fix corner case in merge_vars (#5452)

fixes #5451

lib/compress.js

@@ -6183,8 +6183,11 @@ Compressor.prototype.compress = function(node) {
                 return true;
             }
             if (node instanceof AST_Scope) {
-                var parent = tw.parent();
-                var in_iife = parent && parent.TYPE == "Call" && parent.expression === node;
+                var in_iife = false;
+                if (node instanceof AST_LambdaExpression && !node.name) {
+                    var parent = tw.parent();
+                    in_iife = parent && parent.TYPE == "Call" && parent.expression === node;
+                }
                 if (!in_iife) {
                     push();
                     segment.block = node;

test/compress/const.js

@@ -184,12 +184,12 @@ merge_vars_2: {
     expect: {
         var a = 0;
         1 && --a,
-        a = function f() {
+        b = function f() {
             const c = a && f;
             c.var += 0;
         }(),
-        void console.log(a);
-        var a;
+        void console.log(b);
+        var b;
     }
     expect_stdout: "undefined"
 }

test/compress/let.js

@@ -237,12 +237,12 @@ merge_vars_2: {
         "use strict";
         var a = 0;
         1 && --a,
-        a = function f() {
+        b = function f() {
             let c = a && f;
             c.var += 0;
         }(),
-        void console.log(a);
-        var a;
+        void console.log(b);
+        var b;
     }
     expect_stdout: "undefined"
     node_version: ">=4"

test/compress/merge_vars.js

@@ -3732,3 +3732,25 @@ issue_5420: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5451: {
+    options = {
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        A = 1;
+        var a = 1, b;
+        console.log(function f() {
+            return a-- && f(b = A, b);
+        }());
+    }
+    expect: {
+        A = 1;
+        var a = 1, b;
+        console.log(function f() {
+            return a-- && f(b = A, b);
+        }());
+    }
+    expect_stdout: "0"
+}