fix corner case in if_return (#5675)

Author: alexlamsl

Diff for: lib/compress.js

@@ -1828,12 +1828,11 @@ Compressor.prototype.compress = function(node) {
         return wrap ? make_sequence(orig, [ make_node(AST_Number, orig, { value: 0 }), val ]) : val;
     }
 
-    function merge_expression(base, target, scope) {
+    function merge_expression(base, target) {
         var fixed_by_id = new Dictionary();
         base.walk(new TreeWalker(function(node) {
             if (!(node instanceof AST_SymbolRef)) return;
             var def = node.definition();
-            if (scope && def.scope.resolve() !== scope) return;
             var fixed = node.fixed;
             if (!fixed || !fixed_by_id.has(def.id)) {
                 fixed_by_id.set(def.id, fixed);
@@ -3836,7 +3835,7 @@ Compressor.prototype.compress = function(node) {
                   case 2:
                     value = value.tail_node();
                 }
-                merge_expression(value, jump.value, scope);
+                merge_expression(value, jump.value);
             }
 
             function next_index(i) {

Diff for: test/compress/if_return.js

@@ -1834,7 +1834,7 @@ switch_return_5: {
     ]
 }
 
-merged_references: {
+merged_references_1: {
     options = {
         if_return: true,
         reduce_vars: true,
@@ -1861,6 +1861,36 @@ merged_references: {
     expect_stdout: "PASS"
 }
 
+merged_references_2: {
+    options = {
+        if_return: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        A = "PASS";
+        var a;
+        console.log(function(b) {
+            if (a = b)
+                return console && a;
+            a = FAIL;
+            return console && a;
+        }(A));
+    }
+    expect: {
+        A = "PASS";
+        var a;
+        console.log(function(b) {
+            if (a = b);
+            else
+                a = FAIL;
+            return console && a;
+        }(A));
+    }
+    expect_stdout: "PASS"
+}
+
 issue_5583: {
     options = {
         conditionals: true,