Scopes fix WIP 2

lib/serialize/records.js

@@ -153,28 +153,46 @@ function createScope(id, block, parent) {
 
 /**
  * Update parent for existing scope.
+ *
+ * The new parent may be above or below current parent in hierarchy.
+ * The parentage of previously-recorded scope's scope chain is interleaved with new parent's scope chain.
+ * e.g. If:
+ *   - current scope has block ID 6 with ancestors 4 and 2,
+ *   - new parent has block ID 5 with with ancestors 3 and 1
+ * then the chains are joined 5 -> 4 -> 3 -> 2 -> 1.
+ *
+ * The parent may be in a higher block than current scope.
+ * Current 2 -> 1, parent 3 -> 1 => result 3 -> 2 -> 1.
+ *
+ * NB Block IDs are always higher for nested scopes.
+ * i.e. for `function x() { function y() {} }`, `y` will have higher block ID than `x`.
+ * But scope IDs are essentially random - they are allocated in order functions are called,
+ * which may not be in line with how nested the function is.
+ *
  * @param {Object} scope - Scope object
  * @param {Object} parent - Scope object for new parent
  * @returns {undefined}
  */
 function updateScopeParent(scope, parent) {
-	while (true) { // eslint-disable-line no-constant-condition
-		if (scope === parent) return;
-
-		if (parent.block.id > scope.block.id) [scope, parent] = [parent, scope];
+	if (scope === parent) return;
 
+	while (true) { // eslint-disable-line no-constant-condition
+		// Locate where to insert parent in scope chain
 		let nextScope = scope.parent;
 		while (true) { // eslint-disable-line no-constant-condition
 			if (nextScope === parent) return;
 			if (nextScope.block.id < parent.block.id) break;
 			scope = nextScope;
-			nextScope = nextScope.parent;
+			nextScope = scope.parent;
 		}
 
-		const nextParent = scope.parent;
+		// Insert parent
 		scope.parent = parent;
-		scope = parent;
-		parent = nextParent;
+
+		// Step down scope and repeat.
+		// Swap `parent` and `scope` if necessary to ensure `scope` is nested inside `parent`.
+		scope = nextScope;
+		if (parent.block.id > scope.block.id) [scope, parent] = [parent, scope];
 	}
 }