From 8d98c601d8fb0c7fafe503c6fa22427357eb5080 Mon Sep 17 00:00:00 2001
From: Dhruv Manilawala <dhruvmanila@gmail.com>
Date: Tue, 3 Jun 2025 19:17:47 +0530
Subject: [PATCH] [ty] Infer `list[T]` when unpacking non-tuple type (#18438)

## Summary

Follow-up from #18401, I was looking at whether that would fix the issue
at https://github.com/astral-sh/ty/issues/247#issuecomment-2917656676
and it didn't, which made me realize that the PR only inferred `list[T]`
when the value type was tuple but it could be other types as well.

This PR fixes the actual issue by inferring `list[T]` for the non-tuple
type case.

## Test Plan

Add test cases for starred expression involved with non-tuple type. I
also added a few test cases for list type and list literal.

I also verified that the example in the linked issue comment works:
```py
def _(line: str):
    a, b, *c = line.split(maxsplit=2)
    c.pop()
```
---
 .../resources/mdtest/unpacking.md             | 69 ++++++++++++++++++-
 .../ty_python_semantic/src/types/unpacker.rs  | 11 ++-
 2 files changed, 75 insertions(+), 5 deletions(-)

diff --git a/crates/ty_python_semantic/resources/mdtest/unpacking.md b/crates/ty_python_semantic/resources/mdtest/unpacking.md
index 93d5c314f6..ac94dd99fe 100644
--- a/crates/ty_python_semantic/resources/mdtest/unpacking.md
+++ b/crates/ty_python_semantic/resources/mdtest/unpacking.md
@@ -1,8 +1,8 @@
 # Unpacking
 
-If there are not enough or too many values ​​when unpacking, an error will occur and the types of
-all variables (if nested tuple unpacking fails, only the variables within the failed tuples) is
-inferred to be `Unknown`.
+If there are not enough or too many values when unpacking, an error will occur and the types of all
+variables (if nested tuple unpacking fails, only the variables within the failed tuples) is inferred
+to be `Unknown`.
 
 ## Tuple
 
@@ -207,6 +207,57 @@ reveal_type(c)  # revealed: int
 reveal_type(d)  # revealed: Literal[2]
 ```
 
+## List
+
+### Literal unpacking
+
+```py
+a, b = [1, 2]
+# TODO: should be `int` for both `a` and `b`
+reveal_type(a)  # revealed: Unknown
+reveal_type(b)  # revealed: Unknown
+```
+
+### Simple unpacking
+
+```py
+def _(value: list[int]):
+    a, b = value
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: int
+```
+
+### Nested unpacking
+
+```py
+def _(value: list[list[int]]):
+    a, (b, c) = value
+    reveal_type(a)  # revealed: list[int]
+    reveal_type(b)  # revealed: int
+    reveal_type(c)  # revealed: int
+```
+
+### Invalid nested unpacking
+
+```py
+def _(value: list[int]):
+    # error: [not-iterable] "Object of type `int` is not iterable"
+    a, (b, c) = value
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: Unknown
+    reveal_type(c)  # revealed: Unknown
+```
+
+### Starred expression
+
+```py
+def _(value: list[int]):
+    a, *b, c = value
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: list[int]
+    reveal_type(c)  # revealed: int
+```
+
 ## String
 
 ### Simple unpacking
@@ -293,6 +344,18 @@ reveal_type(b)  # revealed: LiteralString
 reveal_type(c)  # revealed: list[LiteralString]
 ```
 
+### Starred expression (6)
+
+```py
+from typing_extensions import LiteralString
+
+def _(s: LiteralString):
+    a, b, *c = s
+    reveal_type(a)  # revealed: LiteralString
+    reveal_type(b)  # revealed: LiteralString
+    reveal_type(c)  # revealed: list[LiteralString]
+```
+
 ### Unicode
 
 ```py
diff --git a/crates/ty_python_semantic/src/types/unpacker.rs b/crates/ty_python_semantic/src/types/unpacker.rs
index d790a872dd..07c10ce683 100644
--- a/crates/ty_python_semantic/src/types/unpacker.rs
+++ b/crates/ty_python_semantic/src/types/unpacker.rs
@@ -192,8 +192,15 @@ impl<'db> Unpacker<'db> {
                                 err.fallback_element_type(self.db())
                             })
                         };
-                        for target_type in &mut target_types {
-                            target_type.push(ty);
+                        // Both `elts` and `target_types` are guaranteed to have the same length.
+                        for (element, target_type) in elts.iter().zip(&mut target_types) {
+                            if element.is_starred_expr() {
+                                target_type.push(
+                                    KnownClass::List.to_specialized_instance(self.db(), [ty]),
+                                );
+                            } else {
+                                target_type.push(ty);
+                            }
                         }
                     }
                 }