fix usage accounting within assignment

lib/compress.js

@@ -283,10 +283,16 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_VarDef) {
                     var d = node.name.definition();
                     if (d.fixed == null) {
-                        d.fixed = node.value && function() {
-                            return node.value;
-                        };
+                        if (node.value) {
+                            d.fixed = function() {
+                                return node.value;
+                            };
+                            descend();
+                        } else {
+                            d.fixed = null;
+                        }
                         mark_as_safe(d);
+                        return true;
                     } else if (node.value) {
                         d.fixed = false;
                     }
@@ -1167,6 +1173,9 @@ merge(Compressor.prototype, {
         def(AST_UnaryPrefix, function() {
             return this.operator == "void";
         });
+        def(AST_Seq, function(compressor) {
+            return this.cdr.may_eq_null(compressor);
+        });
         def(AST_PropAccess, function(compressor) {
             return !compressor.option("unsafe");
         });

test/compress/reduce_vars.js

@@ -1916,3 +1916,25 @@ side_effects_assign: {
     }
     expect_stdout: "undefined"
 }
+
+pure_getters: {
+    options = {
+        pure_getters: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        try {
+            var a = (a.b, 2);
+        } catch (e) {}
+        console.log(a);
+    }
+    expect: {
+        try {
+            var a = (a.b, 2);
+        } catch (e) {}
+        console.log(a);
+    }
+    expect_stdout: "undefined"
+}