drop assignments to constant expressions only (#2839)

fixes #2838

lib/compress.js

@@ -3469,8 +3469,10 @@ merge(Compressor.prototype, {
             while (left instanceof AST_PropAccess) {
                 left = left.expression;
             }
-            if (left instanceof AST_Symbol) return this;
+            if (left.is_constant_expression(compressor.find_parent(AST_Scope))) {
                 return this.right.drop_side_effect_free(compressor);
+            }
+            return this;
         });
         def(AST_Conditional, function(compressor){
             var consequent = this.consequent.drop_side_effect_free(compressor);

test/compress/pure_getters.js

@@ -694,3 +694,30 @@ issue_2678: {
     }
     expect_stdout: "PASS"
 }
+
+issue_2838: {
+    options = {
+        pure_getters: true,
+        side_effects: true,
+    }
+    input: {
+        function f(a, b) {
+            (a || b).c = "PASS";
+            (function() {
+                return f(a, b);
+            }).prototype.foo = "bar";
+        }
+        var o = {};
+        f(null, o);
+        console.log(o.c);
+    }
+    expect: {
+        function f(a, b) {
+            (a || b).c = "PASS";
+        }
+        var o = {};
+        f(null, o);
+        console.log(o.c);
+    }
+    expect_stdout: "PASS"
+}