fix corner case in merge_vars (#5609)

test/compress/default-values.js

@@ -1718,7 +1718,7 @@ issue_4540: {
     node_version: ">=6"
 }
 
-issue_4548: {
+issue_4548_1: {
     options = {
         merge_vars: true,
         toplevel: true,
@@ -1744,6 +1744,32 @@ issue_4548: {
     node_version: ">=6"
 }
 
+issue_4548_2: {
+    options = {
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        A = "foo";
+        var a = A;
+        var [ b = c = "bar" ] = [ console, console.log(a) ];
+        console.log(c);
+        var c;
+    }
+    expect: {
+        A = "foo";
+        var a = A;
+        var [ b = c = "bar" ] = [ console, console.log(a) ];
+        console.log(c);
+        var c;
+    }
+    expect_stdout: [
+        "foo",
+        "undefined",
+    ]
+    node_version: ">=6"
+}
+
 issue_4588_1_unused: {
     options = {
         unused: true,

test/compress/destructured.js

@@ -1902,8 +1902,8 @@ issue_4288: {
                 console.log(typeof b);
             }()]: a,
         }) {
-            var b = a;
-            b++;
+            var a = a;
+            a++;
         }
         f(0);
     }
@@ -2076,17 +2076,16 @@ issue_4312: {
         console.log(a);
     }
     expect: {
-        var a;
         b = "PASS",
         c = "FAIL",
         [
             {
-                [a = b]: d,
+                [c = b]: d,
             },
         ] = [ c && c ],
         void 0;
         var b, c, d;
-        console.log(a);
+        console.log(c);
     }
     expect_stdout: "PASS"
     node_version: ">=6"