fix corner case in unused (#4907)

fixes #4906

lib/compress.js

@@ -7671,12 +7671,21 @@ merge(Compressor.prototype, {
             return expressions === this.expressions ? this : make_sequence(this, expressions);
         });
         def(AST_Sub, function(compressor, first_in_statement) {
-            if (this.expression.may_throw_on_access(compressor)) return this;
+            var expr = this.expression;
-            var expression = this.expression.drop_side_effect_free(compressor, first_in_statement);
+            var prop = this.property;
-            if (!expression) return this.property.drop_side_effect_free(compressor, first_in_statement);
+            if (expr.may_throw_on_access(compressor)) {
-            var property = this.property.drop_side_effect_free(compressor);
+                if (!this.optional) return this;
-            if (!property) return expression;
+                prop = prop.drop_side_effect_free(compressor);
-            return make_sequence(this, [ expression, property ]);
+                if (!prop) return expr.drop_side_effect_free(compressor, first_in_statement);
+                var node = this.clone();
+                node.property = prop;
+                return node;
+            }
+            expr = expr.drop_side_effect_free(compressor, first_in_statement);
+            if (!expr) return prop.drop_side_effect_free(compressor, first_in_statement);
+            prop = prop.drop_side_effect_free(compressor);
+            if (!prop) return expr;
+            return make_sequence(this, [ expr, prop ]);
         });
         def(AST_SymbolRef, function(compressor) {
             return this.is_declared(compressor) && can_drop_symbol(this, compressor) ? null : this;

test/compress/optional-chains.js

@@ -194,3 +194,20 @@ trim_2: {
     expect_stdout: "PASS"
     node_version: ">=14"
 }
+
+issue_4906: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        do {
+            var a = a?.[42];
+        } while (console.log("PASS"));
+    }
+    expect: {
+        do {} while (console.log("PASS"));
+    }
+    expect_stdout: "PASS"
+    node_version: ">=14"
+}