Handle maybe-unbound __iadd__-like operators in augmented assignments (#14044)

## Summary

One of the follow-ups from augmented assignment inference, now that
`Type::Unbound` has been removed.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

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

@@ -85,7 +85,7 @@ f = Foo()
 # that `Foo.__iadd__` may be unbound as additional context.
 f += "Hello, world!"
 
-reveal_type(f)  # revealed: int
+reveal_type(f)  # revealed: int | @Todo
 ```
 
 ## Partially bound with `__add__`
@@ -104,8 +104,7 @@ class Foo:
 f = Foo()
 f += "Hello, world!"
 
-# TODO(charlie): This should be `int | str`, since `__iadd__` may be unbound.
-reveal_type(f)  # revealed: int
+reveal_type(f)  # revealed: int | str
 ```
 
 ## Partially bound target union

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1550,14 +1550,11 @@ impl<'db> TypeInferenceBuilder<'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 {
-            if let Some(class_member) = class.class_member(self.db, op.in_place_dunder()).as_type()
+            if let Symbol::Type(class_member, boundness) =
+                class.class_member(self.db, op.in_place_dunder())
             {
-                // TODO: Handle the case where boundness is `MayBeUnbound`: fall back
-                // to the binary-op behavior below and union the result with calling
-                // the possibly-unbound in-place dunder.
-
                 let call = class_member.call(self.db, &[target_type, value_type]);
-                return match call.return_ty_result(
+                let augmented_return_ty = match call.return_ty_result(
                     self.db,
                     AnyNodeRef::StmtAugAssign(assignment),
                     &mut self.diagnostics,
@@ -1576,6 +1573,30 @@ impl<'db> TypeInferenceBuilder<'db> {
                         e.return_ty()
                     }
                 };
+
+                return match boundness {
+                    Boundness::Bound => augmented_return_ty,
+                    Boundness::MayBeUnbound => {
+                        let left_ty = target_type;
+                        let right_ty = value_type;
+
+                        let binary_return_ty = self.infer_binary_expression_type(left_ty, right_ty, *op)
+                            .unwrap_or_else(|| {
+                                self.diagnostics.add(
+                                    assignment.into(),
+                                    "unsupported-operator",
+                                    format_args!(
+                                        "Operator `{op}=` is unsupported between objects of type `{}` and `{}`",
+                                        left_ty.display(self.db),
+                                        right_ty.display(self.db)
+                                    ),
+                                );
+                                Type::Unknown
+                            });
+
+                        UnionType::from_elements(self.db, [augmented_return_ty, binary_return_ty])
+                    }
+                };
             }
         }