Optimize conditionals where the condition and consequent are equivalent and have no side effects

lib/compress.js

@@ -2332,6 +2332,15 @@ merge(Compressor.prototype, {
 
             }
         }
+        // x=y?y:z --> x=y||z
+        if (self.condition.equivalent_to(consequent)
+            && !self.condition.has_side_effects(compressor)) {
+            return make_node(AST_Binary, self.condition, {
+                        left: self.condition,
+                        operator: "||",
+                        right: alternative
+                    });
+        }
         return self;
     });
 

test/compress/conditionals.js

@@ -293,3 +293,38 @@ cond_7: {
     }
 }
 
+cond_8: {
+    options = {
+        conditionals: true,
+        evaluate    : true
+    };
+    input: {
+        // compress these
+        a = condition ? condition : b;
+
+        if (condition) {
+            a = condition;
+        } else {
+            a = b;
+        }
+
+        a = condition ? condition : b();
+
+        // Don't compress conditions that have side effects
+        if (condition()) {
+            a = condition();
+        } else {
+            a = b;
+        }
+
+        a = condition() ? condition() : b;
+
+    }
+    expect: {
+        a = condition || b;
+        a = condition || b;
+        a = condition || b();
+        a = condition() ? condition() : b;
+        a = condition() ? condition() : b;
+    }
+}