fix corner case in merge_vars (#5445)

fixes #5444

test/compress/arrows.js

@@ -726,8 +726,8 @@ issue_4401: {
     expect: {
         (function() {
             var a = (b => b(a))(console.log || a);
-            var c = console.log;
-            c && c(typeof b);
+            var a = console.log;
+            a && a(typeof b);
         })();
     }
     expect_stdout: [

test/compress/const.js

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

test/compress/default-values.js

@@ -2239,3 +2239,77 @@ issue_5407: {
     ]
     node_version: ">=6"
 }
+
+issue_5444_1: {
+    options = {
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = 42;
+        var b = function({} = setImmediate(function() {
+            console.log(a++);
+        })) {
+            return this;
+        }();
+        console.log(typeof b);
+    }
+    expect: {
+        var a = 42;
+        var b = function({} = setImmediate(function() {
+            console.log(a++);
+        })) {
+            return this;
+        }();
+        console.log(typeof b);
+    }
+    expect_stdout: [
+        "object",
+        "42",
+    ]
+    node_version: ">=6"
+}
+
+issue_5444_2: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        function f(a, b = a++) {
+            return b;
+        }
+        console.log(f("FAIL") || "PASS");
+    }
+    expect: {
+        function f(a, b = a++) {
+            return b;
+        }
+        console.log(f("FAIL") || "PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
+issue_5444_3: {
+    options = {
+        merge_vars: true,
+    }
+    input: {
+        function f(a, b = function(c = a *= this) {
+            return c;
+        }()) {
+            return b;
+        }
+        console.log(f("FAIL") || "PASS");
+    }
+    expect: {
+        function f(a, b = function(c = a *= this) {
+            return c;
+        }()) {
+            return b;
+        }
+        console.log(f("FAIL") || "PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/compress/destructured.js

@@ -1815,8 +1815,8 @@ issue_4288: {
                 console.log(typeof b);
             }()]: a,
         }) {
-            var b = a;
-            b++;
+            var a = a;
+            a++;
         }
         f(0);
     }

test/compress/let.js

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