enhance `unused` (#4858)

1 files changed, 22 insertions, 7 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 846c391b..8ec9ab88 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -5817,19 +5817,29 @@ merge(Compressor.prototype, {
             } else if (node instanceof AST_Unary) {
                 if (node.write_only) sym = node.expression;
             }
-            if (/strict/.test(compressor.option("pure_getters"))) {
-                while (sym instanceof AST_PropAccess && !sym.expression.may_throw_on_access(compressor)) {
-                    if (sym instanceof AST_Sub) props.unshift(sym.property);
-                    sym = sym.expression;
-                    nested = true;
-                }
-            }
+            if (/strict/.test(compressor.option("pure_getters"))) sym = extract_reference(sym, props);
             if (!(sym instanceof AST_SymbolRef)) return;
             var def = sym.definition();
             if (export_defaults[def.id]) return;
             if (compressor.exposed(def)) return;
             if (!can_drop_symbol(sym, compressor, nested)) return;
             return sym;
+
+            function extract_reference(node, props) {
+                if (node instanceof AST_PropAccess) {
+                    var expr = node.expression;
+                    if (!expr.may_throw_on_access(compressor)) {
+                        nested = true;
+                        if (props && node instanceof AST_Sub) props.unshift(node.property);
+                        return extract_reference(expr, props);
+                    }
+                } else if (node instanceof AST_Assign && node.operator == "=") {
+                    var ref = extract_reference(node.right);
+                    if (props) props.assign = node;
+                    return ref;
+                }
+                return node;
+            }
         };
         var assign_in_use = Object.create(null);
         var export_defaults = Object.create(null);
@@ -6648,6 +6658,11 @@ merge(Compressor.prototype, {
                         }
                     }, true);
                 }))) {
+                if (props.assign) {
+                    props.assign.write_only = true;
+                    props.assign.walk(tw);
+                    delete props.assign.write_only;
+                }
                 props.forEach(function(prop) {
                     prop.walk(tw);
                 });