Use binary semantics when __iadd__ et al are unbound (#13987)

## Summary

I noticed that augmented assignments on floats were yielding "not
supported" diagnostics. If the dunder isn't bound at all, we should use
binary operator semantics, rather than treating it as not-callable.

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

@@ -6,6 +6,10 @@
 x = 3
 x -= 1
 reveal_type(x)  # revealed: Literal[2]
+
+x = 1.0
+x /= 2
+reveal_type(x)  # revealed: float
 ```
 
 ## Dunder methods

crates/red_knot_python_semantic/src/types/infer.rs

@@ -1425,12 +1425,17 @@ impl<'db> TypeInferenceBuilder<'db> {
         };
         let value_type = self.infer_expression(value);
 
+        // 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 class_member = class.class_member(self.db, op.in_place_dunder());
+            if !class_member.is_unbound() {
                 let call = class_member.call(self.db, &[target_type, value_type]);
-
+                return match call.return_ty_result(
-            return match call.return_ty_result(self.db, AnyNodeRef::StmtAugAssign(assignment), self)
+                    self.db,
-            {
+                    AnyNodeRef::StmtAugAssign(assignment),
+                    self,
+                ) {
                     Ok(t) => t,
                     Err(e) => {
                         self.add_diagnostic(
@@ -1446,6 +1451,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                     }
                 };
             }
+        }
 
         let left_ty = target_type;
         let right_ty = value_type;

crates/ruff_benchmark/benches/red_knot.rs

@@ -28,30 +28,22 @@ static EXPECTED_DIAGNOSTICS: &[&str] = &[
     // We don't support terminal statements in control flow yet:
     "/src/tomllib/_parser.py:66:18: Name `s` used when possibly not defined",
     "/src/tomllib/_parser.py:98:12: Name `char` used when possibly not defined",
-    "/src/tomllib/_parser.py:99:13: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:101:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:104:14: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:108:17: Conflicting declared types for `second_char`: Unknown, str | None",
     "/src/tomllib/_parser.py:115:14: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:126:12: Name `char` used when possibly not defined",
-    "/src/tomllib/_parser.py:130:9: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:267:9: Conflicting declared types for `char`: Unknown, str | None",
     "/src/tomllib/_parser.py:348:20: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:353:5: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:353:5: Method `__getitem__` of type `Unbound | @Todo` is not callable on object of type `Unbound | @Todo`",
     "/src/tomllib/_parser.py:364:9: Conflicting declared types for `char`: Unknown, str | None",
-    "/src/tomllib/_parser.py:367:5: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:381:13: Conflicting declared types for `char`: Unknown, str | None",
-    "/src/tomllib/_parser.py:384:9: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:395:9: Conflicting declared types for `char`: Unknown, str | None",
-    "/src/tomllib/_parser.py:429:9: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:453:24: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:455:9: Name `nest` used when possibly not defined",
     "/src/tomllib/_parser.py:455:9: Method `__getitem__` of type `Unbound | @Todo` is not callable on object of type `Unbound | @Todo`",
-    "/src/tomllib/_parser.py:464:9: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:482:16: Name `char` used when possibly not defined",
-    "/src/tomllib/_parser.py:484:13: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
-    "/src/tomllib/_parser.py:545:5: Operator `+=` is unsupported for type `int` with type `Literal[1]`",
     "/src/tomllib/_parser.py:566:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:573:12: Name `char` used when possibly not defined",
     "/src/tomllib/_parser.py:579:12: Name `char` used when possibly not defined",