[ty] Add precise inference for unpacking a TypeVar if the TypeVar has an upper bound with a precise tuple spec (#19985)

crates/ty_python_semantic/resources/mdtest/generics/legacy/functions.md

@@ -455,3 +455,45 @@ def overloaded_outer(t: T | None = None) -> None:
     if t is not None:
         inner(t)
 ```
+
+## Unpacking a TypeVar
+
+We can infer precise heterogeneous types from the result of an unpacking operation applied to a type
+variable if the type variable's upper bound is a type with a precise tuple spec:
+
+```py
+from dataclasses import dataclass
+from typing import NamedTuple, Final, TypeVar, Generic
+
+T = TypeVar("T", bound=tuple[int, str])
+
+def f(x: T) -> T:
+    a, b = x
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: str
+    return x
+
+@dataclass
+class Team(Generic[T]):
+    employees: list[T]
+
+def x(team: Team[T]) -> Team[T]:
+    age, name = team.employees[0]
+    reveal_type(age)  # revealed: int
+    reveal_type(name)  # revealed: str
+    return team
+
+class Age(int): ...
+class Name(str): ...
+
+class Employee(NamedTuple):
+    age: Age
+    name: Name
+
+EMPLOYEES: Final = (Employee(name=Name("alice"), age=Age(42)),)
+team = Team(employees=list(EMPLOYEES))
+reveal_type(team.employees)  # revealed: list[Employee]
+age, name = team.employees[0]
+reveal_type(age)  # revealed: Age
+reveal_type(name)  # revealed: Name
+```

crates/ty_python_semantic/resources/mdtest/generics/pep695/functions.md

@@ -454,3 +454,43 @@ def overloaded_outer[T](t: T | None = None) -> None:
     if t is not None:
         inner(t)
 ```
+
+## Unpacking a TypeVar
+
+We can infer precise heterogeneous types from the result of an unpacking operation applied to a
+TypeVar if the TypeVar's upper bound is a type with a precise tuple spec:
+
+```py
+from dataclasses import dataclass
+from typing import NamedTuple, Final
+
+def f[T: tuple[int, str]](x: T) -> T:
+    a, b = x
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: str
+    return x
+
+@dataclass
+class Team[T: tuple[int, str]]:
+    employees: list[T]
+
+def x[T: tuple[int, str]](team: Team[T]) -> Team[T]:
+    age, name = team.employees[0]
+    reveal_type(age)  # revealed: int
+    reveal_type(name)  # revealed: str
+    return team
+
+class Age(int): ...
+class Name(str): ...
+
+class Employee(NamedTuple):
+    age: Age
+    name: Name
+
+EMPLOYEES: Final = (Employee(name=Name("alice"), age=Age(42)),)
+team = Team(employees=list(EMPLOYEES))
+reveal_type(team.employees)  # revealed: list[Employee]
+age, name = team.employees[0]
+reveal_type(age)  # revealed: Age
+reveal_type(name)  # revealed: Name
+```

crates/ty_python_semantic/src/types.rs

@@ -4870,6 +4870,18 @@ impl<'db> Type<'db> {
             Type::TypeAlias(alias) => {
                 return alias.value_type(db).try_iterate_with_mode(db, mode);
             }
+            Type::NonInferableTypeVar(tvar) => match tvar.typevar(db).bound_or_constraints(db) {
+                Some(TypeVarBoundOrConstraints::UpperBound(bound)) => {
+                    return bound.try_iterate_with_mode(db, mode);
+                }
+                // TODO: could we create a "union of tuple specs"...?
+                // (Same question applies to the `Type::Union()` branch lower down)
+                Some(TypeVarBoundOrConstraints::Constraints(_)) | None => {}
+            },
+            Type::TypeVar(_) => unreachable!(
+                "should not be able to iterate over type variable {} in inferable position",
+                self.display(db)
+            ),
             Type::Dynamic(_)
             | Type::FunctionLiteral(_)
             | Type::GenericAlias(_)
@@ -4895,8 +4907,6 @@ impl<'db> Type<'db> {
             | Type::EnumLiteral(_)
             | Type::LiteralString
             | Type::BytesLiteral(_)
-            | Type::TypeVar(_)
-            | Type::NonInferableTypeVar(_)
             | Type::BoundSuper(_)
             | Type::TypeIs(_)
             | Type::TypedDict(_) => {}