fix corner case in reduce_vars (#4934)

fixes #4933

lib/compress.js

@@ -6096,7 +6096,7 @@ merge(Compressor.prototype, {
             return !compressor.exposed(def) && def.references.length == def.replaced;
         } : function(def) {
             if (!(def.id in in_use_ids)) return true;
-            if (def.orig.length < 2) return false;
+            if (def.orig.length - def.eliminated < 2) return false;
             // function argument will always overshadow its name
             if (def.orig[1] instanceof AST_SymbolFunarg) return true;
             // retain if referenced within destructured object of argument
@@ -8551,7 +8551,7 @@ merge(Compressor.prototype, {
                         def.scope = scope;
                         scope.variables.set(def.name, def);
                     }),
-                    value: defn.value
+                    value: defn.value,
                 });
             })
         });
@@ -10672,6 +10672,14 @@ merge(Compressor.prototype, {
                         lambda_def.recursive_refs = def.recursive_refs;
                     }
                     value.walk(new TreeWalker(function(node) {
+                        if (node instanceof AST_SymbolDeclaration) {
+                            if (node !== name) {
+                                var def = node.definition();
+                                def.orig.push(node);
+                                def.eliminated++;
+                            }
+                            return;
+                        }
                         if (!(node instanceof AST_SymbolRef)) return;
                         var def = node.definition();
                         if (def === defun_def) {

test/compress/varify.js

@@ -523,3 +523,55 @@ default_init: {
     expect_stdout: "PASS"
     node_version: ">=4"
 }
+
+issue_4933_1: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+        varify: true,
+    }
+    input: {
+        console.log(f());
+        function f() {
+            var a;
+            for (console in a = [ f ]) {
+                const b = a;
+            }
+        }
+    }
+    expect: {
+        console.log(function f() {
+            var a;
+            for (console in a = [ f ]) {
+                const b = a;
+            }
+        }());
+    }
+    expect_stdout: "undefined"
+}
+
+issue_4933_2: {
+    options = {
+        passes: 2,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+        varify: true,
+    }
+    input: {
+        console.log(f());
+        function f() {
+            var a;
+            for (console in a = [ f ]) {
+                const b = a;
+            }
+        }
+    }
+    expect: {
+        console.log(function f() {
+            for (console in [ f ]);
+        }());
+    }
+    expect_stdout: "undefined"
+}