Handle unions in augmented assignments (#14045)

## Summary

Removing more TODOs from the augmented assignment test suite. Now, if
the _target_ is a union, we correctly infer the union of results:

```python
if flag:
    f = Foo()
else:
    f = 42.0
f += 12
```

crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md

@@ -126,8 +126,7 @@ else:
     f = 42.0
 f += 12
 
-# TODO(charlie): This should be `str | int | float`
-reveal_type(f)  # revealed: @Todo
+reveal_type(f)  # revealed: int | str | float
 ```
 
 ## Target union
@@ -148,6 +147,36 @@ else:
     f = 42.0
 f += 12
 
-# TODO(charlie): This should be `str | float`.
-reveal_type(f)  # revealed: @Todo
+reveal_type(f)  # revealed: str | float
+```
+
+## Partially bound target union with `__add__`
+
+```py
+def bool_instance() -> bool:
+    return True
+
+flag = bool_instance()
+
+class Foo:
+    def __add__(self, other: int) -> str:
+        return "Hello, world!"
+    if bool_instance():
+        def __iadd__(self, other: int) -> int:
+            return 42
+
+class Bar:
+    def __add__(self, other: int) -> bytes:
+        return b"Hello, world!"
+
+    def __iadd__(self, other: int) -> float:
+        return 42.0
+
+if flag:
+    f = Foo()
+else:
+    f = Bar()
+f += 12
+
+reveal_type(f)  # revealed: int | str | float
 ```

crates/red_knot_python_semantic/src/types.rs

@@ -1902,7 +1902,7 @@ impl<'db> UnionType<'db> {
     pub fn map(
         &self,
         db: &'db dyn Db,
-        transform_fn: impl Fn(&Type<'db>) -> Type<'db>,
+        transform_fn: impl FnMut(&Type<'db>) -> Type<'db>,
     ) -> Type<'db> {
         Self::from_elements(db, self.elements(db).iter().map(transform_fn))
     }

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1516,40 +1516,22 @@ impl<'db> TypeInferenceBuilder<'db> {
         }
     }
 
-    fn infer_augment_assignment_definition(
+    fn infer_augmented_op(
         &mut self,
         assignment: &ast::StmtAugAssign,
-        definition: Definition<'db>,
-    ) {
-        let target_ty = self.infer_augment_assignment(assignment);
-        self.add_binding(assignment.into(), definition, target_ty);
-    }
-
-    fn infer_augment_assignment(&mut self, assignment: &ast::StmtAugAssign) -> Type<'db> {
-        let ast::StmtAugAssign {
-            range: _,
-            target,
-            op,
-            value,
-        } = assignment;
-
-        // Resolve the target type, assuming a load context.
-        let target_type = match &**target {
-            Expr::Name(name) => {
-                self.store_expression_type(target, Type::Never);
-                self.infer_name_load(name)
-            }
-            Expr::Attribute(attr) => {
-                self.store_expression_type(target, Type::Never);
-                self.infer_attribute_load(attr)
-            }
-            _ => self.infer_expression(target),
-        };
-        let value_type = self.infer_expression(value);
-
+        target_type: Type<'db>,
+        value_type: Type<'db>,
+    ) -> Type<'db> {
         // If the target defines, e.g., `__iadd__`, infer the augmented assignment as a call to that
         // dunder.
-        if let Type::Instance(class) = target_type {
+        let op = assignment.op;
+        match target_type {
+            Type::Union(union) => {
+                return union.map(self.db, |&target_type| {
+                    self.infer_augmented_op(assignment, target_type, value_type)
+                })
+            }
+            Type::Instance(class) => {
                 if let Symbol::Type(class_member, boundness) =
                     class.class_member(self.db, op.in_place_dunder())
                 {
@@ -1580,7 +1562,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                             let left_ty = target_type;
                             let right_ty = value_type;
 
-                        let binary_return_ty = self.infer_binary_expression_type(left_ty, right_ty, *op)
+                            let binary_return_ty = self.infer_binary_expression_type(left_ty, right_ty, op)
                                 .unwrap_or_else(|| {
                                     self.diagnostics.add(
                                         assignment.into(),
@@ -1594,16 +1576,22 @@ impl<'db> TypeInferenceBuilder<'db> {
                                     Type::Unknown
                                 });
 
-                        UnionType::from_elements(self.db, [augmented_return_ty, binary_return_ty])
+                            UnionType::from_elements(
+                                self.db,
+                                [augmented_return_ty, binary_return_ty],
+                            )
                         }
                     };
                 }
             }
+            _ => {}
+        }
 
+        // By default, fall back to non-augmented binary operator inference.
         let left_ty = target_type;
         let right_ty = value_type;
 
-        self.infer_binary_expression_type(left_ty, right_ty, *op)
+        self.infer_binary_expression_type(left_ty, right_ty, op)
             .unwrap_or_else(|| {
                 self.diagnostics.add(
                     assignment.into(),
@@ -1618,6 +1606,40 @@ impl<'db> TypeInferenceBuilder<'db> {
             })
     }
 
+    fn infer_augment_assignment_definition(
+        &mut self,
+        assignment: &ast::StmtAugAssign,
+        definition: Definition<'db>,
+    ) {
+        let target_ty = self.infer_augment_assignment(assignment);
+        self.add_binding(assignment.into(), definition, target_ty);
+    }
+
+    fn infer_augment_assignment(&mut self, assignment: &ast::StmtAugAssign) -> Type<'db> {
+        let ast::StmtAugAssign {
+            range: _,
+            target,
+            op: _,
+            value,
+        } = assignment;
+
+        // Resolve the target type, assuming a load context.
+        let target_type = match &**target {
+            Expr::Name(name) => {
+                self.store_expression_type(target, Type::Never);
+                self.infer_name_load(name)
+            }
+            Expr::Attribute(attr) => {
+                self.store_expression_type(target, Type::Never);
+                self.infer_attribute_load(attr)
+            }
+            _ => self.infer_expression(target),
+        };
+        let value_type = self.infer_expression(value);
+
+        self.infer_augmented_op(assignment, target_type, value_type)
+    }
+
     fn infer_type_alias_statement(&mut self, type_alias_statement: &ast::StmtTypeAlias) {
         let ast::StmtTypeAlias {
             range: _,