fix corner case in evaluate (#3906)

fixes #3905

lib/compress.js

@@ -3570,9 +3570,10 @@ merge(Compressor.prototype, {
                 }
                 var args = eval_args(this.args);
                 if (!args && !ignore_side_effects) return this;
-                if (!stat.value) return;
+                var val = stat.value;
+                if (!val) return;
                 var cached_args = [];
-                if (args && !all(fn.argnames, function(sym, i) {
+                if (!args || all(fn.argnames, function(sym, i) {
                     var value = args[i];
                     var def = sym.definition();
                     if (def.orig[def.orig.length - 1] !== sym) return false;
@@ -3583,15 +3584,15 @@ merge(Compressor.prototype, {
                         cached_args.push(node);
                     });
                     return true;
-                }) && !ignore_side_effects) return this;
+                }) || ignore_side_effects) {
                     fn.evaluating = true;
-                var val = stat.value._eval(compressor, ignore_side_effects, cached, depth);
+                    val = val._eval(compressor, ignore_side_effects, cached, depth);
                     delete fn.evaluating;
+                }
                 cached_args.forEach(function(node) {
                     delete node._eval;
                 });
-                if (val === stat.value) return this;
-                return val;
+                return val === stat.value ? this : val;
             } else if (compressor.option("unsafe") && exp instanceof AST_PropAccess) {
                 var key = exp.property;
                 if (key instanceof AST_Node) {

test/compress/evaluate.js

@@ -2444,3 +2444,22 @@ issue_3903: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3905: {
+    options = {
+        evaluate: true,
+        passes: 2,
+        unused: true,
+    }
+    input: {
+        (function(a, a) {
+            return console.log(a = 0), a && console.log("FAIL");
+        })("foo", 42);
+    }
+    expect: {
+        (function(a, a) {
+            return console.log(a = 0), a && console.log("FAIL");
+        })("foo", 42);
+    }
+    expect_stdout: "0"
+}