fix corner case in `reduce_vars` (#3923)

fixes #3922

2 files changed, 28 insertions, 2 deletions

diff --git a/lib/compress.js b/lib/compress.js
index a03b14e9..644746e4 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -738,8 +738,7 @@ merge(Compressor.prototype, {
                         var value = iife.args[i];
                         d.fixed = function() {
                             var j = fn.argnames.indexOf(arg);
-                            if (j < 0) return value;
-                            return iife.args[j] || make_node(AST_Undefined, iife);
+                            return (j < 0 ? value : iife.args[j]) || make_node(AST_Undefined, iife);
                         };
                         tw.loop_ids[d.id] = tw.in_loop;
                         mark(tw, d, true);
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index 08accc45..94852d45 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -7095,3 +7095,30 @@ issue_3894: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3922: {
+    options = {
+        evaluate: true,
+        keep_fargs: "strict",
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        (function(a) {
+            var b;
+            b && b[c];
+            a |= this;
+            console.log("PASS");
+            var c = a.undefined;
+        })();
+    }
+    expect: {
+        (function() {
+            0;
+            console.log("PASS");
+        })();
+    }
+    expect_stdout: "PASS"
+}