enhance `evaluate` (#3339)

fixes #3299

1 files changed, 15 insertions, 9 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 2a4f0df8..b5d155ea 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2727,19 +2727,25 @@ merge(Compressor.prototype, {
                 return typeof function(){};
             }
             if (!non_converting_unary[this.operator]) depth++;
-            e = e._eval(compressor, cached, depth);
-            if (e === this.expression) return this;
+            var v = e._eval(compressor, cached, depth);
+            if (v === this.expression) return this;
             switch (this.operator) {
-              case "!": return !e;
+              case "!": return !v;
               case "typeof":
                 // typeof <RegExp> returns "object" or "function" on different platforms
                 // so cannot evaluate reliably
-                if (e instanceof RegExp) return this;
-                return typeof e;
-              case "void": return void e;
-              case "~": return ~e;
-              case "-": return -e;
-              case "+": return +e;
+                if (v instanceof RegExp) return this;
+                return typeof v;
+              case "void": return void v;
+              case "~": return ~v;
+              case "-": return -v;
+              case "+": return +v;
+              case "++":
+              case "--":
+                if (e instanceof AST_SymbolRef) {
+                    var refs = e.definition().references;
+                    if (refs[refs.length - 1] === e) return v;
+                }
             }
             return this;
         });