fix corner case in evaluate (#5775)

fixes #5774

lib/compress.js

@@ -5431,22 +5431,25 @@ Compressor.prototype.compress = function(node) {
             }
         }
         def(AST_SymbolRef, function(compressor, ignore_side_effects, cached, depth) {
-            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);
+            this._eval = return_this;
+            try {
+                var fixed = this.fixed_value();
+                if (!fixed) return this;
+                var value;
+                if (HOP(fixed, "_eval")) {
+                    value = fixed._eval();
+                } else {
+                    value = fixed._eval(compressor, ignore_side_effects, cached, depth);
+                    if (value === fixed) return this;
+                    fixed._eval = function() {
+                        return value;
+                    };
+                    cached.push(fixed);
+                }
+                return value && typeof value == "object" && !verify_escaped(this, depth) ? this : value;
+            } finally {
                 delete this._eval;
-                if (value === fixed) return this;
-                fixed._eval = function() {
-                    return value;
-                };
-                cached.push(fixed);
             }
-            return value && typeof value == "object" && !verify_escaped(this, depth) ? this : value;
         });
         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"
+}