Use Never instead of None for stores (#13984)

## Summary

See:
#13981 (comment)

Author: charliermarsh

crates/red_knot_python_semantic/resources/mdtest/unpacking.md

@@ -81,8 +81,7 @@ reveal_type(b)  # revealed: Literal[2]
 
 ```py
 # TODO: Add diagnostic (need more values to unpack)
-# TODO: Remove 'not-iterable' diagnostic
-[a, *b, c, d] = (1, 2)  # error: "Object of type `None` is not iterable"
+[a, *b, c, d] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[Any] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -93,7 +92,7 @@ reveal_type(d)  # revealed: Unknown
 ### Starred expression (2)
 
 ```py
-[a, *b, c] = (1, 2)  # error: "Object of type `None` is not iterable"
+[a, *b, c] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[Any] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -103,8 +102,7 @@ reveal_type(c)  # revealed: Literal[2]
 ### Starred expression (3)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-[a, *b, c] = (1, 2, 3)  # error: "Object of type `None` is not iterable"
+[a, *b, c] = (1, 2, 3)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[int] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -114,8 +112,7 @@ reveal_type(c)  # revealed: Literal[3]
 ### Starred expression (4)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-[a, *b, c, d] = (1, 2, 3, 4, 5, 6)  # error: "Object of type `None` is not iterable"
+[a, *b, c, d] = (1, 2, 3, 4, 5, 6)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[int] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -126,8 +123,7 @@ reveal_type(d)  # revealed: Literal[6]
 ### Starred expression (5)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-[a, b, *c] = (1, 2, 3, 4)  # error: "Object of type `None` is not iterable"
+[a, b, *c] = (1, 2, 3, 4)
 reveal_type(a)  # revealed: Literal[1]
 reveal_type(b)  # revealed: Literal[2]
 # TODO: Should be list[int] once support for assigning to starred expression is added
@@ -215,8 +211,7 @@ reveal_type(b)  # revealed: LiteralString
 
 ```py
 # TODO: Add diagnostic (need more values to unpack)
-# TODO: Remove 'not-iterable' diagnostic
-(a, *b, c, d) = "ab"  # error: "Object of type `None` is not iterable"
+(a, *b, c, d) = "ab"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -227,7 +222,7 @@ reveal_type(d)  # revealed: Unknown
 ### Starred expression (2)
 
 ```py
-(a, *b, c) = "ab"  # error: "Object of type `None` is not iterable"
+(a, *b, c) = "ab"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[Any] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -237,8 +232,7 @@ reveal_type(c)  # revealed: LiteralString
 ### Starred expression (3)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-(a, *b, c) = "abc"  # error: "Object of type `None` is not iterable"
+(a, *b, c) = "abc"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -248,8 +242,7 @@ reveal_type(c)  # revealed: LiteralString
 ### Starred expression (4)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-(a, *b, c, d) = "abcdef"  # error: "Object of type `None` is not iterable"
+(a, *b, c, d) = "abcdef"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
 reveal_type(b)  # revealed: @Todo
@@ -260,8 +253,7 @@ reveal_type(d)  # revealed: LiteralString
 ### Starred expression (5)
 
 ```py
-# TODO: Remove 'not-iterable' diagnostic
-(a, b, *c) = "abcd"  # error: "Object of type `None` is not iterable"
+(a, b, *c) = "abcd"
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: LiteralString
 # TODO: Should be list[int] once support for assigning to starred expression is added

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1414,11 +1414,11 @@ impl<'db> TypeInferenceBuilder<'db> {
         // Resolve the target type, assuming a load context.
         let target_type = match &**target {
             Expr::Name(name) => {
-                self.store_expression_type(target, Type::None);
+                self.store_expression_type(target, Type::Never);
                 self.infer_name_load(name)
             }
             Expr::Attribute(attr) => {
-                self.store_expression_type(target, Type::None);
+                self.store_expression_type(target, Type::Never);
                 self.infer_attribute_load(attr)
             }
             _ => self.infer_expression(target),
@@ -2506,7 +2506,7 @@ impl<'db> TypeInferenceBuilder<'db> {
     fn infer_name_expression(&mut self, name: &ast::ExprName) -> Type<'db> {
         match name.ctx {
             ExprContext::Load => self.infer_name_load(name),
-            ExprContext::Store | ExprContext::Del => Type::None,
+            ExprContext::Store | ExprContext::Del => Type::Never,
             ExprContext::Invalid => Type::Unknown,
         }
     }
@@ -2536,7 +2536,7 @@ impl<'db> TypeInferenceBuilder<'db> {
             ExprContext::Load => self.infer_attribute_load(attribute),
             ExprContext::Store | ExprContext::Del => {
                 self.infer_expression(value);
-                Type::None
+                Type::Never
             }
             ExprContext::Invalid => {
                 self.infer_expression(value);