas2001 Jul 3, 2025
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34 changes: 30 additions & 4 deletions crates/ty_python_semantic/resources/mdtest/directives/assert_never.md
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Expand Up @@ -2,27 +2,53 @@

## Basic functionality

<!-- snapshot-diagnostics -->
`assert_never` makes sure that the type of the argument is `Never`.

`assert_never` makes sure that the type of the argument is `Never`. If it is not, a
`type-assertion-failure` diagnostic is emitted.
### Correct usage

```py
from typing_extensions import assert_never, Never, Any
from ty_extensions import Unknown

def _(never: Never, any_: Any, unknown: Unknown, flag: bool):
def _(never: Never):
assert_never(never) # fine
```

### Diagnostics

<!-- snapshot-diagnostics -->

If it is not, a `type-assertion-failure` diagnostic is emitted.

```py
from typing_extensions import assert_never, Never, Any
from ty_extensions import Unknown

def _():
assert_never(0) # error: [type-assertion-failure]

def _():
assert_never("") # error: [type-assertion-failure]

def _():
assert_never(None) # error: [type-assertion-failure]

def _():
assert_never([]) # error: [type-assertion-failure]

def _():
assert_never({}) # error: [type-assertion-failure]

def _():
assert_never(()) # error: [type-assertion-failure]

def _(flag: bool, never: Never):
assert_never(1 if flag else never) # error: [type-assertion-failure]

def _(any_: Any):
assert_never(any_) # error: [type-assertion-failure]

def _(unknown: Unknown):
assert_never(unknown) # error: [type-assertion-failure]
```

Expand Down
124 changes: 66 additions & 58 deletions ...sic_functionality_(78e7b52096b8d36c).snap → ...ity_-_Diagnostics_(be8f5d8b0718ee54).snap
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Expand Up @@ -3,7 +3,7 @@ source: crates/ty_test/src/lib.rs
expression: snapshot
---
---
mdtest name: assert_never.md - `assert_never` - Basic functionality
mdtest name: assert_never.md - `assert_never` - Basic functionality - Diagnostics
mdtest path: crates/ty_python_semantic/resources/mdtest/directives/assert_never.md
---

Expand All @@ -15,35 +15,47 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/directives/assert_never.
1 | from typing_extensions import assert_never, Never, Any
2 | from ty_extensions import Unknown
3 |
4 | def _(never: Never, any_: Any, unknown: Unknown, flag: bool):
5 | assert_never(never) # fine
4 | def _():
5 | assert_never(0) # error: [type-assertion-failure]
6 |
7 | assert_never(0) # error: [type-assertion-failure]
7 | def _():
8 | assert_never("") # error: [type-assertion-failure]
9 | assert_never(None) # error: [type-assertion-failure]
10 | assert_never([]) # error: [type-assertion-failure]
11 | assert_never({}) # error: [type-assertion-failure]
12 | assert_never(()) # error: [type-assertion-failure]
13 | assert_never(1 if flag else never) # error: [type-assertion-failure]
14 |
15 | assert_never(any_) # error: [type-assertion-failure]
16 | assert_never(unknown) # error: [type-assertion-failure]
9 |
10 | def _():
11 | assert_never(None) # error: [type-assertion-failure]
12 |
13 | def _():
14 | assert_never([]) # error: [type-assertion-failure]
15 |
16 | def _():
17 | assert_never({}) # error: [type-assertion-failure]
18 |
19 | def _():
20 | assert_never(()) # error: [type-assertion-failure]
21 |
22 | def _(flag: bool, never: Never):
23 | assert_never(1 if flag else never) # error: [type-assertion-failure]
24 |
25 | def _(any_: Any):
26 | assert_never(any_) # error: [type-assertion-failure]
27 |
28 | def _(unknown: Unknown):
29 | assert_never(unknown) # error: [type-assertion-failure]
```

# Diagnostics

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:7:5
--> src/mdtest_snippet.py:5:5
|
5 | assert_never(never) # fine
6 |
7 | assert_never(0) # error: [type-assertion-failure]
4 | def _():
5 | assert_never(0) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^-^
| |
| Inferred type of argument is `Literal[0]`
8 | assert_never("") # error: [type-assertion-failure]
9 | assert_never(None) # error: [type-assertion-failure]
6 |
7 | def _():
|
info: `Never` and `Literal[0]` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -54,13 +66,13 @@ info: rule `type-assertion-failure` is enabled by default
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:8:5
|
7 | assert_never(0) # error: [type-assertion-failure]
7 | def _():
8 | assert_never("") # error: [type-assertion-failure]
| ^^^^^^^^^^^^^--^
| |
| Inferred type of argument is `Literal[""]`
9 | assert_never(None) # error: [type-assertion-failure]
10 | assert_never([]) # error: [type-assertion-failure]
9 |
10 | def _():
|
info: `Never` and `Literal[""]` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -69,16 +81,15 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:9:5
--> src/mdtest_snippet.py:11:5
|
7 | assert_never(0) # error: [type-assertion-failure]
8 | assert_never("") # error: [type-assertion-failure]
9 | assert_never(None) # error: [type-assertion-failure]
10 | def _():
11 | assert_never(None) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^----^
| |
| Inferred type of argument is `None`
10 | assert_never([]) # error: [type-assertion-failure]
11 | assert_never({}) # error: [type-assertion-failure]
12 |
13 | def _():
|
info: `Never` and `None` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -87,16 +98,15 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:10:5
--> src/mdtest_snippet.py:14:5
|
8 | assert_never("") # error: [type-assertion-failure]
9 | assert_never(None) # error: [type-assertion-failure]
10 | assert_never([]) # error: [type-assertion-failure]
13 | def _():
14 | assert_never([]) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^--^
| |
| Inferred type of argument is `list[Unknown]`
11 | assert_never({}) # error: [type-assertion-failure]
12 | assert_never(()) # error: [type-assertion-failure]
15 |
16 | def _():
|
info: `Never` and `list[Unknown]` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -105,16 +115,15 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:11:5
--> src/mdtest_snippet.py:17:5
|
9 | assert_never(None) # error: [type-assertion-failure]
10 | assert_never([]) # error: [type-assertion-failure]
11 | assert_never({}) # error: [type-assertion-failure]
16 | def _():
17 | assert_never({}) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^--^
| |
| Inferred type of argument is `dict[Unknown, Unknown]`
12 | assert_never(()) # error: [type-assertion-failure]
13 | assert_never(1 if flag else never) # error: [type-assertion-failure]
18 |
19 | def _():
|
info: `Never` and `dict[Unknown, Unknown]` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -123,15 +132,15 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:12:5
--> src/mdtest_snippet.py:20:5
|
10 | assert_never([]) # error: [type-assertion-failure]
11 | assert_never({}) # error: [type-assertion-failure]
12 | assert_never(()) # error: [type-assertion-failure]
19 | def _():
20 | assert_never(()) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^--^
| |
| Inferred type of argument is `tuple[()]`
13 | assert_never(1 if flag else never) # error: [type-assertion-failure]
21 |
22 | def _(flag: bool, never: Never):
|
info: `Never` and `tuple[()]` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -140,16 +149,15 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:13:5
--> src/mdtest_snippet.py:23:5
|
11 | assert_never({}) # error: [type-assertion-failure]
12 | assert_never(()) # error: [type-assertion-failure]
13 | assert_never(1 if flag else never) # error: [type-assertion-failure]
22 | def _(flag: bool, never: Never):
23 | assert_never(1 if flag else never) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^--------------------^
| |
| Inferred type of argument is `Literal[1]`
14 |
15 | assert_never(any_) # error: [type-assertion-failure]
24 |
25 | def _(any_: Any):
|
info: `Never` and `Literal[1]` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -158,15 +166,15 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:15:5
--> src/mdtest_snippet.py:26:5
|
13 | assert_never(1 if flag else never) # error: [type-assertion-failure]
14 |
15 | assert_never(any_) # error: [type-assertion-failure]
25 | def _(any_: Any):
26 | assert_never(any_) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^----^
| |
| Inferred type of argument is `Any`
16 | assert_never(unknown) # error: [type-assertion-failure]
27 |
28 | def _(unknown: Unknown):
|
info: `Never` and `Any` are not equivalent types
info: rule `type-assertion-failure` is enabled by default
Expand All @@ -175,10 +183,10 @@ info: rule `type-assertion-failure` is enabled by default

