fix corner case in evaluate (#5775)

fixes #5774

lib/compress.js

@@ -5431,15 +5431,15 @@ Compressor.prototype.compress = function(node) {
             }
         }
         def(AST_SymbolRef, function(compressor, ignore_side_effects, cached, depth) {
+            this._eval = return_this;
+            try {
                 var fixed = this.fixed_value();
                 if (!fixed) return this;
                 var value;
                 if (HOP(fixed, "_eval")) {
                     value = fixed._eval();
                 } else {
-                this._eval = return_this;
                     value = fixed._eval(compressor, ignore_side_effects, cached, depth);
-                delete this._eval;
                     if (value === fixed) return this;
                     fixed._eval = function() {
                         return value;
@@ -5447,6 +5447,9 @@ Compressor.prototype.compress = function(node) {
                     cached.push(fixed);
                 }
                 return value && typeof value == "object" && !verify_escaped(this, depth) ? this : value;
+            } finally {
+                delete this._eval;
+            }
         });
         var global_objs = {
             Array: Array,

test/compress/default-values.js

@@ -3069,3 +3069,41 @@ issue_5651: {
     expect_stdout: true
     node_version: ">=6"
 }
+
+issue_5774: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        evaluate: true,
+        join_vars: true,
+        reduce_vars: true,
+        sequences: true,
+        unsafe: true,
+    }
+    input: {
+        (function() {
+            while (console.log("PASS")) {
+                if (console) {
+                    a = void 0;
+                    var b = void 0;
+                    var c = void 0;
+                    ([ a = 0 ] = [ b, b ]);
+                    var a;
+                }
+            }
+        })();
+    }
+    expect: {
+        (function() {
+            while (console.log("PASS")) {
+                var a, b, c, a;
+                console && (
+                    c = b = a = void 0,
+                    [ a = 0 ] = [ a, a ]
+                );
+            }
+        })();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}