fix corner case in merge_vars (#5661)

fixes #5660

lib/compress.js

@@ -6560,16 +6560,19 @@ Compressor.prototype.compress = function(node) {
                     if (node instanceof AST_SymbolVar) {
                         mark(node);
                     } else {
-                        references[node.definition().id] = false;
+                        node.walk(tw);
                     }
                 } : function(node) {
-                    var id = node.definition().id;
-                    if (!(node instanceof AST_SymbolVar)) {
-                        references[id] = false;
-                    } else if (!(id in references)) {
-                        declarations.add(id, node);
-                    } else if (references[id]) {
-                        references[id].push(node);
+                    if (node instanceof AST_SymbolVar) {
+                        var id = node.definition().id;
+                        var refs = references[id];
+                        if (refs) {
+                            refs.push(node);
+                        } else if (!(id in references)) {
+                            declarations.add(id, node);
+                        }
+                    } else {
+                        node.walk(tw);
                     }
                 }, node.name);
                 return true;
@@ -6650,8 +6653,8 @@ Compressor.prototype.compress = function(node) {
                     || (head_refs.start.loop || !same_scope(def)) && !mergeable(tail_refs, head_refs)
                     || compressor.option("webkit") && is_funarg(def) !== is_funarg(head.definition)
                     || head.definition.const_redefs
-                    || !all(head_refs, function(sym) {
-                        return sym.scope.find_variable(def.name) === def;
+                    || !all(head_refs.scopes, function(scope) {
+                        return scope.find_variable(def.name) === def;
                     })) {
                     skipped.push(head);
                     continue;
@@ -6733,8 +6736,7 @@ Compressor.prototype.compress = function(node) {
                 var refs = references[def.id];
                 if (!refs) return;
                 if (refs.start.block !== seg.block) return references[def.id] = false;
-                sym.scope = find_scope(tw);
-                refs.push(sym);
+                push_ref(sym);
                 refs.end = seg;
                 if (def.id in prev) {
                     last[prev[def.id]] = null;
@@ -6748,8 +6750,8 @@ Compressor.prototype.compress = function(node) {
                 return references[def.id] = false;
             } else {
                 var refs = declarations.get(def.id) || [];
-                sym.scope = find_scope(tw);
-                refs.push(sym);
+                refs.scopes = [];
+                push_ref(sym);
                 references[def.id] = refs;
                 if (!read) {
                     refs.start = seg;
@@ -6766,6 +6768,13 @@ Compressor.prototype.compress = function(node) {
                 index: index++,
                 definition: def,
             });
+
+            function push_ref(sym) {
+                refs.push(sym);
+                push_uniq(refs.scopes, sym.scope);
+                var scope = find_scope(tw);
+                if (scope !== sym.scope) push_uniq(refs.scopes, scope);
+            }
         }
 
         function insert(target) {

test/compress/const.js

@@ -2169,3 +2169,38 @@ issue_5656: {
     }
     expect_stdout: true
 }
+
+issue_5660: {
+    options = {
+        merge_vars: true,
+        side_effects: true,
+    }
+    input: {
+        function f() {
+            try {
+                a;
+                var b;
+                return b;
+            } catch (e) {
+                var a = "FAIL";
+                const b = null;
+                return a;
+            }
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            try {
+                var b;
+                return b;
+            } catch (e) {
+                var a = "FAIL";
+                const b = null;
+                return a;
+            }
+        }
+        console.log(f());
+    }
+    expect_stdout: true
+}