```
error[type-assertion-failure]: Argument does not have asserted type `Never`
--> src/mdtest_snippet.py:16:5
--> src/mdtest_snippet.py:29:5
|
15 | assert_never(any_) # error: [type-assertion-failure]
16 | assert_never(unknown) # error: [type-assertion-failure]
28 | def _(unknown: Unknown):
29 | assert_never(unknown) # error: [type-assertion-failure]
| ^^^^^^^^^^^^^-------^
| |
| Inferred type of argument is `Unknown`
Expand Down
59 changes: 59 additions & 0 deletions crates/ty_python_semantic/resources/mdtest/terminal_statements.md
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Expand Up @@ -570,6 +570,65 @@ def f():
reveal_type(x) # revealed: Literal[1]
```

## Calls to functions returning `Never` / `NoReturn`

### No implicit return

If we see a call to a function returning `Never`, we should be able to understand that the function
cannot implicitly return `None`. In the below examples, verify that there are no errors emitted for
invalid return type.

```py
from typing import NoReturn
import sys

def f() -> NoReturn:
sys.exit(1)
```

Let's try cases where the function annotated with `NoReturn` is some sub-expression.

```py
from typing import NoReturn
import sys

def _() -> NoReturn:
3 + sys.exit(1)

def _() -> NoReturn:
3 if sys.exit(1) else 4
```

### Type narrowing

```py
from typing import NoReturn
import sys

def g(x: int | None):
if x is None:
sys.exit(1)

# TODO: should be just int, not int | None
reveal_type(x) # revealed: int | None
```

### Bindings after call

These should be understood to be unreachable.

```py
import sys

def _():
x = 3

sys.exit(1)

x = 4
reveal_type(x) # revealed: Never
```

## Nested functions

Free references inside of a function body refer to variables defined in the containing scope.
Expand Down
3 changes: 3 additions & 0 deletions crates/ty_python_semantic/resources/primer/bad.txt
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Expand Up @@ -8,6 +8,7 @@ hydpy # too many iterations
ibis # too many iterations
jax # too many iterations
mypy # too many iterations (self-recursive type alias)
nox # too many iterations (because of packaging)
packaging # too many iterations
pandas # slow (9s)
pandera # too many iterations
Expand All @@ -19,4 +20,6 @@ setuptools # vendors packaging, see above
spack # slow, success, but mypy-primer hangs processing the output
spark # too many iterations
steam.py # hangs (single threaded)
streamlit # too many iterations (because of packaging)
tornado # bad use-def map (https://github.com/astral-sh/ty/issues/365)
xarray # too many iterations
2 changes: 0 additions & 2 deletions crates/ty_python_semantic/resources/primer/good.txt
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Expand Up @@ -63,7 +63,6 @@ more-itertools
mypy-protobuf
mypy_primer
nionutils
nox
openlibrary
operator
optuna
Expand Down Expand Up @@ -107,7 +106,6 @@ starlette
static-frame
stone
strawberry
streamlit
svcs
sympy
tornado
Expand Down
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