fix corner case in collapse_vars (#3521)

fixes #3520

lib/compress.js

@@ -1128,6 +1128,8 @@ merge(Compressor.prototype, {
                 if (!stop_if_hit && in_conditional(node, parent)) {
                     stop_if_hit = parent;
                 }
+                // Skip transient nodes caused by single-use variable replacement
+                if (node.single_use && parent instanceof AST_VarDef && parent.value === node) return node;
                 // Replace variable with assignment when found
                 var hit_rhs;
                 if (can_replace
@@ -3737,9 +3739,7 @@ merge(Compressor.prototype, {
                         def.value = null;
                         head.push(def);
                     } else {
-                        var value = def.value
-                            && !def.value.single_use
-                            && def.value.drop_side_effect_free(compressor);
+                        var value = def.value && def.value.drop_side_effect_free(compressor);
                         if (value) {
                             AST_Node.warn("Side effects in initialization of unused variable {name} [{file}:{line},{col}]", template(def.name));
                             side_effects.push(value);

test/compress/collapse_vars.js

@@ -6258,3 +6258,41 @@ cond_sequence_return: {
     }
     expect_stdout: "2"
 }
+
+issue_3520: {
+    options = {
+        collapse_vars: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = 0;
+        var b = function(c) {
+            for (var i = 2; --i >= 0;) {
+                (function f() {
+                    c = 0;
+                    var i = void 0;
+                    var f = f && f[i];
+                })();
+                a += b;
+                c && b++;
+            }
+        }(b = 1);
+        console.log(a);
+    }
+    expect: {
+        var a = 0;
+        var b = function(c) {
+            for (var i = 2; --i >= 0;) {
+                (function() {
+                    c = 0;
+                    var f = f && f[void 0];
+                })();
+                a += b;
+                c && b++;
+            }
+        }(b = 1);
+        console.log(a);
+    }
+    expect_stdout: "2"
+}

test/compress/drop-unused.js

@@ -2121,7 +2121,8 @@ issue_3515_1: {
     expect: {
         var c = 0;
         (function() {
-            for (var key20 in !(this[c++] = 0));
+            this[c++] = 0;
+            for (var key20 in !0);
         })();
         console.log(c);
     }