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Oct 23, 2025
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[ty] Improve invalid-argument-type diagnostics where a union type was provided #21044

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[ty] Improve `invalid-argument-type` diagnostics where a union type w…
AlexWaygood Oct 23, 2025
40036e5
simplify impl
AlexWaygood Oct 23, 2025
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24 changes: 24 additions & 0 deletions crates/ty_python_semantic/resources/mdtest/call/function.md
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Original file line number Diff line number Diff line change
Expand Up @@ -820,6 +820,30 @@ def f(x: int = 1, y: str = "foo") -> int:
reveal_type(f(y=2, x="bar")) # revealed: int
```

### Diagnostics for union types where the union is not assignable

<!-- snapshot-diagnostics -->

```py
from typing import Sized

class Foo: ...
class Bar: ...
class Baz: ...

def f(x: Sized): ...
def g(
a: str | Foo,
b: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo,
c: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo | Bar,
d: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo | Bar | Baz,
):
f(a) # error: [invalid-argument-type]
f(b) # error: [invalid-argument-type]
f(c) # error: [invalid-argument-type]
f(d) # error: [invalid-argument-type]
```

## Too many positional arguments

### One too many
Expand Down
135 changes: 135 additions & 0 deletions ...d_-_Call_expression_-_Wrong_argument_type_-_Diagnostics_for_unio…_(5396a8f9e7f88f71).snap
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Original file line number Diff line number Diff line change
@@ -0,0 +1,135 @@
---
source: crates/ty_test/src/lib.rs
expression: snapshot
---
---
mdtest name: function.md - Call expression - Wrong argument type - Diagnostics for union types where the union is not assignable
mdtest path: crates/ty_python_semantic/resources/mdtest/call/function.md
---

# Python source files

## mdtest_snippet.py

```
1 | from typing import Sized
2 |
3 | class Foo: ...
4 | class Bar: ...
5 | class Baz: ...
6 |
7 | def f(x: Sized): ...
8 | def g(
9 | a: str | Foo,
10 | b: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo,
11 | c: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo | Bar,
12 | d: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo | Bar | Baz,
13 | ):
14 | f(a) # error: [invalid-argument-type]
15 | f(b) # error: [invalid-argument-type]
16 | f(c) # error: [invalid-argument-type]
17 | f(d) # error: [invalid-argument-type]
```

# Diagnostics

```
error[invalid-argument-type]: Argument to function `f` is incorrect
--> src/mdtest_snippet.py:14:7
|
12 | d: list[str] | str | dict[str, str] | tuple[str, ...] | bytes | frozenset[str] | set[str] | Foo | Bar | Baz,
13 | ):
14 | f(a) # error: [invalid-argument-type]
| ^ Expected `Sized`, found `str | Foo`
15 | f(b) # error: [invalid-argument-type]
16 | f(c) # error: [invalid-argument-type]
|
info: Element `Foo` of this union is not assignable to `Sized`
info: Function defined here
--> src/mdtest_snippet.py:7:5
|
5 | class Baz: ...
6 |
7 | def f(x: Sized): ...
| ^ -------- Parameter declared here
8 | def g(
9 | a: str | Foo,
|
info: rule `invalid-argument-type` is enabled by default

```

```
error[invalid-argument-type]: Argument to function `f` is incorrect
--> src/mdtest_snippet.py:15:7
|
13 | ):
14 | f(a) # error: [invalid-argument-type]
15 | f(b) # error: [invalid-argument-type]
| ^ Expected `Sized`, found `list[str] | str | dict[str, str] | ... omitted 5 union elements`
16 | f(c) # error: [invalid-argument-type]
17 | f(d) # error: [invalid-argument-type]
|
info: Element `Foo` of this union is not assignable to `Sized`
info: Function defined here
--> src/mdtest_snippet.py:7:5
|
5 | class Baz: ...
6 |
7 | def f(x: Sized): ...
| ^ -------- Parameter declared here
8 | def g(
9 | a: str | Foo,
|
info: rule `invalid-argument-type` is enabled by default

```

