enhance comparisons (#3379)

lib/compress.js

@@ -2296,17 +2296,14 @@ merge(Compressor.prototype, {
         def(AST_Node, is_strict);
         def(AST_Array, return_false);
         def(AST_Assign, function(compressor) {
-            return this.operator == "="
-                && this.right._dot_throw(compressor);
-        })
+            return this.operator == "=" && this.right._dot_throw(compressor);
+        });
         def(AST_Binary, function(compressor) {
-            return (this.operator == "&&" || this.operator == "||")
-                && (this.left._dot_throw(compressor) || this.right._dot_throw(compressor));
-        })
+            return lazy_op[this.operator] && (this.left._dot_throw(compressor) || this.right._dot_throw(compressor));
+        });
         def(AST_Conditional, function(compressor) {
-            return this.consequent._dot_throw(compressor)
-                || this.alternative._dot_throw(compressor);
-        })
+            return this.consequent._dot_throw(compressor) || this.alternative._dot_throw(compressor);
+        });
         def(AST_Constant, return_false);
         def(AST_Dot, function(compressor) {
             if (!is_strict(compressor)) return false;
@@ -2346,6 +2343,51 @@ merge(Compressor.prototype, {
         node.DEFMETHOD("_dot_throw", func);
     });
 
+    (function(def) {
+        def(AST_Node, return_false);
+        def(AST_Array, return_true);
+        def(AST_Assign, function(compressor) {
+            return this.operator != "=" || this.right.is_defined(compressor);
+        });
+        def(AST_Binary, function(compressor) {
+            switch (this.operator) {
+              case "&&":
+                return this.left.is_defined(compressor) && this.right.is_defined(compressor);
+              case "||":
+                return this.left.is_defined(compressor) || this.right.is_defined(compressor);
+              default:
+                return true;
+            }
+        });
+        def(AST_Conditional, function(compressor) {
+            return this.consequent.is_defined(compressor) && this.alternative.is_defined(compressor);
+        });
+        def(AST_Constant, return_true);
+        def(AST_Lambda, return_true);
+        def(AST_Object, return_true);
+        def(AST_Sequence, function(compressor) {
+            return this.tail_node().is_defined(compressor);
+        });
+        def(AST_SymbolRef, function(compressor) {
+            if (this.is_undefined) return false;
+            if (is_undeclared_ref(this) && this.is_declared(compressor)) return true;
+            if (this.is_immutable()) return true;
+            var fixed = this.fixed_value();
+            if (!fixed) return false;
+            this.is_defined = return_true;
+            var result = fixed.is_defined(compressor);
+            delete this.is_defined;
+            return result;
+        });
+        def(AST_UnaryPrefix, function() {
+            return this.operator != "void";
+        });
+        def(AST_UnaryPostfix, return_true);
+        def(AST_Undefined, return_false);
+    })(function(node, func) {
+        node.DEFMETHOD("is_defined", func);
+    });
+
     /* -----[ boolean/negation helpers ]----- */
 
     // methods to determine whether an expression has a boolean result type
@@ -5490,6 +5532,13 @@ merge(Compressor.prototype, {
         if (compressor.option("comparisons")) switch (self.operator) {
           case "===":
           case "!==":
+            if (is_undefined(self.left, compressor) && self.right.is_defined(compressor)) {
+                compressor.warn("Expression always defined [{file}:{line},{col}]", self.start);
+                return make_sequence(self, [
+                    self.right,
+                    make_node(self.operator == "===" ? AST_False : AST_True, self)
+                ]).optimize(compressor);
+            }
             var is_strict_comparison = true;
             if ((self.left.is_string(compressor) && self.right.is_string(compressor)) ||
                 (self.left.is_number(compressor) && self.right.is_number(compressor)) ||
@@ -5528,6 +5577,8 @@ merge(Compressor.prototype, {
             break;
           case "&&":
           case "||":
+            // void 0 !== x && null !== x => null != x
+            // void 0 === x || null === x => null == x
             var lhs = self.left;
             if (lhs.operator == self.operator) {
                 lhs = lhs.right;

test/compress/comparisons.js

@@ -345,3 +345,23 @@ is_boolean_var: {
     }
     expect_stdout: "1"
 }
+
+is_defined: {
+    options = {
+        comparisons: true,
+    }
+    input: {
+        console.log(function a() {
+            return void 0 === a;
+        }());
+    }
+    expect: {
+        console.log(function a() {
+            return a, false;
+        }());
+    }
+    expect_stdout: "false"
+    expect_warnings: [
+        "WARN: Expression always defined [test/compress/comparisons.js:2,19]",
+    ]
+}