enhance `side_effects` (#4638)

1 files changed, 23 insertions, 3 deletions

diff --git a/lib/compress.js b/lib/compress.js
index a0e018e3..5a2a0543 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -6922,9 +6922,29 @@ merge(Compressor.prototype, {
                 exprs = trim(exprs, compressor, first_in_statement, array_spread);
                 return exprs && make_sequence(self, exprs.map(convert_spread));
             }
-            var def;
-            if ((is_arrow(exp) && !exp.value || exp instanceof AST_AsyncFunction || exp instanceof AST_Function)
-                && !(exp.name && (def = exp.name.definition()).references.length > def.replaced)) {
+            if (compressor.option("yields") && is_generator(exp)) {
+                var call = self.clone();
+                call.expression = make_node(AST_Function, exp, exp);
+                call.expression.body = [];
+                var opt = call.transform(compressor);
+                if (opt !== call) return opt.drop_side_effect_free(compressor, first_in_statement);
+            }
+            var drop_body = false;
+            if (compressor.option("arrows") && is_arrow(exp)) {
+                if (exp.value) {
+                    exp.value = exp.value.drop_side_effect_free(compressor);
+                } else {
+                    drop_body = true;
+                }
+            } else if (exp instanceof AST_AsyncFunction || exp instanceof AST_Function) {
+                if (exp.name) {
+                    var def = exp.name.definition();
+                    drop_body = def.references.length == def.replaced;
+                } else {
+                    drop_body = true;
+                }
+            }
+            if (drop_body) {
                 exp.process_expression(false, function(node) {
                     var value = node.value && node.value.drop_side_effect_free(compressor, true);
                     return value ? make_node(AST_SimpleStatement, node, {