```
error[invalid-argument-type]: Argument to function `f` is incorrect
--> src/mdtest_snippet.py:16:7
|
14 | f(a) # error: [invalid-argument-type]
15 | f(b) # error: [invalid-argument-type]
16 | f(c) # error: [invalid-argument-type]
| ^ Expected `Sized`, found `list[str] | str | dict[str, str] | ... omitted 6 union elements`
17 | f(d) # error: [invalid-argument-type]
|
info: Union elements `Foo` and `Bar` are not assignable to `Sized`
info: Function defined here
--> src/mdtest_snippet.py:7:5
|
5 | class Baz: ...
6 |
7 | def f(x: Sized): ...
| ^ -------- Parameter declared here
8 | def g(
9 | a: str | Foo,
|
info: rule `invalid-argument-type` is enabled by default

```

```
error[invalid-argument-type]: Argument to function `f` is incorrect
--> src/mdtest_snippet.py:17:7
|
15 | f(b) # error: [invalid-argument-type]
16 | f(c) # error: [invalid-argument-type]
17 | f(d) # error: [invalid-argument-type]
| ^ Expected `Sized`, found `list[str] | str | dict[str, str] | ... omitted 7 union elements`
|
info: Union element `Foo`, and 2 more union elements, are not assignable to `Sized`
info: Function defined here
--> src/mdtest_snippet.py:7:5
|
5 | class Baz: ...
6 |
7 | def f(x: Sized): ...
| ^ -------- Parameter declared here
8 | def g(
9 | a: str | Foo,
|
info: rule `invalid-argument-type` is enabled by default

```
30 changes: 30 additions & 0 deletions crates/ty_python_semantic/src/types/call/bind.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3519,6 +3519,36 @@ impl<'db> BindingError<'db> {
"Expected `{expected_ty_display}`, found `{provided_ty_display}`"
));

if let Type::Union(union) = provided_ty {
let union_elements = union.elements(context.db());
let invalid_elements: Vec<Type<'db>> = union
.elements(context.db())
.iter()
.filter(|element| !element.is_assignable_to(context.db(), *expected_ty))
.copied()
.collect();
let first_invalid_element = invalid_elements[0].display(context.db());
if invalid_elements.len() < union_elements.len() {
match &invalid_elements[1..] {
[] => diag.info(format_args!(
"Element `{first_invalid_element}` of this union \
is not assignable to `{expected_ty_display}`",
)),
[single] => diag.info(format_args!(
"Union elements `{first_invalid_element}` and `{}` \
are not assignable to `{expected_ty_display}`",
single.display(context.db()),
)),
rest => diag.info(format_args!(
"Union element `{first_invalid_element}`, \
and {} more union elements, \
are not assignable to `{expected_ty_display}`",
rest.len(),
)),
}
}
}
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@sharkdp sharkdp Oct 27, 2025

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I assumed we would fix this somewhere else, e.g. when actually checking assignability in the first place. This code here only affects invalid-argument-type, but we should do the same thing in other situations, like for actual assignments to variables. Consider:

from typing import Sized

class Foo: ...

def g(
    b: list[str]
    | str
    | dict[str, str]
    | tuple[str, ...]
    | bytes
    | frozenset[str]
    | set[str]
    | Foo,
):
    y: Sized = b  # still only shows the truncated union

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@AlexWaygood AlexWaygood Oct 27, 2025

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A more general solution would probably be #19580 (so that we always propagate up the reason why an assignability/subtyping relation did not hold). I do think we need something like that eventually, for better diagnostics in cases where Protocol subtyping doesn't hold (our diagnostics are currently terrible for protocols with lots of members — you can't tell which specific member of the protocol wasn't satisfied). But that would be a big change, and I'm not yet sure how it would fit in with Doug's work on the new constraint solver.

We already do ad-hoc special-casing of union types like this for several other diagnostics. I agree it's not great but I think it's the best we can do for now, and it's better than nothing in the short term.


if let Some(matching_overload) = matching_overload {
if let Some((name_span, parameter_span)) =
matching_overload.get(context.db()).and_then(|overload| {
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