collapse assignment with adjacent subsequent usage (#1553)

- consolidate `cascade` optimisations
- support ++/-- postfixes
- remove redundant optimisation identified in #1460

fixes #368

Author: alexlamsl

lib/compress.js

@@ -607,10 +607,7 @@ merge(Compressor.prototype, {
             return statements;
 
             function is_lvalue(node, parent) {
-                return node instanceof AST_SymbolRef && (
-                    (parent instanceof AST_Assign && node === parent.left)
-                    || (parent instanceof AST_Unary && parent.expression === node
-                        && (parent.operator == "++" || parent.operator == "--")));
+                return node instanceof AST_SymbolRef && isLHS(node, parent);
             }
             function replace_var(node, parent, is_constant) {
                 if (is_lvalue(node, parent)) return node;
@@ -1152,7 +1149,7 @@ merge(Compressor.prototype, {
     });
 
     function isLHS(node, parent) {
-        return parent instanceof AST_Unary && (parent.operator === "++" || parent.operator === "--")
+        return parent instanceof AST_Unary && (parent.operator == "++" || parent.operator == "--")
             || parent instanceof AST_Assign && parent.left === node;
     }
 
@@ -2832,21 +2829,35 @@ merge(Compressor.prototype, {
         self.car = self.car.drop_side_effect_free(compressor, first_in_statement(compressor));
         if (!self.car) return maintain_this_binding(compressor.parent(), self, self.cdr);
         if (compressor.option("cascade")) {
+            var left;
             if (self.car instanceof AST_Assign
                 && !self.car.left.has_side_effects(compressor)) {
-                if (self.car.left.equivalent_to(self.cdr)) {
-                    return self.car;
-                }
-                if (self.cdr instanceof AST_Call
-                    && self.cdr.expression.equivalent_to(self.car.left)) {
-                    self.cdr.expression = self.car;
-                    return self.cdr;
+                left = self.car.left;
+            } else if (self.car instanceof AST_UnaryPostfix
+                && (self.car.operator == "++" || self.car.operator == "--")) {
+                left = self.car.expression;
+            }
+            if (left) {
+                var parent, field;
+                var cdr = self.cdr;
+                while (true) {
+                    if (cdr.equivalent_to(left)) {
+                        if (parent) {
+                            parent[field] = self.car;
+                            return self.cdr;
+                        }
+                        return self.car;
+                    }
+                    if (cdr instanceof AST_Binary && !(cdr instanceof AST_Assign)) {
+                        field = cdr.left.is_constant() ? "right" : "left";
+                    } else if (cdr instanceof AST_Call
+                        || cdr instanceof AST_Unary && cdr.operator != "++" && cdr.operator != "--") {
+                        field = "expression";
+                    } else break;
+                    parent = cdr;
+                    cdr = cdr[field];
                 }
             }
-            if (!self.car.has_side_effects(compressor)
-                && self.car.equivalent_to(self.cdr)) {
-                return self.car;
-            }
         }
         if (is_undefined(self.cdr)) {
             return make_node(AST_UnaryPrefix, self, {

test/compress/issue-368.js

@@ -0,0 +1,41 @@
+collapse: {
+    options = {
+        cascade: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        function f1() {
+            var a;
+            a = typeof b === 'function' ? b() : b;
+            return a !== undefined && c();
+        }
+        function f2(b) {
+            var a;
+            b = c();
+            a = typeof b === 'function' ? b() : b;
+            return 'stirng' == typeof a && d();
+        }
+        function f3(c) {
+            var a;
+            a = b(a / 2);
+            if (a < 0) {
+                c++;
+                return c / 2;
+            }
+        }
+    }
+    expect: {
+        function f1() {
+            return void 0 !== ('function' === typeof b ? b() : b) && c();
+        }
+        function f2(b) {
+            return b = c(), 'stirng' == typeof ('function' === typeof b ? b() : b) && d();
+        }
+        function f3(c) {
+            var a;
+            if ((a = b(a / 2)) < 0) return c++ / 2;
+        }
+    }
+}