fix corner case in merge_vars (#5466)

fixes #5465

test/compress/default-values.js

@@ -2421,3 +2421,30 @@ issue_5463: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5465: {
+    options = {
+        inline: true,
+        merge_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function f(a, b) {
+            (function(c = b = "FAIL 2") {
+                this && console.log(b || "PASS");
+            })(42 - a && a);
+        }
+        f("FAIL 1");
+    }
+    expect: {
+        a = "FAIL 1",
+        void function(c = b = "FAIL 2") {
+            this && console.log(b || "PASS");
+        }(42 - a && a);
+        var a, b;
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/compress/destructured.js

@@ -1815,8 +1815,8 @@ issue_4288: {
                 console.log(typeof b);
             }()]: a,
         }) {
-            var a = a;
-            a++;
+            var b = a;
+            b++;
         }
         f(0);
     }
@@ -1848,8 +1848,8 @@ issue_4294: {
             }) {}({
                 [a]: 0,
             });
-            var a = A;
-            console.log(a);
+            var b = A;
+            console.log(b);
         })();
     }
     expect_stdout: "PASS"