improve handling of self-referential classes (#1504)

lib/ast.js

@@ -2271,6 +2271,24 @@ var AST_Class = DEFNODE("Class", "name extends properties", function AST_Class(p
             }
         });
     },
+    is_self_referential: function() {
+        const this_id = this.name && this.name.definition().id;
+        let found = false;
+        let class_this = true;
+        this.visit_nondeferred_class_parts(new TreeWalker((node, descend) => {
+            if (found) return true;
+            if (node instanceof AST_This) return (found = class_this);
+            if (node instanceof AST_SymbolRef) return (found = node.definition().id === this_id);
+            if (node instanceof AST_Lambda && !(node instanceof AST_Arrow)) {
+                const class_this_save = class_this;
+                class_this = false;
+                descend();
+                class_this = class_this_save;
+                return true;
+            }
+        }));
+        return found;
+    },
 }, AST_Scope /* TODO a class might have a scope but it's not a scope */);
 
 var AST_ClassProperty = DEFNODE("ClassProperty", "static quote", function AST_ClassProperty(props) {

lib/compress/drop-side-effect-free.js

@@ -155,9 +155,13 @@ def_drop_side_effect_free(AST_Arrow, return_null);
 
 def_drop_side_effect_free(AST_Class, function (compressor) {
     const with_effects = [];
+
+    if (this.is_self_referential() && this.has_side_effects(compressor)) {
+        return this;
+    }
+
     const trimmed_extends = this.extends && this.extends.drop_side_effect_free(compressor);
-    if (trimmed_extends)
-        with_effects.push(trimmed_extends);
+    if (trimmed_extends) with_effects.push(trimmed_extends);
 
     for (const prop of this.properties) {
         if (prop instanceof AST_ClassStaticBlock) {
@@ -166,11 +170,7 @@ def_drop_side_effect_free(AST_Class, function (compressor) {
             }
         } else {
             const trimmed_prop = prop.drop_side_effect_free(compressor);
-            if (trimmed_prop) {
-                if (trimmed_prop.contains_this()) return this;
-
-                with_effects.push(trimmed_prop);
-            }
+            if (trimmed_prop) with_effects.push(trimmed_prop);
         }
     }
 

lib/compress/drop-unused.js

@@ -70,7 +70,6 @@ import {
     AST_SymbolFunarg,
     AST_SymbolRef,
     AST_SymbolVar,
-    AST_This,
     AST_Toplevel,
     AST_Unary,
     AST_Var,
@@ -127,7 +126,6 @@ AST_Scope.DEFMETHOD("drop_unused", function(compressor) {
             return node.expression;
         }
     };
-    var this_def = null;
     var in_use_ids = new Map();
     var fixed_ids = new Map();
     if (self instanceof AST_Toplevel && compressor.top_retain) {
@@ -139,6 +137,7 @@ AST_Scope.DEFMETHOD("drop_unused", function(compressor) {
     }
     var var_defs_by_id = new Map();
     var initializations = new Map();
+    var self_referential_classes = new Set();
 
     // pass 1: find out which symbols are directly used in
     // this scope (not in nested scopes).
@@ -152,6 +151,10 @@ AST_Scope.DEFMETHOD("drop_unused", function(compressor) {
             });
         }
         if (node === self) return;
+        if (node instanceof AST_Class && node.has_side_effects(compressor)) {
+            if (node.is_self_referential()) self_referential_classes.add(node);
+            node.visit_nondeferred_class_parts(tw);
+        }
         if (node instanceof AST_Defun || node instanceof AST_DefClass) {
             var node_def = node.name.definition();
             const in_export = tw.parent() instanceof AST_Export;
@@ -161,22 +164,11 @@ AST_Scope.DEFMETHOD("drop_unused", function(compressor) {
                 }
             }
 
-            if (node instanceof AST_DefClass && node.has_side_effects(compressor)) {
-                const save_this_def = this_def;
-                this_def = node_def;
-                node.visit_nondeferred_class_parts(tw);
-                this_def = save_this_def;
-            }
-
             map_add(initializations, node_def.id, node);
             return true; // don't go in nested scopes
         }
         // In the root scope, we drop things. In inner scopes, we just check for uses.
         const in_root_scope = scope === self;
-        if (node instanceof AST_This && this_def && in_root_scope) {
-            in_use_ids.set(this_def.id, this_def);
-            return true;
-        }
         if (node instanceof AST_SymbolFunarg && in_root_scope) {
             map_add(var_defs_by_id, node.definition().id, node);
         }
@@ -225,6 +217,9 @@ AST_Scope.DEFMETHOD("drop_unused", function(compressor) {
             init.walk(tw);
         });
     });
+    self_referential_classes.forEach(function (cls) {
+        cls.walk(tw);
+    });
     // pass 3: we should drop declarations not in_use
     var tt = new TreeTransformer(
         function before(node, descend, in_list) {